## Slice

`struct Slice<Base>`

A view into a subsequence of elements of another collection.

A slice stores a base collection and the start and end indices of the view. It does not copy the elements from the collection into separate storage. Thus, creating a slice has O(1) complexity.

## Slices Share Indices

Indices of a slice can be used interchangeably with indices of the base collection. An element of a slice is located under the same index in the slice and in the base collection, as long as neither the collection nor the slice has been mutated since the slice was created.

For example, suppose you have an array holding the number of absences from each class during a session.

``var absences = [0, 2, 0, 4, 0, 3, 1, 0]``

You're tasked with finding the day with the most absences in the second half of the session. To find the index of the day in question, follow these setps:

1) Create a slice of the `absences` array that holds the second half of the days. 2) Use the `max(by:)` method to determine the index of the day with the most absences. 3) Print the result using the index found in step 2 on the original `absences` array.

Here's an implementation of those steps:

``````let secondHalf = absences.suffix(absences.count / 2)
if let i = secondHalf.indices.max(by: { secondHalf[\$0] < secondHalf[\$1] }) {
print("Highest second-half absences: \(absences[i])")
}
// Prints "Highest second-half absences: 3"``````

## Slices Inherit Semantics

A slice inherits the value or reference semantics of its base collection. That is, if a `Slice` instance is wrapped around a mutable collection that has value semantics, such as an array, mutating the original collection would trigger a copy of that collection, and not affect the base collection stored inside of the slice.

For example, if you update the last element of the `absences` array from `0` to `2`, the `secondHalf` slice is unchanged.

``````absences[7] = 2
print(absences)
// Prints "[0, 2, 0, 4, 0, 3, 1, 2]"
print(secondHalf)
// Prints "[0, 3, 1, 0]"``````

Use slices only for transient computation. A slice may hold a reference to the entire storage of a larger collection, not just to the portion it presents, even after the base collection's lifetime ends. Long-term storage of a slice may therefore prolong the lifetime of elements that are no longer otherwise accessible, which can erroneously appear to be memory leakage.

Note: Using a `Slice` instance with a mutable collection requires that the base collection's `subscript(_: Index)` setter does not invalidate indices. If mutations need to invalidate indices in your custom collection type, don't use `Slice` as its subsequence type. Instead, define your own subsequence type that takes your index invalidation requirements into account.

Inheritance `BidirectionalCollection, Collection, LazyCollectionProtocol, LazySequenceProtocol, MutableCollection, RandomAccessCollection, RangeReplaceableCollection, Sequence` View Protocol Hierarchy → `Index = Base.Index` A type that represents a position in the collection. Valid indices consist of the position of every element and a "past the end" position that's not valid for use as a subscript argument. `Indices = Base.Indices` A type that represents the indices that are valid for subscripting the collection, in ascending order. `Element = Base.Element` A type representing the sequence's elements. `SubSequence = Slice` A type that represents a subsequence of some of the sequence's elements. `Iterator = IndexingIterator>` A type that provides the sequence's iteration interface and encapsulates its iteration state. `import Swift`

### Initializers

init()

Creates a new, empty collection.

#### Declaration

`init()`
init(_:)

Creates a new instance of a collection containing the elements of a sequence.

`elements`: The sequence of elements for the new collection. `elements` must be finite.

#### Declaration

`init<S>(_ elements: S)`

#### Declared In

`Slice` , `RangeReplaceableCollection`
init(base:bounds:)

Creates a view into the given collection that allows access to elements within the specified range.

It is unusual to need to call this method directly. Instead, create a slice of a collection by using the collection's range-based subscript or by using methods that return a subsequence.

``````let singleDigits = 0...9
let subSequence = singleDigits.dropFirst(5)
print(Array(subSequence))
// Prints "[5, 6, 7, 8, 9]"``````

In this example, the expression `singleDigits.dropFirst(5))` is equivalent to calling this initializer with `singleDigits` and a range covering the last five items of `singleDigits.indices`.

Parameters: base: The collection to create a view into. bounds: The range of indices to allow access to in the new slice.

#### Declaration

`init(base: Base, bounds: Range<Base.Index>)`
init(repeating:count:)

Creates a new collection containing the specified number of a single, repeated value.

The following example creates an array initialized with five strings containing the letter Z.

``````let fiveZs = Array(repeating: "Z", count: 5)
print(fiveZs)
// Prints "["Z", "Z", "Z", "Z", "Z"]"``````

Parameters: repeatedValue: The element to repeat. count: The number of times to repeat the value passed in the `repeating` parameter. `count` must be zero or greater.

#### Declaration

`init(repeating repeatedValue: Base.Element, count: Int)`

#### Declared In

`Slice` , `RangeReplaceableCollection`

### Instance Variables

var base: Base

The underlying collection of the slice.

You can use a slice's `base` property to access its base collection. The following example declares `singleDigits`, a range of single digit integers, and then drops the first element to create a slice of that range, `singleNonZeroDigits`. The `base` property of the slice is equal to `singleDigits`.

``````let singleDigits = 0..<10
let singleNonZeroDigits = singleDigits.dropFirst()
// singleNonZeroDigits is a Slice<Range<Int>>

print(singleNonZeroDigits.count)
// Prints "9"
prints(singleNonZeroDigits.base.count)
// Prints "10"
print(singleDigits == singleNonZeroDigits.base)
// Prints "true"``````

#### Declaration

`var base: Base { get }`
var count: Int

The number of elements in the collection.

To check whether a collection is empty, use its `isEmpty` property instead of comparing `count` to zero. Unless the collection guarantees random-access performance, calculating `count` can be an O(n) operation.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the length of the collection.

#### Declaration

`var count: Int { get }`

#### Declared In

`LazyCollectionProtocol` , `MutableCollection` , `RandomAccessCollection` , `RangeReplaceableCollection` , `Collection` , `BidirectionalCollection`
var endIndex: Base.Index

The collection's "past the end" position---that is, the position one greater than the last valid subscript argument.

When you need a range that includes the last element of a collection, use the half-open range operator (`..<`) with `endIndex`. The `..<` operator creates a range that doesn't include the upper bound, so it's always safe to use with `endIndex`. For example:

``````let numbers = [10, 20, 30, 40, 50]
if let index = numbers.firstIndex(of: 30) {
print(numbers[index ..< numbers.endIndex])
}
// Prints "[30, 40, 50]"``````

If the collection is empty, `endIndex` is equal to `startIndex`.

#### Declaration

`var endIndex: Base.Index { get }`
var first: Slice<Base>.Element?

The first element of the collection.

If the collection is empty, the value of this property is `nil`.

``````let numbers = [10, 20, 30, 40, 50]
if let firstNumber = numbers.first {
print(firstNumber)
}
// Prints "10"``````

#### Declaration

`var first: Slice<Base>.Element? { get }`

#### Declared In

`LazyCollectionProtocol` , `MutableCollection` , `RandomAccessCollection` , `RangeReplaceableCollection` , `Collection` , `BidirectionalCollection`
var indices: Base.Indices

The indices that are valid for subscripting the collection, in ascending order.

A collection's `indices` property can hold a strong reference to the collection itself, causing the collection to be nonuniquely referenced. If you mutate the collection while iterating over its indices, a strong reference can result in an unexpected copy of the collection. To avoid the unexpected copy, use the `index(after:)` method starting with `startIndex` to produce indices instead.

``````var c = MyFancyCollection([10, 20, 30, 40, 50])
var i = c.startIndex
while i != c.endIndex {
c[i] /= 5
i = c.index(after: i)
}
// c == MyFancyCollection([2, 4, 6, 8, 10])``````

#### Declaration

`var indices: Base.Indices { get }`
var isEmpty: Bool

A Boolean value indicating whether the collection is empty.

When you need to check whether your collection is empty, use the `isEmpty` property instead of checking that the `count` property is equal to zero. For collections that don't conform to `RandomAccessCollection`, accessing the `count` property iterates through the elements of the collection.

``````let horseName = "Silver"
if horseName.isEmpty {
print("I've been through the desert on a horse with no name.")
} else {
print("Hi ho, \(horseName)!")
}
// Prints "Hi ho, Silver!")``````

Complexity: O(1)

#### Declaration

`var isEmpty: Bool { get }`

#### Declared In

`LazyCollectionProtocol` , `MutableCollection` , `RandomAccessCollection` , `RangeReplaceableCollection` , `Collection` , `BidirectionalCollection`
var last: Slice<Base>.Element?

The last element of the collection.

If the collection is empty, the value of this property is `nil`.

``````let numbers = [10, 20, 30, 40, 50]
if let lastNumber = numbers.last {
print(lastNumber)
}
// Prints "50"``````

#### Declaration

`var last: Slice<Base>.Element? { get }`

#### Declared In

`RandomAccessCollection` , `BidirectionalCollection`
var lazy: LazyCollection<Slice<Base>.Elements>

#### Declaration

`var lazy: LazyCollection<Slice<Base>.Elements> { get }`

#### Declared In

`LazyCollectionProtocol`
var startIndex: Base.Index

The position of the first element in a nonempty collection.

If the collection is empty, `startIndex` is equal to `endIndex`.

#### Declaration

`var startIndex: Base.Index { get }`
var underestimatedCount: Int

A value less than or equal to the number of elements in the collection.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the length of the collection.

#### Declaration

`var underestimatedCount: Int { get }`

#### Declared In

`LazyCollectionProtocol` , `MutableCollection` , `RandomAccessCollection` , `RangeReplaceableCollection` , `Collection` , `LazySequenceProtocol` , `BidirectionalCollection` , `Sequence`

### Subscripts

subscript(_: (UnboundedRange_)

#### Declaration

`subscript(x: (UnboundedRange_) -> ()) -> Slice<Base>.SubSequence { get }`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
subscript(_: Range<Slice<Base>.Index>)

Accesses a contiguous subrange of the collection's elements.

The accessed slice uses the same indices for the same elements as the original collection. Always use the slice's `startIndex` property instead of assuming that its indices start at a particular value.

This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.

``````let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2 ..< streets.endIndex]
print(streetsSlice)
// Prints "["Channing", "Douglas", "Evarts"]"

let index = streetsSlice.firstIndex(of: "Evarts")    // 4
streets[index!] = "Eustace"
print(streets[index!])
// Prints "Eustace"``````

`bounds`: A range of the collection's indices. The bounds of the range must be valid indices of the collection.

#### Declaration

`subscript(bounds: Range<Slice<Base>.Index>) -> Slice<Base>`

#### Declared In

`Slice`, `MutableCollection`
subscript(_: Range<Slice<Base>.Index>)

Accesses a contiguous subrange of the collection's elements.

For example, using a `PartialRangeFrom` range expression with an array accesses the subrange from the start of the range expression until the end of the array.

``````let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2..<5]
print(streetsSlice)
// ["Channing", "Douglas", "Evarts"]``````

The accessed slice uses the same indices for the same elements as the original collection. This example searches `streetsSlice` for one of the strings in the slice, and then uses that index in the original array.

``````let index = streetsSlice.firstIndex(of: "Evarts")!    // 4
print(streets[index])
// "Evarts"``````

Always use the slice's `startIndex` property instead of assuming that its indices start at a particular value. Attempting to access an element by using an index outside the bounds of the slice may result in a runtime error, even if that index is valid for the original collection.

``````print(streetsSlice.startIndex)
// 2
print(streetsSlice[2])
// "Channing"

print(streetsSlice[0])
// error: Index out of bounds``````

`bounds`: A range of the collection's indices. The bounds of the range must be valid indices of the collection.

Complexity: O(1)

#### Declaration

`subscript(bounds: Range<Slice<Base>.Index>) -> Slice<Base> { get }`
subscript(_: Slice<Base>.Index)

Accesses the element at the specified position.

For example, you can replace an element of an array by using its subscript.

``````var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
streets[1] = "Butler"
print(streets[1])
// Prints "Butler"``````

You can subscript a collection with any valid index other than the collection's end index. The end index refers to the position one past the last element of a collection, so it doesn't correspond with an element.

`position`: The position of the element to access. `position` must be a valid index of the collection that is not equal to the `endIndex` property.

#### Declaration

`subscript(index: Slice<Base>.Index) -> Base.Element`
subscript(_: Slice<Base>.Index)

Accesses the element at the specified position.

The following example accesses an element of an array through its subscript to print its value:

``````var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
print(streets[1])
// Prints "Bryant"``````

You can subscript a collection with any valid index other than the collection's end index. The end index refers to the position one past the last element of a collection, so it doesn't correspond with an element.

`position`: The position of the element to access. `position` must be a valid index of the collection that is not equal to the `endIndex` property.

Complexity: O(1)

#### Declaration

`subscript(index: Slice<Base>.Index) -> Base.Element { get }`
subscript<R>(_: R)

Accesses the contiguous subrange of the collection's elements specified by a range expression.

The range expression is converted to a concrete subrange relative to this collection. For example, using a `PartialRangeFrom` range expression with an array accesses the subrange from the start of the range expression until the end of the array.

``````let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2...]
print(streetsSlice)
// ["Channing", "Douglas", "Evarts"]``````

The accessed slice uses the same indices for the same elements as the original collection uses. This example searches `streetsSlice` for one of the strings in the slice, and then uses that index in the original array.

``````let index = streetsSlice.firstIndex(of: "Evarts")    // 4
print(streets[index!])
// "Evarts"``````

Always use the slice's `startIndex` property instead of assuming that its indices start at a particular value. Attempting to access an element by using an index outside the bounds of the slice's indices may result in a runtime error, even if that index is valid for the original collection.

``````print(streetsSlice.startIndex)
// 2
print(streetsSlice[2])
// "Channing"

print(streetsSlice[0])
// error: Index out of bounds``````

`bounds`: A range of the collection's indices. The bounds of the range must be valid indices of the collection.

Complexity: O(1)

#### Declaration

`subscript<R>(r: R) -> Slice<Base>.SubSequence where R : RangeExpression, Slice<Base>.Index == R.Bound { get }`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`

### Instance Methods

func allSatisfy(_:)

Returns a Boolean value indicating whether every element of a sequence satisfies a given predicate.

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value that indicates whether the passed element satisfies a condition. Returns: `true` if the sequence contains only elements that satisfy `predicate`; otherwise, `false`.

#### Declaration

`func allSatisfy(_ predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> Bool`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
mutating func append(_:)

Adds an element to the end of the collection.

If the collection does not have sufficient capacity for another element, additional storage is allocated before appending `newElement`. The following example adds a new number to an array of integers:

``````var numbers = [1, 2, 3, 4, 5]
numbers.append(100)

print(numbers)
// Prints "[1, 2, 3, 4, 5, 100]"``````

`newElement`: The element to append to the collection.

Complexity: O(1) on average, over many additions to the same collection.

#### Declaration

`mutating func append(_ newElement: Slice<Base>.Element)`

#### Declared In

`RangeReplaceableCollection`
mutating func append(contentsOf:)

Adds the elements of a sequence or collection to the end of this collection.

The collection being appended to allocates any additional necessary storage to hold the new elements.

The following example appends the elements of a `Range<Int>` instance to an array of integers:

``````var numbers = [1, 2, 3, 4, 5]
numbers.append(contentsOf: 10...15)
print(numbers)
// Prints "[1, 2, 3, 4, 5, 10, 11, 12, 13, 14, 15]"``````

`newElements`: The elements to append to the collection.

Complexity: O(n), where n is the length of the resulting collection.

#### Declaration

`mutating func append<S>(contentsOf newElements: S)`

#### Declared In

`RangeReplaceableCollection`
func compactMap(_:)

Returns the non-`nil` results of mapping the given transformation over this collection.

Use this method to receive a collection of nonoptional values when your transformation produces an optional value.

`transform`: A closure that accepts an element of this collection as its argument and returns an optional value.

Complexity: O(1)

#### Declaration

`func compactMap<ElementOfResult>(_ transform: @escaping (Slice<Base>.Elements.Element) -> ElementOfResult?) -> LazyMapCollection<LazyFilterCollection<LazyMapCollection<Slice<Base>.Elements, ElementOfResult?>>, ElementOfResult>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func contains(where:)

Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate.

You can use the predicate to check for an element of a type that doesn't conform to the `Equatable` protocol, such as the `HTTPResponse` enumeration in this example.

``````enum HTTPResponse {
case ok
case error(Int)
}

let lastThreeResponses: [HTTPResponse] = [.ok, .ok, .error(404)]
let hadError = lastThreeResponses.contains { element in
if case .error = element {
return true
} else {
return false
}
}

Alternatively, a predicate can be satisfied by a range of `Equatable` elements or a general condition. This example shows how you can check an array for an expense greater than \$100.

``````let expenses = [21.37, 55.21, 9.32, 10.18, 388.77, 11.41]
let hasBigPurchase = expenses.contains { \$0 > 100 }
// 'hasBigPurchase' == true``````

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value that indicates whether the passed element represents a match. Returns: `true` if the sequence contains an element that satisfies `predicate`; otherwise, `false`.

#### Declaration

`func contains(where predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> Bool`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func distance(from: Slice<Base>.Index, to: Slice<Base>.Index)

Returns the distance between two indices.

Unless the collection conforms to the `BidirectionalCollection` protocol, `start` must be less than or equal to `end`.

Parameters: start: A valid index of the collection. end: Another valid index of the collection. If `end` is equal to `start`, the result is zero. Returns: The distance between `start` and `end`. The result can be negative only if the collection conforms to the `BidirectionalCollection` protocol.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the resulting distance.

#### Declaration

`func distance(from start: Slice<Base>.Index, to end: Slice<Base>.Index) -> Int`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func distance(from:to:)

Returns the distance between two indices.

Unless the collection conforms to the `BidirectionalCollection` protocol, `start` must be less than or equal to `end`.

Parameters: start: A valid index of the collection. end: Another valid index of the collection. If `end` is equal to `start`, the result is zero. Returns: The distance between `start` and `end`. The result can be negative only if the collection conforms to the `BidirectionalCollection` protocol.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the resulting distance.

#### Declaration

`func distance(from start: Slice<Base>.Index, to end: Slice<Base>.Index) -> Int`

#### Declared In

`Slice`, `RandomAccessCollection`, `BidirectionalCollection`
func distance<T>(from: Slice<Base>.Index, to: Slice<Base>.Index)

Deprecated: all index distances are now of type Int.

#### Declaration

`func distance<T>(from start: Slice<Base>.Index, to end: Slice<Base>.Index) -> T where T : BinaryInteger`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func drop(while:)

Returns a lazy collection that skips any initial elements that satisfy `predicate`.

`predicate`: A closure that takes an element of the collection as its argument and returns `true` if the element should be skipped or `false` otherwise. Once `predicate` returns `false` it will not be called again.

#### Declaration

`func drop(while predicate: @escaping (Slice<Base>.Elements.Element) -> Bool) -> LazyDropWhileCollection<Slice<Base>.Elements>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`
func dropFirst()

Returns a subsequence containing all but the first element of the sequence.

The following example drops the first element from an array of integers.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst())
// Prints "[2, 3, 4, 5]"``````

If the sequence has no elements, the result is an empty subsequence.

``````let empty: [Int] = []
print(empty.dropFirst())
// Prints "[]"``````

Returns: A subsequence starting after the first element of the sequence.

Complexity: O(1)

#### Declaration

`func dropFirst() -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func dropFirst(_:)

Returns a subsequence containing all but the given number of initial elements.

If the number of elements to drop exceeds the number of elements in the collection, the result is an empty subsequence.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"``````

`n`: The number of elements to drop from the beginning of the collection. `n` must be greater than or equal to zero. Returns: A subsequence starting after the specified number of elements.

Complexity: O(n), where n is the number of elements to drop from the beginning of the collection.

#### Declaration

`func dropFirst(_ n: Int) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func dropLast()

Returns a subsequence containing all but the last element of the sequence.

The sequence must be finite.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast())
// Prints "[1, 2, 3, 4]"``````

If the sequence has no elements, the result is an empty subsequence.

``````let empty: [Int] = []
print(empty.dropLast())
// Prints "[]"``````

Returns: A subsequence leaving off the last element of the sequence.

Complexity: O(n), where n is the length of the sequence.

#### Declaration

`func dropLast() -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func dropLast(_:)

Returns a subsequence containing all but the specified number of final elements.

If the number of elements to drop exceeds the number of elements in the collection, the result is an empty subsequence.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"``````

`n`: The number of elements to drop off the end of the collection. `n` must be greater than or equal to zero. Returns: A subsequence that leaves off the specified number of elements at the end.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func dropLast(_ n: Int) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func elementsEqual(_:by:)

Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements in the same order, using the given predicate as the equivalence test.

At least one of the sequences must be finite.

The predicate must be a equivalence relation over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areEquivalent(a, a)` is always `true`. (Reflexivity)
• `areEquivalent(a, b)` implies `areEquivalent(b, a)`. (Symmetry)
• If `areEquivalent(a, b)` and `areEquivalent(b, c)` are both `true`, then `areEquivalent(a, c)` is also `true`. (Transitivity)

Parameters: other: A sequence to compare to this sequence. areEquivalent: A predicate that returns `true` if its two arguments are equivalent; otherwise, `false`. Returns: `true` if this sequence and `other` contain equivalent items, using `areEquivalent` as the equivalence test; otherwise, `false.`

#### Declaration

`func elementsEqual<OtherSequence>(_ other: OtherSequence, by areEquivalent: (Slice<Base>.Element, OtherSequence.Element) throws -> Bool) rethrows -> Bool where OtherSequence : Sequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func enumerated()

Returns a sequence of pairs (n, x), where n represents a consecutive integer starting at zero and x represents an element of the sequence.

This example enumerates the characters of the string "Swift" and prints each character along with its place in the string.

``````for (n, c) in "Swift".enumerated() {
print("\(n): '\(c)'")
}
// Prints "0: 'S'"
// Prints "1: 'w'"
// Prints "2: 'i'"
// Prints "3: 'f'"
// Prints "4: 't'"``````

When you enumerate a collection, the integer part of each pair is a counter for the enumeration, but is not necessarily the index of the paired value. These counters can be used as indices only in instances of zero-based, integer-indexed collections, such as `Array` and `ContiguousArray`. For other collections the counters may be out of range or of the wrong type to use as an index. To iterate over the elements of a collection with its indices, use the `zip(_:_:)` function.

This example iterates over the indices and elements of a set, building a list consisting of indices of names with five or fewer letters.

``````let names: Set = ["Sofia", "Camilla", "Martina", "Mateo", "Nicolás"]
var shorterIndices: [SetIndex<String>] = []
for (i, name) in zip(names.indices, names) {
if name.count <= 5 {
shorterIndices.append(i)
}
}``````

Now that the `shorterIndices` array holds the indices of the shorter names in the `names` set, you can use those indices to access elements in the set.

``````for i in shorterIndices {
print(names[i])
}
// Prints "Sofia"
// Prints "Mateo"``````

Returns: A sequence of pairs enumerating the sequence.

#### Declaration

`func enumerated() -> EnumeratedSequence<Slice<Base>>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func filter(_:)

Returns the elements of `self` that satisfy `predicate`.

Note: The elements of the result are computed on-demand, as the result is used. No buffering storage is allocated and each traversal step invokes `predicate` on one or more underlying elements.

#### Declaration

`func filter(_ isIncluded: @escaping (Slice<Base>.Elements.Element) -> Bool) -> LazyFilterCollection<Slice<Base>.Elements>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func first(where:)

Returns the first element of the sequence that satisfies the given predicate.

The following example uses the `first(where:)` method to find the first negative number in an array of integers:

``````let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
if let firstNegative = numbers.first(where: { \$0 < 0 }) {
print("The first negative number is \(firstNegative).")
}
// Prints "The first negative number is -2."``````

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element is a match. Returns: The first element of the sequence that satisfies `predicate`, or `nil` if there is no element that satisfies `predicate`.

#### Declaration

`func first(where predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Element?`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func firstIndex(where:)

Returns the first index in which an element of the collection satisfies the given predicate.

You can use the predicate to find an element of a type that doesn't conform to the `Equatable` protocol or to find an element that matches particular criteria. Here's an example that finds a student name that begins with the letter "A":

``````let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
if let i = students.firstIndex(where: { \$0.hasPrefix("A") }) {
print("\(students[i]) starts with 'A'!")
}
// Prints "Abena starts with 'A'!"``````

`predicate`: A closure that takes an element as its argument and returns a Boolean value that indicates whether the passed element represents a match. Returns: The index of the first element for which `predicate` returns `true`. If no elements in the collection satisfy the given predicate, returns `nil`.

#### Declaration

`func firstIndex(where predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Index?`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func flatMap(_:)

#### Declaration

`func flatMap(_ transform: (Slice<Base>.Element) throws -> String?) rethrows -> [String]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func flatMap<ElementOfResult>(_: (Slice<Base>.Element) throws -> ElementOfResult?)

Returns an array containing the non-`nil` results of calling the given transformation with each element of this sequence.

Use this method to receive an array of nonoptional values when your transformation produces an optional value.

In this example, note the difference in the result of using `map` and `flatMap` with a transformation that returns an optional `Int` value.

``````let possibleNumbers = ["1", "2", "three", "///4///", "5"]

let mapped: [Int?] = possibleNumbers.map { str in Int(str) }
// [1, 2, nil, nil, 5]

let flatMapped: [Int] = possibleNumbers.flatMap { str in Int(str) }
// [1, 2, 5]``````

`transform`: A closure that accepts an element of this sequence as its argument and returns an optional value. Returns: An array of the non-`nil` results of calling `transform` with each element of the sequence.

Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.

#### Declaration

`func flatMap<ElementOfResult>(_ transform: (Slice<Base>.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func flatMap<ElementOfResult>(_: @escaping (Slice<Base>.Elements.Element) -> ElementOfResult?)

Returns the non-`nil` results of mapping the given transformation over this collection.

Use this method to receive a collection of nonoptional values when your transformation produces an optional value.

`transform`: A closure that accepts an element of this collection as its argument and returns an optional value.

Complexity: O(1)

#### Declaration

`func flatMap<ElementOfResult>(_ transform: @escaping (Slice<Base>.Elements.Element) -> ElementOfResult?) -> LazyMapCollection<LazyFilterCollection<LazyMapCollection<Slice<Base>.Elements, ElementOfResult?>>, ElementOfResult>`

#### Declared In

`LazyCollectionProtocol`, `LazySequenceProtocol`
func flatMap<SegmentOfResult>(_: (Slice<Base>.Element) throws -> SegmentOfResult)

Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.

Use this method to receive a single-level collection when your transformation produces a sequence or collection for each element.

In this example, note the difference in the result of using `map` and `flatMap` with a transformation that returns an array.

``````let numbers = [1, 2, 3, 4]

let mapped = numbers.map { Array(repeating: \$0, count: \$0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]

let flatMapped = numbers.flatMap { Array(repeating: \$0, count: \$0) }
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]``````

In fact, `s.flatMap(transform)` is equivalent to `Array(s.map(transform).joined())`.

`transform`: A closure that accepts an element of this sequence as its argument and returns a sequence or collection. Returns: The resulting flattened array.

Complexity: O(m + n), where m is the length of this sequence and n is the length of the result.

#### Declaration

`func flatMap<SegmentOfResult>(_ transform: (Slice<Base>.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func flatMap<SegmentOfResult>(_: @escaping (Slice<Base>.Elements.Element) -> SegmentOfResult)

Returns the concatenated results of mapping the given transformation over this collection.

Use this method to receive a single-level collection when your transformation produces a collection for each element. Calling `flatMap(_:)` on a collection `c` is equivalent to calling `c.map(transform).joined()`.

Complexity: O(1)

#### Declaration

`func flatMap<SegmentOfResult>(_ transform: @escaping (Slice<Base>.Elements.Element) -> SegmentOfResult) -> LazyCollection<FlattenCollection<LazyMapCollection<Slice<Base>.Elements, SegmentOfResult>>> where SegmentOfResult : Collection`

#### Declared In

`LazyCollectionProtocol`, `LazySequenceProtocol`
func forEach(_:)

Calls the given closure on each element in the sequence in the same order as a `for`-`in` loop.

The two loops in the following example produce the same output:

``````let numberWords = ["one", "two", "three"]
for word in numberWords {
print(word)
}
// Prints "one"
// Prints "two"
// Prints "three"

numberWords.forEach { word in
print(word)
}
// Same as above``````

Using the `forEach` method is distinct from a `for`-`in` loop in two important ways:

1. You cannot use a `break` or `continue` statement to exit the current call of the `body` closure or skip subsequent calls.
2. Using the `return` statement in the `body` closure will exit only from the current call to `body`, not from any outer scope, and won't skip subsequent calls.

`body`: A closure that takes an element of the sequence as a parameter.

#### Declaration

`func forEach(_ body: (Slice<Base>.Element) throws -> Void) rethrows`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func formIndex(_: inout Slice<Base>.Index, offsetBy: Int)

Offsets the given index by the specified distance.

The value passed as `n` must not offset `i` beyond the bounds of the collection.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func formIndex(_ i: inout Slice<Base>.Index, offsetBy n: Int)`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func formIndex<T>(_: inout Slice<Base>.Index, offsetBy: T)

Deprecated: all index distances are now of type Int.

#### Declaration

`func formIndex<T>(_ i: inout Slice<Base>.Index, offsetBy n: T)`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func formIndex(_: inout Slice<Base>.Index, offsetBy: Int, limitedBy: Slice<Base>.Index)

Offsets the given index by the specified distance, or so that it equals the given limiting index.

The value passed as `n` must not offset `i` beyond the bounds of the collection, unless the index passed as `limit` prevents offsetting beyond those bounds.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. limit: A valid index of the collection to use as a limit. If `n > 0`, a limit that is less than `i` has no effect. Likewise, if `n < 0`, a limit that is greater than `i` has no effect. Returns: `true` if `i` has been offset by exactly `n` steps without going beyond `limit`; otherwise, `false`. When the return value is `false`, the value of `i` is equal to `limit`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func formIndex(_ i: inout Slice<Base>.Index, offsetBy n: Int, limitedBy limit: Slice<Base>.Index) -> Bool`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func formIndex<T>(_: inout Slice<Base>.Index, offsetBy: T, limitedBy: Slice<Base>.Index)

Deprecated: all index distances are now of type Int.

#### Declaration

`func formIndex<T>(_ i: inout Slice<Base>.Index, offsetBy n: T, limitedBy limit: Slice<Base>.Index) -> Bool where T : BinaryInteger`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func formIndex(after:)

Replaces the given index with its successor.

`i`: A valid index of the collection. `i` must be less than `endIndex`.

#### Declaration

`func formIndex(after i: inout Slice<Base>.Index)`

#### Declared In

`Slice`, `LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func formIndex(before:)

Replaces the given index with its predecessor.

`i`: A valid index of the collection. `i` must be greater than `startIndex`.

#### Declaration

`func formIndex(before i: inout Slice<Base>.Index)`

#### Declared In

`Slice`, `RandomAccessCollection`, `BidirectionalCollection`
func index(_: Slice<Base>.Index, offsetBy: Int)

Returns an index that is the specified distance from the given index.

The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position.

``````let s = "Swift"
let i = s.index(s.startIndex, offsetBy: 4)
print(s[i])
// Prints "t"``````

The value passed as `n` must not offset `i` beyond the bounds of the collection.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. Returns: An index offset by `n` from the index `i`. If `n` is positive, this is the same value as the result of `n` calls to `index(after:)`. If `n` is negative, this is the same value as the result of `-n` calls to `index(before:)`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: Slice<Base>.Index, offsetBy n: Int) -> Slice<Base>.Index`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func index(_:offsetBy:)

Returns an index that is the specified distance from the given index.

The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position.

``````let s = "Swift"
let i = s.index(s.startIndex, offsetBy: 4)
print(s[i])
// Prints "t"``````

The value passed as `n` must not offset `i` beyond the bounds of the collection.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. Returns: An index offset by `n` from the index `i`. If `n` is positive, this is the same value as the result of `n` calls to `index(after:)`. If `n` is negative, this is the same value as the result of `-n` calls to `index(before:)`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: Slice<Base>.Index, offsetBy n: Int) -> Slice<Base>.Index`

#### Declared In

`Slice`, `RandomAccessCollection`, `BidirectionalCollection`
func index<T>(_: Slice<Base>.Index, offsetBy: T)

Deprecated: all index distances are now of type Int.

#### Declaration

`func index<T>(_ i: Slice<Base>.Index, offsetBy n: T) -> Slice<Base>.Index where T : BinaryInteger`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func index(_: Slice<Base>.Index, offsetBy: Int, limitedBy: Slice<Base>.Index)

Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.

The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position. The operation doesn't require going beyond the limiting `s.endIndex` value, so it succeeds.

``````let s = "Swift"
if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
print(s[i])
}
// Prints "t"``````

The next example attempts to retrieve an index six positions from `s.startIndex` but fails, because that distance is beyond the index passed as `limit`.

``````let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
print(j)
// Prints "nil"``````

The value passed as `n` must not offset `i` beyond the bounds of the collection, unless the index passed as `limit` prevents offsetting beyond those bounds.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. limit: A valid index of the collection to use as a limit. If `n > 0`, a limit that is less than `i` has no effect. Likewise, if `n < 0`, a limit that is greater than `i` has no effect. Returns: An index offset by `n` from the index `i`, unless that index would be beyond `limit` in the direction of movement. In that case, the method returns `nil`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: Slice<Base>.Index, offsetBy n: Int, limitedBy limit: Slice<Base>.Index) -> Slice<Base>.Index?`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func index(_:offsetBy:limitedBy:)

Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.

The following example obtains an index advanced four positions from a string's starting index and then prints the character at that position. The operation doesn't require going beyond the limiting `s.endIndex` value, so it succeeds.

``````let s = "Swift"
if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
print(s[i])
}
// Prints "t"``````

The next example attempts to retrieve an index six positions from `s.startIndex` but fails, because that distance is beyond the index passed as `limit`.

``````let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
print(j)
// Prints "nil"``````

The value passed as `n` must not offset `i` beyond the bounds of the collection, unless the index passed as `limit` prevents offsetting beyond those bounds.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. `n` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. limit: A valid index of the collection to use as a limit. If `n > 0`, a limit that is less than `i` has no effect. Likewise, if `n < 0`, a limit that is greater than `i` has no effect. Returns: An index offset by `n` from the index `i`, unless that index would be beyond `limit` in the direction of movement. In that case, the method returns `nil`.

Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: Slice<Base>.Index, offsetBy n: Int, limitedBy limit: Slice<Base>.Index) -> Slice<Base>.Index?`

#### Declared In

`Slice`, `RandomAccessCollection`, `BidirectionalCollection`
func index<T>(_: Slice<Base>.Index, offsetBy: T, limitedBy: Slice<Base>.Index)

Deprecated: all index distances are now of type Int.

#### Declaration

`func index<T>(_ i: Slice<Base>.Index, offsetBy n: T, limitedBy limit: Slice<Base>.Index) -> Slice<Base>.Index? where T : BinaryInteger`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func index(after:)

Returns the position immediately after the given index.

The successor of an index must be well defined. For an index `i` into a collection `c`, calling `c.index(after: i)` returns the same index every time.

`i`: A valid index of the collection. `i` must be less than `endIndex`. Returns: The index value immediately after `i`.

#### Declaration

`func index(after i: Slice<Base>.Index) -> Slice<Base>.Index`
func index(before:)

Returns the position immediately before the given index.

`i`: A valid index of the collection. `i` must be greater than `startIndex`. Returns: The index value immediately before `i`.

#### Declaration

`func index(before i: Slice<Base>.Index) -> Slice<Base>.Index`
mutating func insert(_: Base.Element, at: Slice<Base>.Index)

#### Declaration

`mutating func insert(_ newElement: Base.Element, at i: Slice<Base>.Index)`
mutating func insert(_: Base.Element, at: Slice<Base>.Index)

Inserts a new element into the collection at the specified position.

The new element is inserted before the element currently at the specified index. If you pass the collection's `endIndex` property as the `index` parameter, the new element is appended to the collection.

``````var numbers = [1, 2, 3, 4, 5]
numbers.insert(100, at: 3)
numbers.insert(200, at: numbers.endIndex)

print(numbers)
// Prints "[1, 2, 3, 100, 4, 5, 200]"``````

Calling this method may invalidate any existing indices for use with this collection.

`newElement`: The new element to insert into the collection.

`i`: The position at which to insert the new element. `index` must be a valid index into the collection.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func insert(_ newElement: Base.Element, at i: Slice<Base>.Index)`
mutating func insert(_:at:)

Inserts a new element into the collection at the specified position.

The new element is inserted before the element currently at the specified index. If you pass the collection's `endIndex` property as the `index` parameter, the new element is appended to the collection.

``````var numbers = [1, 2, 3, 4, 5]
numbers.insert(100, at: 3)
numbers.insert(200, at: numbers.endIndex)

print(numbers)
// Prints "[1, 2, 3, 100, 4, 5, 200]"``````

Calling this method may invalidate any existing indices for use with this collection.

`newElement`: The new element to insert into the collection.

`i`: The position at which to insert the new element. `index` must be a valid index into the collection.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func insert(_ newElement: Slice<Base>.Element, at i: Slice<Base>.Index)`

#### Declared In

`RangeReplaceableCollection`
mutating func insert(contentsOf:at:)

Inserts the elements of a sequence into the collection at the specified position.

The new elements are inserted before the element currently at the specified index. If you pass the collection's `endIndex` property as the `index` parameter, the new elements are appended to the collection.

Here's an example of inserting a range of integers into an array of the same type:

``````var numbers = [1, 2, 3, 4, 5]
numbers.insert(contentsOf: 100...103, at: 3)
print(numbers)
// Prints "[1, 2, 3, 100, 101, 102, 103, 4, 5]"``````

Calling this method may invalidate any existing indices for use with this collection.

`newElements`: The new elements to insert into the collection.

`i`: The position at which to insert the new elements. `index` must be a valid index of the collection.

Complexity: O(m), where m is the combined length of the collection and `newElements`. If `i` is equal to the collection's `endIndex` property, the complexity is O(n), where n is the length of `newElements`.

#### Declaration

`mutating func insert<C>(contentsOf newElements: C, at i: Slice<Base>.Index)`

#### Declared In

`RangeReplaceableCollection`
mutating func insert<S>(contentsOf: S, at: Slice<Base>.Index)

Inserts the elements of a sequence into the collection at the specified position.

The new elements are inserted before the element currently at the specified index. If you pass the collection's `endIndex` property as the `index` parameter, the new elements are appended to the collection.

Here's an example of inserting a range of integers into an array of the same type:

``````var numbers = [1, 2, 3, 4, 5]
numbers.insert(contentsOf: 100...103, at: 3)
print(numbers)
// Prints "[1, 2, 3, 100, 101, 102, 103, 4, 5]"``````

Calling this method may invalidate any existing indices for use with this collection.

`newElements`: The new elements to insert into the collection.

`i`: The position at which to insert the new elements. `index` must be a valid index of the collection.

Complexity: O(m), where m is the combined length of the collection and `newElements`. If `i` is equal to the collection's `endIndex` property, the complexity is O(n), where n is the length of `newElements`.

#### Declaration

`mutating func insert<S>(contentsOf newElements: S, at i: Slice<Base>.Index)`
mutating func insert<S>(contentsOf: S, at: Slice<Base>.Index)

#### Declaration

`mutating func insert<S>(contentsOf newElements: S, at i: Slice<Base>.Index)`
func last(where:)

Returns the last element of the sequence that satisfies the given predicate.

This example uses the `last(where:)` method to find the last negative number in an array of integers:

``````let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
if let lastNegative = numbers.last(where: { \$0 < 0 }) {
print("The last negative number is \(firstNegative).")
}
// Prints "The last negative number is -6."``````

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element is a match. Returns: The last element of the sequence that satisfies `predicate`, or `nil` if there is no element that satisfies `predicate`.

#### Declaration

`func last(where predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Element?`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`
func lastIndex(where:)

Returns the index of the last element in the collection that matches the given predicate.

You can use the predicate to find an element of a type that doesn't conform to the `Equatable` protocol or to find an element that matches particular criteria. This example finds the index of the last name that begins with the letter "A":

``````let students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
if let i = students.lastIndex(where: { \$0.hasPrefix("A") }) {
print("\(students[i]) starts with 'A'!")
}
// Prints "Akosua starts with 'A'!"``````

`predicate`: A closure that takes an element as its argument and returns a Boolean value that indicates whether the passed element represents a match. Returns: The index of the last element in the collection that matches `predicate`, or `nil` if no elements match.

#### Declaration

`func lastIndex(where predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Index?`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`
func lexicographicallyPrecedes(_:by:)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements.

The predicate must be a strict weak ordering over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

Parameters: other: A sequence to compare to this sequence. areInIncreasingOrder: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: `true` if this sequence precedes `other` in a dictionary ordering as ordered by `areInIncreasingOrder`; otherwise, `false`.

Note: This method implements the mathematical notion of lexicographical ordering, which has no connection to Unicode. If you are sorting strings to present to the end user, use `String` APIs that perform localized comparison instead.

#### Declaration

`func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence, by areInIncreasingOrder: (Slice<Base>.Element, Slice<Base>.Element) throws -> Bool) rethrows -> Bool where OtherSequence : Sequence, Slice<Base>.Element == OtherSequence.Element`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func map(_:)

Returns a `LazyMapCollection` over this `Collection`. The elements of the result are computed lazily, each time they are read, by calling `transform` function on a base element.

#### Declaration

`func map<U>(_ transform: @escaping (Slice<Base>.Elements.Element) -> U) -> LazyMapCollection<Slice<Base>.Elements, U>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
@warn_unqualified_access func max(by:)

Returns the maximum element in the sequence, using the given predicate as the comparison between elements.

The predicate must be a strict weak ordering over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

This example shows how to use the `max(by:)` method on a dictionary to find the key-value pair with the highest value.

``````let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156]
let greatestHue = hues.max { a, b in a.value < b.value }
print(greatestHue)
// Prints "Optional(("Heliotrope", 296))"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: The sequence's maximum element if the sequence is not empty; otherwise, `nil`.

#### Declaration

```@warn_unqualified_access func max(by areInIncreasingOrder: (Slice<Base>.Element, Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Element?```

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func min(by:)

Returns the minimum element in the sequence, using the given predicate as the comparison between elements.

The predicate must be a strict weak ordering over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

This example shows how to use the `min(by:)` method on a dictionary to find the key-value pair with the lowest value.

``````let hues = ["Heliotrope": 296, "Coral": 16, "Aquamarine": 156]
let leastHue = hues.min { a, b in a.value < b.value }
print(leastHue)
// Prints "Optional(("Coral", 16))"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: The sequence's minimum element, according to `areInIncreasingOrder`. If the sequence has no elements, returns `nil`.

#### Declaration

`func min(by areInIncreasingOrder: (Slice<Base>.Element, Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Element?`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
mutating func partition(by: (Slice<Base>.Element) throws -> Bool)

Reorders the elements of the collection such that all the elements that match the given predicate are after all the elements that don't match.

After partitioning a collection, there is a pivot index `p` where no element before `p` satisfies the `belongsInSecondPartition` predicate and every element at or after `p` satisfies `belongsInSecondPartition`.

In the following example, an array of numbers is partitioned by a predicate that matches elements greater than 30.

``````var numbers = [30, 40, 20, 30, 30, 60, 10]
let p = numbers.partition(by: { \$0 > 30 })
// p == 5
// numbers == [30, 10, 20, 30, 30, 60, 40]``````

The `numbers` array is now arranged in two partitions. The first partition, `numbers[..<p]`, is made up of the elements that are not greater than 30. The second partition, `numbers[p...]`, is made up of the elements that are greater than 30.

``````let first = numbers[..<p]
// first == [30, 10, 20, 30, 30]
let second = numbers[p...]
// second == [60, 40]``````

`belongsInSecondPartition`: A predicate used to partition the collection. All elements satisfying this predicate are ordered after all elements not satisfying it. Returns: The index of the first element in the reordered collection that matches `belongsInSecondPartition`. If no elements in the collection match `belongsInSecondPartition`, the returned index is equal to the collection's `endIndex`.

Complexity: O(n)

#### Declaration

`mutating func partition(by belongsInSecondPartition: (Slice<Base>.Element) throws -> Bool) rethrows -> Slice<Base>.Index`

#### Declared In

`MutableCollection`
mutating func popLast()

Removes and returns the last element of the collection.

Calling this method may invalidate all saved indices of this collection. Do not rely on a previously stored index value after altering a collection with any operation that can change its length.

Returns: The last element of the collection if the collection is not empty; otherwise, `nil`.

Complexity: O(1)

#### Declaration

`mutating func popLast() -> Slice<Base>.Element?`

#### Declared In

`RangeReplaceableCollection`
func prefix(_:)

Returns a subsequence, up to the specified maximum length, containing the initial elements of the collection.

If the maximum length exceeds the number of elements in the collection, the result contains all the elements in the collection.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"``````

`maxLength`: The maximum number of elements to return. `maxLength` must be greater than or equal to zero. Returns: A subsequence starting at the beginning of this collection with at most `maxLength` elements.

#### Declaration

`func prefix(_ maxLength: Int) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func prefix(through:)

Returns a subsequence from the start of the collection through the specified position.

The resulting subsequence includes the element at the position `end`. The following example searches for the index of the number `40` in an array of integers, and then prints the prefix of the array up to, and including, that index:

``````let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
print(numbers.prefix(through: i))
}
// Prints "[10, 20, 30, 40]"``````

Using the `prefix(through:)` method is equivalent to using a partial closed range as the collection's subscript. The subscript notation is preferred over `prefix(through:)`.

``````if let i = numbers.firstIndex(of: 40) {
print(numbers[...i])
}
// Prints "[10, 20, 30, 40]"``````

`end`: The index of the last element to include in the resulting subsequence. `end` must be a valid index of the collection that is not equal to the `endIndex` property. Returns: A subsequence up to, and including, the `end` position.

Complexity: O(1)

#### Declaration

`func prefix(through position: Slice<Base>.Index) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func prefix(upTo:)

Returns a subsequence from the start of the collection up to, but not including, the specified position.

The resulting subsequence does not include the element at the position `end`. The following example searches for the index of the number `40` in an array of integers, and then prints the prefix of the array up to, but not including, that index:

``````let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
print(numbers.prefix(upTo: i))
}
// Prints "[10, 20, 30]"``````

Passing the collection's starting index as the `end` parameter results in an empty subsequence.

``````print(numbers.prefix(upTo: numbers.startIndex))
// Prints "[]"``````

Using the `prefix(upTo:)` method is equivalent to using a partial half-open range as the collection's subscript. The subscript notation is preferred over `prefix(upTo:)`.

``````if let i = numbers.firstIndex(of: 40) {
print(numbers[..<i])
}
// Prints "[10, 20, 30]"``````

`end`: The "past the end" index of the resulting subsequence. `end` must be a valid index of the collection. Returns: A subsequence up to, but not including, the `end` position.

Complexity: O(1)

#### Declaration

`func prefix(upTo end: Slice<Base>.Index) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func prefix(while:)

Returns a lazy collection of the initial consecutive elements that satisfy `predicate`.

`predicate`: A closure that takes an element of the collection as its argument and returns `true` if the element should be included or `false` otherwise. Once `predicate` returns `false` it will not be called again.

#### Declaration

`func prefix(while predicate: @escaping (Slice<Base>.Element) -> Bool) -> LazyPrefixWhileCollection<Slice<Base>.Elements>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`
func randomElement()

Returns a random element of the collection.

Call `randomElement()` to select a random element from an array or another collection. This example picks a name at random from an array:

``````let names = ["Zoey", "Chloe", "Amani", "Amaia"]
let randomName = names.randomElement()!
// randomName == "Amani"``````

This method uses the default random generator, `Random.default`. The call to `names.randomElement()` above is equivalent to calling `names.randomElement(using: &Random.default)`.

Returns: A random element from the collection. If the collection is empty, the method returns `nil`.

#### Declaration

`func randomElement() -> Slice<Base>.Element?`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func randomElement(using:)

Returns a random element of the collection, using the given generator as a source for randomness.

Call `randomElement(using:)` to select a random element from an array or another collection when you are using a custom random number generator. This example picks a name at random from an array:

``````let names = ["Zoey", "Chloe", "Amani", "Amaia"]
let randomName = names.randomElement(using: &myGenerator)!
// randomName == "Amani"``````

`generator`: The random number generator to use when choosing a random element. Returns: A random element from the collection. If the collection is empty, the method returns `nil`.

#### Declaration

`func randomElement<T>(using generator: inout T) -> Slice<Base>.Element? where T : RandomNumberGenerator`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func reduce(_:_:)

Returns the result of combining the elements of the sequence using the given closure.

Use the `reduce(_:_:)` method to produce a single value from the elements of an entire sequence. For example, you can use this method on an array of numbers to find their sum or product.

The `nextPartialResult` closure is called sequentially with an accumulating value initialized to `initialResult` and each element of the sequence. This example shows how to find the sum of an array of numbers.

``````let numbers = [1, 2, 3, 4]
let numberSum = numbers.reduce(0, { x, y in
x + y
})
// numberSum == 10``````

When `numbers.reduce(_:_:)` is called, the following steps occur:

1. The `nextPartialResult` closure is called with `initialResult`---`0` in this case---and the first element of `numbers`, returning the sum: `1`.
2. The closure is called again repeatedly with the previous call's return value and each element of the sequence.
3. When the sequence is exhausted, the last value returned from the closure is returned to the caller.

If the sequence has no elements, `nextPartialResult` is never executed and `initialResult` is the result of the call to `reduce(_:_:)`.

Parameters: initialResult: The value to use as the initial accumulating value. `initialResult` is passed to `nextPartialResult` the first time the closure is executed. nextPartialResult: A closure that combines an accumulating value and an element of the sequence into a new accumulating value, to be used in the next call of the `nextPartialResult` closure or returned to the caller. Returns: The final accumulated value. If the sequence has no elements, the result is `initialResult`.

#### Declaration

`func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Slice<Base>.Element) throws -> Result) rethrows -> Result`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func reduce(into:_:)

Returns the result of combining the elements of the sequence using the given closure.

Use the `reduce(into:_:)` method to produce a single value from the elements of an entire sequence. For example, you can use this method on an array of integers to filter adjacent equal entries or count frequencies.

This method is preferred over `reduce(_:_:)` for efficiency when the result is a copy-on-write type, for example an Array or a Dictionary.

The `updateAccumulatingResult` closure is called sequentially with a mutable accumulating value initialized to `initialResult` and each element of the sequence. This example shows how to build a dictionary of letter frequencies of a string.

``````let letters = "abracadabra"
let letterCount = letters.reduce(into: [:]) { counts, letter in
counts[letter, default: 0] += 1
}
// letterCount == ["a": 5, "b": 2, "r": 2, "c": 1, "d": 1]``````

When `letters.reduce(into:_:)` is called, the following steps occur:

1. The `updateAccumulatingResult` closure is called with the initial accumulating value---`[:]` in this case---and the first character of `letters`, modifying the accumulating value by setting `1` for the key `"a"`.
2. The closure is called again repeatedly with the updated accumulating value and each element of the sequence.
3. When the sequence is exhausted, the accumulating value is returned to the caller.

If the sequence has no elements, `updateAccumulatingResult` is never executed and `initialResult` is the result of the call to `reduce(into:_:)`.

Parameters: initialResult: The value to use as the initial accumulating value. updateAccumulatingResult: A closure that updates the accumulating value with an element of the sequence. Returns: The final accumulated value. If the sequence has no elements, the result is `initialResult`.

#### Declaration

`func reduce<Result>(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Slice<Base>.Element) throws -> ()) rethrows -> Result`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
mutating func remove(at: Slice<Base>.Index)

#### Declaration

`mutating func remove(at i: Slice<Base>.Index) -> Base.Element`
mutating func remove(at: Slice<Base>.Index)

Removes and returns the element at the specified position.

All the elements following the specified position are moved to close the gap. This example removes the middle element from an array of measurements.

``````var measurements = [1.2, 1.5, 2.9, 1.2, 1.6]
let removed = measurements.remove(at: 2)
print(measurements)
// Prints "[1.2, 1.5, 1.2, 1.6]"``````

Calling this method may invalidate any existing indices for use with this collection.

`i`: The position of the element to remove. `index` must be a valid index of the collection that is not equal to the collection's end index. Returns: The removed element.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func remove(at i: Slice<Base>.Index) -> Base.Element`
mutating func remove(at:)

Removes and returns the element at the specified position.

All the elements following the specified position are moved to close the gap. This example removes the middle element from an array of measurements.

``````var measurements = [1.2, 1.5, 2.9, 1.2, 1.6]
let removed = measurements.remove(at: 2)
print(measurements)
// Prints "[1.2, 1.5, 1.2, 1.6]"``````

Calling this method may invalidate any existing indices for use with this collection.

`position`: The position of the element to remove. `position` must be a valid index of the collection that is not equal to the collection's end index. Returns: The removed element.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func remove(at position: Slice<Base>.Index) -> Slice<Base>.Element`

#### Declared In

`RangeReplaceableCollection`
mutating func removeAll(keepingCapacity:)

Removes all elements from the collection.

Calling this method may invalidate any existing indices for use with this collection.

`keepCapacity`: Pass `true` to request that the collection avoid releasing its storage. Retaining the collection's storage can be a useful optimization when you're planning to grow the collection again. The default value is `false`.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func removeAll(keepingCapacity keepCapacity: Bool = default)`

#### Declared In

`RangeReplaceableCollection`
mutating func removeAll(where: (Slice<Base>.Element) throws -> Bool)

Removes all the elements that satisfy the given predicate.

Use this method to remove every element in a collection that meets particular criteria. This example removes all the odd values from an array of numbers:

``````var numbers = [5, 6, 7, 8, 9, 10, 11]
numbers.removeAll(where: { \$0 % 2 == 1 })
// numbers == [6, 8, 10]``````

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be removed from the collection.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func removeAll(where predicate: (Slice<Base>.Element) throws -> Bool) rethrows`

#### Declared In

`RangeReplaceableCollection`
mutating func removeFirst()

Removes and returns the first element of the collection.

The collection must not be empty.

``````var bugs = ["Aphid", "Bumblebee", "Cicada", "Damselfly", "Earwig"]
bugs.removeFirst()
print(bugs)
// Prints "["Bumblebee", "Cicada", "Damselfly", "Earwig"]"``````

Calling this method may invalidate any existing indices for use with this collection.

Returns: The removed element.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func removeFirst() -> Slice<Base>.Element`

#### Declared In

`RangeReplaceableCollection`
mutating func removeFirst(_:)

Removes the specified number of elements from the beginning of the collection.

``````var bugs = ["Aphid", "Bumblebee", "Cicada", "Damselfly", "Earwig"]
bugs.removeFirst(3)
print(bugs)
// Prints "["Damselfly", "Earwig"]"``````

Calling this method may invalidate any existing indices for use with this collection.

`n`: The number of elements to remove from the collection. `n` must be greater than or equal to zero and must not exceed the number of elements in the collection.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func removeFirst(_ n: Int)`

#### Declared In

`RangeReplaceableCollection`
mutating func removeLast()

Removes and returns the last element of the collection.

The collection must not be empty.

Calling this method may invalidate all saved indices of this collection. Do not rely on a previously stored index value after altering a collection with any operation that can change its length.

Returns: The last element of the collection.

Complexity: O(1)

#### Declaration

`mutating func removeLast() -> Slice<Base>.Element`

#### Declared In

`RangeReplaceableCollection`
mutating func removeLast(_:)

Removes the specified number of elements from the end of the collection.

Attempting to remove more elements than exist in the collection triggers a runtime error.

Calling this method may invalidate all saved indices of this collection. Do not rely on a previously stored index value after altering a collection with any operation that can change its length.

`n`: The number of elements to remove from the collection. `n` must be greater than or equal to zero and must not exceed the number of elements in the collection.

Complexity: O(n), where n is the specified number of elements.

#### Declaration

`mutating func removeLast(_ n: Int)`

#### Declared In

`RangeReplaceableCollection`
mutating func removeSubrange(_: Range<Slice<Base>.Index>)

Removes the specified subrange of elements from the collection.

``````var bugs = ["Aphid", "Bumblebee", "Cicada", "Damselfly", "Earwig"]
bugs.removeSubrange(1...3)
print(bugs)
// Prints "["Aphid", "Earwig"]"``````

Calling this method may invalidate any existing indices for use with this collection.

`bounds`: The subrange of the collection to remove. The bounds of the range must be valid indices of the collection.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func removeSubrange(_ bounds: Range<Slice<Base>.Index>)`

#### Declared In

`Slice`, `RangeReplaceableCollection`
mutating func removeSubrange(_: Range<Slice<Base>.Index>)

#### Declaration

`mutating func removeSubrange(_ bounds: Range<Slice<Base>.Index>)`
mutating func removeSubrange<R>(_: R)

Removes the elements in the specified subrange from the collection.

All the elements following the specified position are moved to close the gap. This example removes three elements from the middle of an array of measurements.

``````var measurements = [1.2, 1.5, 2.9, 1.2, 1.5]
measurements.removeSubrange(1..<4)
print(measurements)
// Prints "[1.2, 1.5]"``````

Calling this method may invalidate any existing indices for use with this collection.

`bounds`: The range of the collection to be removed. The bounds of the range must be valid indices of the collection.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`mutating func removeSubrange<R>(_ bounds: R)`

#### Declared In

`RangeReplaceableCollection`
mutating func replaceSubrange<C>(_: Range<Slice<Base>.Index>, with: C)

Replaces the specified subrange of elements with the given collection.

This method has the effect of removing the specified range of elements from the collection and inserting the new elements at the same location. The number of new elements need not match the number of elements being removed.

In this example, three elements in the middle of an array of integers are replaced by the five elements of a `Repeated<Int>` instance.

`````` var nums = [10, 20, 30, 40, 50]
nums.replaceSubrange(1...3, with: repeatElement(1, count: 5))
print(nums)
// Prints "[10, 1, 1, 1, 1, 1, 50]"``````

If you pass a zero-length range as the `subrange` parameter, this method inserts the elements of `newElements` at `subrange.startIndex`. Calling the `insert(contentsOf:at:)` method instead is preferred.

Likewise, if you pass a zero-length collection as the `newElements` parameter, this method removes the elements in the given subrange without replacement. Calling the `removeSubrange(_:)` method instead is preferred.

Calling this method may invalidate any existing indices for use with this collection.

Parameters: subrange: The subrange of the collection to replace. The bounds of the range must be valid indices of the collection. newElements: The new elements to add to the collection.

Complexity: O(m), where m is the combined length of the collection and `newElements`. If the call to `replaceSubrange` simply appends the contents of `newElements` to the collection, the complexity is O(n), where n is the length of `newElements`.

#### Declaration

`mutating func replaceSubrange<C>(_ subRange: Range<Slice<Base>.Index>, with newElements: C)`
mutating func replaceSubrange<C>(_: Range<Slice<Base>.Index>, with: C)

#### Declaration

`mutating func replaceSubrange<C>(_ subRange: Range<Slice<Base>.Index>, with newElements: C)`
mutating func replaceSubrange(_:with:)

Replaces the specified subrange of elements with the given collection.

This method has the effect of removing the specified range of elements from the collection and inserting the new elements at the same location. The number of new elements need not match the number of elements being removed.

In this example, three elements in the middle of an array of integers are replaced by the five elements of a `Repeated<Int>` instance.

`````` var nums = [10, 20, 30, 40, 50]
nums.replaceSubrange(1...3, with: repeatElement(1, count: 5))
print(nums)
// Prints "[10, 1, 1, 1, 1, 1, 50]"``````

If you pass a zero-length range as the `subrange` parameter, this method inserts the elements of `newElements` at `subrange.startIndex`. Calling the `insert(contentsOf:at:)` method instead is preferred.

Likewise, if you pass a zero-length collection as the `newElements` parameter, this method removes the elements in the given subrange without replacement. Calling the `removeSubrange(_:)` method instead is preferred.

Calling this method may invalidate any existing indices for use with this collection.

Parameters: subrange: The subrange of the collection to replace. The bounds of the range must be valid indices of the collection. newElements: The new elements to add to the collection.

Complexity: O(m), where m is the combined length of the collection and `newElements`. If the call to `replaceSubrange` simply appends the contents of `newElements` to the collection, the complexity is O(n), where n is the length of `newElements`.

#### Declaration

`mutating func replaceSubrange<C, R>(_ subrange: R, with newElements: C)`

#### Declared In

`RangeReplaceableCollection`
mutating func reserveCapacity(_:)

Prepares the collection to store the specified number of elements, when doing so is appropriate for the underlying type.

If you will be adding a known number of elements to a collection, use this method to avoid multiple reallocations. A type that conforms to `RangeReplaceableCollection` can choose how to respond when this method is called. Depending on the type, it may make sense to allocate more or less storage than requested or to take no action at all.

`n`: The requested number of elements to store.

#### Declaration

`mutating func reserveCapacity(_ n: Int)`

#### Declared In

`RangeReplaceableCollection`
mutating func reverse()

Reverses the elements of the collection in place.

The following example reverses the elements of an array of characters:

``````var characters: [Character] = ["C", "a", "f", "é"]
characters.reverse()
print(characters)
// Prints "["é", "f", "a", "C"]``````

Complexity: O(n), where n is the number of elements in the collection.

#### Declaration

`mutating func reverse()`

#### Declared In

`MutableCollection`
func reversed()

Returns an array containing the elements of this sequence in reverse order.

The sequence must be finite.

Complexity: O(n), where n is the length of the sequence.

Returns: An array containing the elements of this sequence in reverse order.

#### Declaration

`func reversed() -> [Slice<Base>.Element]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
mutating func shuffle()

Shuffles the collection in place.

Use the `shuffle()` method to randomly reorder the elements of an array.

``````var names = ["Alejandro", "Camila", "Diego", "Luciana", "Luis", "Sofía"]
names.shuffle(using: myGenerator)
// names == ["Luis", "Camila", "Luciana", "Sofía", "Alejandro", "Diego"]``````

This method uses the default random generator, `Random.default`. The call to `names.shuffle()` above is equivalent to calling `names.shuffle(using: &Random.default)`.

Complexity: O(n)

#### Declaration

`mutating func shuffle()`

#### Declared In

`MutableCollection`
mutating func shuffle(using:)

Shuffles the collection in place, using the given generator as a source for randomness.

You use this method to randomize the elements of a collection when you are using a custom random number generator. For example, you can use the `shuffle(using:)` method to randomly reorder the elements of an array.

``````var names = ["Alejandro", "Camila", "Diego", "Luciana", "Luis", "Sofía"]
names.shuffle(using: &myGenerator)
// names == ["Sofía", "Alejandro", "Camila", "Luis", "Diego", "Luciana"]``````

`generator`: The random number generator to use when shuffling the collection.

Complexity: O(n)

#### Declaration

`mutating func shuffle<T>(using generator: inout T)`

#### Declared In

`MutableCollection`
func shuffled()

Returns the elements of the sequence, shuffled.

For example, you can shuffle the numbers between `0` and `9` by calling the `shuffled()` method on that range:

``````let numbers = 0...9
let shuffledNumbers = numbers.shuffled()
// shuffledNumbers == [1, 7, 6, 2, 8, 9, 4, 3, 5, 0]``````

This method uses the default random generator, `Random.default`. The call to `numbers.shuffled()` above is equivalent to calling `numbers.shuffled(using: &Random.default)`.

Returns: A shuffled array of this sequence's elements.

Complexity: O(n)

#### Declaration

`func shuffled() -> [Slice<Base>.Element]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func shuffled(using:)

Returns the elements of the sequence, shuffled using the given generator as a source for randomness.

You use this method to randomize the elements of a sequence when you are using a custom random number generator. For example, you can shuffle the numbers between `0` and `9` by calling the `shuffled(using:)` method on that range:

``````let numbers = 0...9
let shuffledNumbers = numbers.shuffled(using: &myGenerator)
// shuffledNumbers == [8, 9, 4, 3, 2, 6, 7, 0, 5, 1]``````

`generator`: The random number generator to use when shuffling the sequence. Returns: An array of this sequence's elements in a shuffled order.

Complexity: O(n)

#### Declaration

`func shuffled<T>(using generator: inout T) -> [Slice<Base>.Element] where T : RandomNumberGenerator`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
mutating func sort(by:)

Sorts the collection in place, using the given predicate as the comparison between elements.

When you want to sort a collection of elements that doesn't conform to the `Comparable` protocol, pass a closure to this method that returns `true` when the first element passed should be ordered before the second.

The predicate must be a strict weak ordering over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements for which `areInIncreasingOrder` does not establish an order.

In the following example, the closure provides an ordering for an array of a custom enumeration that describes an HTTP response. The predicate orders errors before successes and sorts the error responses by their error code.

``````enum HTTPResponse {
case ok
case error(Int)
}

var responses: [HTTPResponse] = [.error(500), .ok, .ok, .error(404), .error(403)]
responses.sort {
switch (\$0, \$1) {
// Order errors by code
case let (.error(aCode), .error(bCode)):
return aCode < bCode

// All successes are equivalent, so none is before any other
case (.ok, .ok): return false

// Order errors before successes
case (.error, .ok): return true
case (.ok, .error): return false
}
}
print(responses)
// Prints "[.error(403), .error(404), .error(500), .ok, .ok]"``````

Alternatively, use this method to sort a collection of elements that do conform to `Comparable` when you want the sort to be descending instead of ascending. Pass the greater-than operator (`>`) operator as the predicate.

``````var students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
students.sort(by: >)
print(students)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. If `areInIncreasingOrder` throws an error during the sort, the elements may be in a different order, but none will be lost.

#### Declaration

`mutating func sort(by areInIncreasingOrder: (Slice<Base>.Element, Slice<Base>.Element) throws -> Bool) rethrows`

#### Declared In

`MutableCollection`
func sorted(by:)

Returns the elements of the sequence, sorted using the given predicate as the comparison between elements.

When you want to sort a sequence of elements that don't conform to the `Comparable` protocol, pass a predicate to this method that returns `true` when the first element passed should be ordered before the second. The elements of the resulting array are ordered according to the given predicate.

The predicate must be a strict weak ordering over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areInIncreasingOrder(a, a)` is always `false`. (Irreflexivity)
• If `areInIncreasingOrder(a, b)` and `areInIncreasingOrder(b, c)` are both `true`, then `areInIncreasingOrder(a, c)` is also `true`. (Transitive comparability)
• Two elements are incomparable if neither is ordered before the other according to the predicate. If `a` and `b` are incomparable, and `b` and `c` are incomparable, then `a` and `c` are also incomparable. (Transitive incomparability)

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements for which `areInIncreasingOrder` does not establish an order.

In the following example, the predicate provides an ordering for an array of a custom `HTTPResponse` type. The predicate orders errors before successes and sorts the error responses by their error code.

``````enum HTTPResponse {
case ok
case error(Int)
}

let responses: [HTTPResponse] = [.error(500), .ok, .ok, .error(404), .error(403)]
let sortedResponses = responses.sorted {
switch (\$0, \$1) {
// Order errors by code
case let (.error(aCode), .error(bCode)):
return aCode < bCode

// All successes are equivalent, so none is before any other
case (.ok, .ok): return false

// Order errors before successes
case (.error, .ok): return true
case (.ok, .error): return false
}
}
print(sortedResponses)
// Prints "[.error(403), .error(404), .error(500), .ok, .ok]"``````

You also use this method to sort elements that conform to the `Comparable` protocol in descending order. To sort your sequence in descending order, pass the greater-than operator (`>`) as the `areInIncreasingOrder` parameter.

``````let students: Set = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
let descendingStudents = students.sorted(by: >)
print(descendingStudents)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"``````

Calling the related `sorted()` method is equivalent to calling this method and passing the less-than operator (`<`) as the predicate.

``````print(students.sorted())
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"
print(students.sorted(by: <))
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"``````

`areInIncreasingOrder`: A predicate that returns `true` if its first argument should be ordered before its second argument; otherwise, `false`. Returns: A sorted array of the sequence's elements.

#### Declaration

`func sorted(by areInIncreasingOrder: (Slice<Base>.Element, Slice<Base>.Element) throws -> Bool) rethrows -> [Slice<Base>.Element]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func split(_:omittingEmptySubsequences:whereSeparator:)

Returns the longest possible subsequences of the collection, in order, that don't contain elements satisfying the given predicate.

The resulting array consists of at most `maxSplits + 1` subsequences. Elements that are used to split the sequence are not returned as part of any subsequence.

The following examples show the effects of the `maxSplits` and `omittingEmptySubsequences` parameters when splitting a string using a closure that matches spaces. The first use of `split` returns each word that was originally separated by one or more spaces.

``````let line = "BLANCHE:   I don't want realism. I want magic!"
print(line.split(whereSeparator: { \$0 == " " }))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

The second example passes `1` for the `maxSplits` parameter, so the original string is split just once, into two new strings.

``````print(line.split(maxSplits: 1, whereSeparator: { \$0 == " " }))
// Prints "["BLANCHE:", "  I don\'t want realism. I want magic!"]"``````

The final example passes `false` for the `omittingEmptySubsequences` parameter, so the returned array contains empty strings where spaces were repeated.

``````print(line.split(omittingEmptySubsequences: false, whereSeparator: { \$0 == " " }))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

Parameters: maxSplits: The maximum number of times to split the collection, or one less than the number of subsequences to return. If `maxSplits + 1` subsequences are returned, the last one is a suffix of the original collection containing the remaining elements. `maxSplits` must be greater than or equal to zero. The default value is `Int.max`. omittingEmptySubsequences: If `false`, an empty subsequence is returned in the result for each pair of consecutive elements satisfying the `isSeparator` predicate and for each element at the start or end of the collection satisfying the `isSeparator` predicate. The default value is `true`. isSeparator: A closure that takes an element as an argument and returns a Boolean value indicating whether the collection should be split at that element. Returns: An array of subsequences, split from this collection's elements.

#### Declaration

`func split(maxSplits: Int = default, omittingEmptySubsequences: Bool = default, whereSeparator isSeparator: (Slice<Base>.Element) throws -> Bool) rethrows -> [Slice<Base>.SubSequence]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func starts(with:by:)

Returns a Boolean value indicating whether the initial elements of the sequence are equivalent to the elements in another sequence, using the given predicate as the equivalence test.

The predicate must be a equivalence relation over the elements. That is, for any elements `a`, `b`, and `c`, the following conditions must hold:

• `areEquivalent(a, a)` is always `true`. (Reflexivity)
• `areEquivalent(a, b)` implies `areEquivalent(b, a)`. (Symmetry)
• If `areEquivalent(a, b)` and `areEquivalent(b, c)` are both `true`, then `areEquivalent(a, c)` is also `true`. (Transitivity)

Parameters: possiblePrefix: A sequence to compare to this sequence. areEquivalent: A predicate that returns `true` if its two arguments are equivalent; otherwise, `false`. Returns: `true` if the initial elements of the sequence are equivalent to the elements of `possiblePrefix`; otherwise, `false`. If `possiblePrefix` has no elements, the return value is `true`.

#### Declaration

`func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix, by areEquivalent: (Slice<Base>.Element, PossiblePrefix.Element) throws -> Bool) rethrows -> Bool where PossiblePrefix : Sequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func suffix(_:)

Returns a subsequence, up to the given maximum length, containing the final elements of the collection.

If the maximum length exceeds the number of elements in the collection, the result contains all the elements in the collection.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.suffix(2))
// Prints "[4, 5]"
print(numbers.suffix(10))
// Prints "[1, 2, 3, 4, 5]"``````

`maxLength`: The maximum number of elements to return. The value of `maxLength` must be greater than or equal to zero. Returns: A subsequence terminating at the end of the collection with at most `maxLength` elements.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func suffix(_ maxLength: Int) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func suffix(from:)

Returns a subsequence from the specified position to the end of the collection.

The following example searches for the index of the number `40` in an array of integers, and then prints the suffix of the array starting at that index:

``````let numbers = [10, 20, 30, 40, 50, 60]
if let i = numbers.firstIndex(of: 40) {
print(numbers.suffix(from: i))
}
// Prints "[40, 50, 60]"``````

Passing the collection's `endIndex` as the `start` parameter results in an empty subsequence.

``````print(numbers.suffix(from: numbers.endIndex))
// Prints "[]"``````

Using the `suffix(from:)` method is equivalent to using a partial range from the index as the collection's subscript. The subscript notation is preferred over `suffix(from:)`.

``````if let i = numbers.firstIndex(of: 40) {
print(numbers[i...])
}
// Prints "[40, 50, 60]"``````

`start`: The index at which to start the resulting subsequence. `start` must be a valid index of the collection. Returns: A subsequence starting at the `start` position.

Complexity: O(1)

#### Declaration

`func suffix(from start: Slice<Base>.Index) -> Slice<Base>.SubSequence`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
mutating func swapAt(_:_:)

Exchanges the values at the specified indices of the collection.

Both parameters must be valid indices of the collection that are not equal to `endIndex`. Calling `swapAt(_:_:)` with the same index as both `i` and `j` has no effect.

Parameters: i: The index of the first value to swap. j: The index of the second value to swap.

#### Declaration

`mutating func swapAt(_ i: Slice<Base>.Index, _ j: Slice<Base>.Index)`

#### Declared In

`MutableCollection`

### Conditionally Inherited Items

The initializers, methods, and properties listed below may be available on this type under certain conditions (such as methods that are available on `Array` when its elements are `Equatable`) or may not ever be available if that determination is beyond SwiftDoc.org's capabilities. Please open an issue on GitHub if you see something out of place!

#### Where Element : Collection

func joined()

A concatenation of the elements of `self`.

#### Declaration

`func joined() -> LazyCollection<FlattenCollection<Slice<Base>.Elements>>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`

#### Where Element : Comparable

func lexicographicallyPrecedes(_:)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the less-than operator (`<`) to compare elements.

This example uses the `lexicographicallyPrecedes` method to test which array of integers comes first in a lexicographical ordering.

``````let a = [1, 2, 2, 2]
let b = [1, 2, 3, 4]

print(a.lexicographicallyPrecedes(b))
// Prints "true"
print(b.lexicographicallyPrecedes(b))
// Prints "false"``````

`other`: A sequence to compare to this sequence. Returns: `true` if this sequence precedes `other` in a dictionary ordering; otherwise, `false`.

Note: This method implements the mathematical notion of lexicographical ordering, which has no connection to Unicode. If you are sorting strings to present to the end user, use `String` APIs that perform localized comparison.

#### Declaration

`func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence) -> Bool where OtherSequence : Sequence, Slice<Base>.Element == OtherSequence.Element`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
@warn_unqualified_access func max()

Returns the maximum element in the sequence.

This example finds the largest value in an array of height measurements.

``````let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let greatestHeight = heights.max()
print(greatestHeight)
// Prints "Optional(67.5)"``````

Returns: The sequence's maximum element. If the sequence has no elements, returns `nil`.

#### Declaration

```@warn_unqualified_access func max() -> Slice<Base>.Element?```

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
@warn_unqualified_access func min()

Returns the minimum element in the sequence.

This example finds the smallest value in an array of height measurements.

``````let heights = [67.5, 65.7, 64.3, 61.1, 58.5, 60.3, 64.9]
let lowestHeight = heights.min()
print(lowestHeight)
// Prints "Optional(58.5)"``````

Returns: The sequence's minimum element. If the sequence has no elements, returns `nil`.

#### Declaration

```@warn_unqualified_access func min() -> Slice<Base>.Element?```

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
mutating func sort()

Sorts the collection in place.

You can sort any mutable collection of elements that conform to the `Comparable` protocol by calling this method. Elements are sorted in ascending order.

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements that compare equal.

Here's an example of sorting a list of students' names. Strings in Swift conform to the `Comparable` protocol, so the names are sorted in ascending order according to the less-than operator (`<`).

``````var students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
students.sort()
print(students)
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"``````

To sort the elements of your collection in descending order, pass the greater-than operator (`>`) to the `sort(by:)` method.

``````students.sort(by: >)
print(students)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"``````

#### Declaration

`mutating func sort()`

#### Declared In

`MutableCollection`
func sorted()

Returns the elements of the sequence, sorted.

You can sort any sequence of elements that conform to the `Comparable` protocol by calling this method. Elements are sorted in ascending order.

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements that compare equal.

Here's an example of sorting a list of students' names. Strings in Swift conform to the `Comparable` protocol, so the names are sorted in ascending order according to the less-than operator (`<`).

``````let students: Set = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
let sortedStudents = students.sorted()
print(sortedStudents)
// Prints "["Abena", "Akosua", "Kofi", "Kweku", "Peter"]"``````

To sort the elements of your sequence in descending order, pass the greater-than operator (`>`) to the `sorted(by:)` method.

``````let descendingStudents = students.sorted(by: >)
print(descendingStudents)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"``````

Returns: A sorted array of the sequence's elements.

#### Declaration

`func sorted() -> [Slice<Base>.Element]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`

#### Where Element : Equatable

func contains(_:)

Returns a Boolean value indicating whether the sequence contains the given element.

This example checks to see whether a favorite actor is in an array storing a movie's cast.

``````let cast = ["Vivien", "Marlon", "Kim", "Karl"]
print(cast.contains("Marlon"))
// Prints "true"
print(cast.contains("James"))
// Prints "false"``````

`element`: The element to find in the sequence. Returns: `true` if the element was found in the sequence; otherwise, `false`.

#### Declaration

`func contains(_ element: Slice<Base>.Element) -> Bool`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func elementsEqual(_:)

Returns a Boolean value indicating whether this sequence and another sequence contain the same elements in the same order.

At least one of the sequences must be finite.

This example tests whether one countable range shares the same elements as another countable range and an array.

``````let a = 1...3
let b = 1...10

print(a.elementsEqual(b))
// Prints "false"
print(a.elementsEqual([1, 2, 3]))
// Prints "true"``````

`other`: A sequence to compare to this sequence. Returns: `true` if this sequence and `other` contain the same elements in the same order.

#### Declaration

`func elementsEqual<OtherSequence>(_ other: OtherSequence) -> Bool where OtherSequence : Sequence, Slice<Base>.Element == OtherSequence.Element`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func firstIndex(of:)

Returns the first index where the specified value appears in the collection.

After using `firstIndex(of:)` to find the position of a particular element in a collection, you can use it to access the element by subscripting. This example shows how you can modify one of the names in an array of students.

``````var students = ["Ben", "Ivy", "Jordell", "Maxime"]
if let i = students.firstIndex(of: "Maxime") {
students[i] = "Max"
}
print(students)
// Prints "["Ben", "Ivy", "Jordell", "Max"]"``````

`element`: An element to search for in the collection. Returns: The first index where `element` is found. If `element` is not found in the collection, returns `nil`.

#### Declaration

`func firstIndex(of element: Slice<Base>.Element) -> Slice<Base>.Index?`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`
func lastIndex(of:)

Returns the last index where the specified value appears in the collection.

After using `lastIndex(of:)` to find the position of the last instance of a particular element in a collection, you can use it to access the element by subscripting. This example shows how you can modify one of the names in an array of students.

``````var students = ["Ben", "Ivy", "Jordell", "Ben", "Maxime"]
if let i = students.lastIndex(of: "Ben") {
students[i] = "Benjamin"
}
print(students)
// Prints "["Ben", "Ivy", "Jordell", "Benjamin", "Max"]"``````

`element`: An element to search for in the collection. Returns: The last index where `element` is found. If `element` is not found in the collection, returns `nil`.

#### Declaration

`func lastIndex(of element: Slice<Base>.Element) -> Slice<Base>.Index?`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`
func split(_:maxSplits:omittingEmptySubsequences:)

Returns the longest possible subsequences of the collection, in order, around elements equal to the given element.

The resulting array consists of at most `maxSplits + 1` subsequences. Elements that are used to split the collection are not returned as part of any subsequence.

The following examples show the effects of the `maxSplits` and `omittingEmptySubsequences` parameters when splitting a string at each space character (" "). The first use of `split` returns each word that was originally separated by one or more spaces.

``````let line = "BLANCHE:   I don't want realism. I want magic!"
print(line.split(separator: " "))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

The second example passes `1` for the `maxSplits` parameter, so the original string is split just once, into two new strings.

``````print(line.split(separator: " ", maxSplits: 1))
// Prints "["BLANCHE:", "  I don\'t want realism. I want magic!"]"``````

The final example passes `false` for the `omittingEmptySubsequences` parameter, so the returned array contains empty strings where spaces were repeated.

``````print(line.split(separator: " ", omittingEmptySubsequences: false))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

Parameters: separator: The element that should be split upon. maxSplits: The maximum number of times to split the collection, or one less than the number of subsequences to return. If `maxSplits + 1` subsequences are returned, the last one is a suffix of the original collection containing the remaining elements. `maxSplits` must be greater than or equal to zero. The default value is `Int.max`. omittingEmptySubsequences: If `false`, an empty subsequence is returned in the result for each consecutive pair of `separator` elements in the collection and for each instance of `separator` at the start or end of the collection. If `true`, only nonempty subsequences are returned. The default value is `true`. Returns: An array of subsequences, split from this collection's elements.

#### Declaration

`func split(separator: Slice<Base>.Element, maxSplits: Int = default, omittingEmptySubsequences: Bool = default) -> [Slice<Base>.SubSequence]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func starts(with:)

Returns a Boolean value indicating whether the initial elements of the sequence are the same as the elements in another sequence.

This example tests whether one countable range begins with the elements of another countable range.

``````let a = 1...3
let b = 1...10

print(b.starts(with: a))
// Prints "true"``````

Passing a sequence with no elements or an empty collection as `possiblePrefix` always results in `true`.

``````print(b.starts(with: []))
// Prints "true"``````

`possiblePrefix`: A sequence to compare to this sequence. Returns: `true` if the initial elements of the sequence are the same as the elements of `possiblePrefix`; otherwise, `false`. If `possiblePrefix` has no elements, the return value is `true`.

#### Declaration

`func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix) -> Bool where PossiblePrefix : Sequence, Slice<Base>.Element == PossiblePrefix.Element`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`

#### Where Element : Sequence

func joined()

Returns the elements of this sequence of sequences, concatenated.

In this example, an array of three ranges is flattened so that the elements of each range can be iterated in turn.

``````let ranges = [0..<3, 8..<10, 15..<17]

// A for-in loop over 'ranges' accesses each range:
for range in ranges {
print(range)
}
// Prints "0..<3"
// Prints "8..<10"
// Prints "15..<17"

// Use 'joined()' to access each element of each range:
for index in ranges.joined() {
print(index, terminator: " ")
}
// Prints: "0 1 2 8 9 15 16"``````

Returns: A flattened view of the elements of this sequence of sequences.

#### Declaration

`func joined() -> FlattenSequence<Slice<Base>>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func joined(_:)

Returns the concatenated elements of this sequence of sequences, inserting the given separator between each element.

This example shows how an array of `[Int]` instances can be joined, using another `[Int]` instance as the separator:

``````let nestedNumbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let joined = nestedNumbers.joined(separator: [-1, -2])
print(Array(joined))
// Prints "[1, 2, 3, -1, -2, 4, 5, 6, -1, -2, 7, 8, 9]"``````

`separator`: A sequence to insert between each of this sequence's elements. Returns: The joined sequence of elements.

#### Declaration

`func joined<Separator>(separator: Separator) -> JoinedSequence<Slice<Base>> where Separator : Sequence, Separator.Element == Slice<Base>.Element.Element`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`

#### Where Element : StringProtocol

func joined(_:)

Returns a new string by concatenating the elements of the sequence, adding the given separator between each element.

The following example shows how an array of strings can be joined to a single, comma-separated string:

``````let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let list = cast.joined(separator: ", ")
print(list)
// Prints "Vivien, Marlon, Kim, Karl"``````

`separator`: A string to insert between each of the elements in this sequence. The default separator is an empty string. Returns: A single, concatenated string.

#### Declaration

`func joined(separator: String = default) -> String`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`

#### Where Elements : BidirectionalCollection

func reversed()

Returns the elements of the collection in reverse order.

Complexity: O(1)

#### Declaration

`func reversed() -> LazyCollection<ReversedCollection<Slice<Base>.Elements>>`

#### Declared In

`LazyCollectionProtocol`

#### Where Elements : LazyCollectionProtocol

var lazy: Slice<Base>.Elements

#### Declaration

`var lazy: Slice<Base>.Elements { get }`

#### Declared In

`LazyCollectionProtocol`

#### Where Elements : LazySequenceProtocol

var lazy: Slice<Base>.Elements

#### Declaration

`var lazy: Slice<Base>.Elements { get }`

#### Declared In

`LazyCollectionProtocol` , `LazySequenceProtocol`

#### Where Index : Strideable, Indices == Range, Index.Stride == Int

var indices: Range<Slice<Base>.Index>

The indices that are valid for subscripting the collection, in ascending order.

#### Declaration

`var indices: Range<Slice<Base>.Index> { get }`

#### Declared In

`RandomAccessCollection`
func distance(from:to:)

Returns the distance between two indices.

Parameters: start: A valid index of the collection. end: Another valid index of the collection. If `end` is equal to `start`, the result is zero. Returns: The distance between `start` and `end`.

Complexity: O(1)

#### Declaration

`func distance(from start: Slice<Base>.Index, to end: Slice<Base>.Index) -> Slice<Base>.Index.Stride`

#### Declared In

`RandomAccessCollection`
func index(_:offsetBy:)

Returns an index that is the specified distance from the given index.

The following example obtains an index advanced four positions from an array's starting index and then prints the element at that position.

``````let numbers = [10, 20, 30, 40, 50]
let i = numbers.index(numbers.startIndex, offsetBy: 4)
print(numbers[i])
// Prints "50"``````

The value passed as `n` must not offset `i` beyond the bounds of the collection.

Parameters: i: A valid index of the collection. n: The distance to offset `i`. Returns: An index offset by `n` from the index `i`. If `n` is positive, this is the same value as the result of `n` calls to `index(after:)`. If `n` is negative, this is the same value as the result of `-n` calls to `index(before:)`.

Complexity: O(1)

#### Declaration

`func index(_ i: Slice<Base>.Index, offsetBy n: Slice<Base>.Index.Stride) -> Slice<Base>.Index`

#### Declared In

`RandomAccessCollection`
func index(after:)

Returns the position immediately after the given index.

`i`: A valid index of the collection. `i` must be less than `endIndex`. Returns: The index value immediately after `i`.

#### Declaration

`func index(after i: Slice<Base>.Index) -> Slice<Base>.Index`

#### Declared In

`RandomAccessCollection`
func index(before:)

Returns the position immediately after the given index.

`i`: A valid index of the collection. `i` must be greater than `startIndex`. Returns: The index value immediately before `i`.

#### Declaration

`func index(before i: Slice<Base>.Index) -> Slice<Base>.Index`

#### Declared In

`RandomAccessCollection`

#### Where Indices == DefaultIndices

var indices: DefaultIndices<Slice<Base>>

The indices that are valid for subscripting the collection, in ascending order.

A collection's `indices` property can hold a strong reference to the collection itself, causing the collection to be non-uniquely referenced. If you mutate the collection while iterating over its indices, a strong reference can cause an unexpected copy of the collection. To avoid the unexpected copy, use the `index(after:)` method starting with `startIndex` to produce indices instead.

``````var c = MyFancyCollection([10, 20, 30, 40, 50])
var i = c.startIndex
while i != c.endIndex {
c[i] /= 5
i = c.index(after: i)
}
// c == MyFancyCollection([2, 4, 6, 8, 10])``````

#### Declaration

`var indices: DefaultIndices<Slice<Base>> { get }`

#### Declared In

`LazyCollectionProtocol` , `MutableCollection` , `RandomAccessCollection` , `RangeReplaceableCollection` , `Collection` , `BidirectionalCollection`

#### Where Iterator == IndexingIterator

func makeIterator()

Returns an iterator over the elements of the collection.

#### Declaration

`func makeIterator() -> IndexingIterator<Slice<Base>>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`

#### Where SubSequence == AnySequence

func drop(while:)

Returns a subsequence by skipping the initial, consecutive elements that satisfy the given predicate.

The following example uses the `drop(while:)` method to skip over the positive numbers at the beginning of the `numbers` array. The result begins with the first element of `numbers` that does not satisfy `predicate`.

``````let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
let startingWithNegative = numbers.drop(while: { \$0 > 0 })
// startingWithNegative == [-2, 9, -6, 10, 1]``````

If `predicate` matches every element in the sequence, the result is an empty sequence.

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result. Returns: A subsequence starting after the initial, consecutive elements that satisfy `predicate`.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func drop(while predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> AnySequence<Slice<Base>.Element>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func dropFirst(_:)

Returns a subsequence containing all but the given number of initial elements.

If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty subsequence.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.dropFirst(2))
// Prints "[3, 4, 5]"
print(numbers.dropFirst(10))
// Prints "[]"``````

`n`: The number of elements to drop from the beginning of the sequence. `n` must be greater than or equal to zero. Returns: A subsequence starting after the specified number of elements.

Complexity: O(1).

#### Declaration

`func dropFirst(_ n: Int) -> AnySequence<Slice<Base>.Element>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func dropLast(_:)

Returns a subsequence containing all but the given number of final elements.

The sequence must be finite. If the number of elements to drop exceeds the number of elements in the sequence, the result is an empty subsequence.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.dropLast(2))
// Prints "[1, 2, 3]"
print(numbers.dropLast(10))
// Prints "[]"``````

`n`: The number of elements to drop off the end of the sequence. `n` must be greater than or equal to zero. Returns: A subsequence leaving off the specified number of elements.

Complexity: O(n), where n is the length of the sequence.

#### Declaration

`func dropLast(_ n: Int) -> AnySequence<Slice<Base>.Element>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func prefix(_:)

Returns a subsequence, up to the specified maximum length, containing the initial elements of the sequence.

If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.prefix(2))
// Prints "[1, 2]"
print(numbers.prefix(10))
// Prints "[1, 2, 3, 4, 5]"``````

`maxLength`: The maximum number of elements to return. The value of `maxLength` must be greater than or equal to zero. Returns: A subsequence starting at the beginning of this sequence with at most `maxLength` elements.

Complexity: O(1)

#### Declaration

`func prefix(_ maxLength: Int) -> AnySequence<Slice<Base>.Element>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func prefix(while:)

Returns a subsequence containing the initial, consecutive elements that satisfy the given predicate.

The following example uses the `prefix(while:)` method to find the positive numbers at the beginning of the `numbers` array. Every element of `numbers` up to, but not including, the first negative value is included in the result.

``````let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
let positivePrefix = numbers.prefix(while: { \$0 > 0 })
// positivePrefix == [3, 7, 4]``````

If `predicate` matches every element in the sequence, the resulting sequence contains every element of the sequence.

`predicate`: A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result. Returns: A subsequence of the initial, consecutive elements that satisfy `predicate`.

Complexity: O(n), where n is the length of the collection.

#### Declaration

`func prefix(while predicate: (Slice<Base>.Element) throws -> Bool) rethrows -> AnySequence<Slice<Base>.Element>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func split(_:omittingEmptySubsequences:whereSeparator:)

Returns the longest possible subsequences of the sequence, in order, that don't contain elements satisfying the given predicate. Elements that are used to split the sequence are not returned as part of any subsequence.

The following examples show the effects of the `maxSplits` and `omittingEmptySubsequences` parameters when splitting a string using a closure that matches spaces. The first use of `split` returns each word that was originally separated by one or more spaces.

``````let line = "BLANCHE:   I don't want realism. I want magic!"
print(line.split(whereSeparator: { \$0 == " " })
.map(String.init))
// Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

The second example passes `1` for the `maxSplits` parameter, so the original string is split just once, into two new strings.

``````print(
line.split(maxSplits: 1, whereSeparator: { \$0 == " " })
.map(String.init))
// Prints "["BLANCHE:", "  I don\'t want realism. I want magic!"]"``````

The final example passes `true` for the `allowEmptySlices` parameter, so the returned array contains empty strings where spaces were repeated.

``````print(
line.split(
omittingEmptySubsequences: false,
whereSeparator: { \$0 == " " }
).map(String.init))
// Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"``````

Parameters: maxSplits: The maximum number of times to split the sequence, or one less than the number of subsequences to return. If `maxSplits + 1` subsequences are returned, the last one is a suffix of the original sequence containing the remaining elements. `maxSplits` must be greater than or equal to zero. The default value is `Int.max`. omittingEmptySubsequences: If `false`, an empty subsequence is returned in the result for each pair of consecutive elements satisfying the `isSeparator` predicate and for each element at the start or end of the sequence satisfying the `isSeparator` predicate. If `true`, only nonempty subsequences are returned. The default value is `true`. isSeparator: A closure that returns `true` if its argument should be used to split the sequence; otherwise, `false`. Returns: An array of subsequences, split from this sequence's elements.

#### Declaration

`func split(maxSplits: Int = default, omittingEmptySubsequences: Bool = default, whereSeparator isSeparator: (Slice<Base>.Element) throws -> Bool) rethrows -> [AnySequence<Slice<Base>.Element>]`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`
func suffix(_:)

Returns a subsequence, up to the given maximum length, containing the final elements of the sequence.

The sequence must be finite. If the maximum length exceeds the number of elements in the sequence, the result contains all the elements in the sequence.

``````let numbers = [1, 2, 3, 4, 5]
print(numbers.suffix(2))
// Prints "[4, 5]"
print(numbers.suffix(10))
// Prints "[1, 2, 3, 4, 5]"``````

`maxLength`: The maximum number of elements to return. The value of `maxLength` must be greater than or equal to zero. Complexity: O(n), where n is the length of the sequence.

#### Declaration

`func suffix(_ maxLength: Int) -> AnySequence<Slice<Base>.Element>`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `LazySequenceProtocol`, `BidirectionalCollection`, `Sequence`

#### Where SubSequence == Slice

subscript(_: Range<Slice<Base>.Index>)

Accesses a contiguous subrange of the collection's elements.

The accessed slice uses the same indices for the same elements as the original collection. Always use the slice's `startIndex` property instead of assuming that its indices start at a particular value.

This example demonstrates getting a slice of an array of strings, finding the index of one of the strings in the slice, and then using that index in the original array.

``````let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let streetsSlice = streets[2 ..< streets.endIndex]
print(streetsSlice)
// Prints "["Channing", "Douglas", "Evarts"]"

let index = streetsSlice.firstIndex(of: "Evarts")    // 4
print(streets[index!])
// Prints "Evarts"``````

`bounds`: A range of the collection's indices. The bounds of the range must be valid indices of the collection.

Complexity: O(1)

#### Declaration

`subscript(bounds: Range<Slice<Base>.Index>) -> Slice<Slice<Base>> { get }`

#### Declared In

`LazyCollectionProtocol`, `MutableCollection`, `RandomAccessCollection`, `RangeReplaceableCollection`, `Collection`, `BidirectionalCollection`