## RandomAccessSlice

`struct RandomAccessSlice<Base where Base : _RandomAccessIndexable>`

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 `RandomAccessSlice` 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]"``````

Important: 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.

Inheritance `BidirectionalCollection, Collection, RandomAccessCollection, 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. See Also: endIndex `IndexDistance = Base.IndexDistance` A type that represents the number of steps between a pair of indices. `SubSequence = RandomAccessSlice` A sequence that represents a contiguous subrange of the collection's elements. This associated type appears as a requirement in the `Sequence` protocol, but it is restated here with stricter constraints. In a collection, the subsequence should also conform to `Collection`. `Iterator = IndexingIterator>` Type alias inferred. `Element = Base._Element` Type alias inferred. `Index = Base.Index` Type alias inferred. `import Swift`

### Initializers

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<RandomAccessSlice.Index>)`

### 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 RandomAccessSlice<CountableRange<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: Base.IndexDistance

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: Base.IndexDistance { get }`

#### Declared In

`RandomAccessCollection` , `BidirectionalCollection` , `Collection`
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.index(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: 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: Base._Element? { get }`

#### Declared In

`RandomAccessCollection` , `BidirectionalCollection` , `Collection`
var indices: DefaultRandomAccessIndices<RandomAccessSlice<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: DefaultRandomAccessIndices<RandomAccessSlice<Base>> { get }`

#### Declared In

`RandomAccessCollection`
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.characters.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

`RandomAccessCollection` , `BidirectionalCollection` , `Collection`
var last: 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: Base._Element? { get }`

#### Declared In

`RandomAccessCollection` , `BidirectionalCollection`
var lazy: LazyRandomAccessCollection<RandomAccessSlice<Base>>

A view onto this collection that provides lazy implementations of normally eager operations, such as `map` and `filter`.

Use the `lazy` property when chaining operations to prevent intermediate operations from allocating storage, or when you only need a part of the final collection to avoid unnecessary computation.

See Also: `LazySequenceProtocol`, `LazyCollectionProtocol`.

#### Declaration

`var lazy: LazyRandomAccessCollection<RandomAccessSlice<Base>> { get }`

#### Declared In

`RandomAccessCollection`
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

`RandomAccessCollection` , `BidirectionalCollection` , `Collection` , `Sequence`

### Subscripts

subscript(_: 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: Base.Index) -> Base._Element { get }`
subscript(_: Range<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 uses. 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.index(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<Base.Index>) -> RandomAccessSlice<Base> { get }`

#### Declared In

`RandomAccessSlice`, `RandomAccessCollection`

### Instance Methods

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: (Base._Element) throws -> Bool) rethrows -> Bool`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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: RandomAccessSlice.Index, to end: RandomAccessSlice.Index) -> RandomAccessSlice.IndexDistance`
func drop(while:)

Returns a subsequence by skipping elements while `predicate` returns `true` and returning the remaining elements.

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

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

#### Declaration

`func drop(while predicate: (Base._Element) throws -> Bool) rethrows -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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() -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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) -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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() -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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 `n` elements from the end.

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

#### Declaration

`func dropLast(_ n: Int) -> RandomAccessSlice<Base>`

#### Declared In

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

Returns a Boolean value indicating whether this sequence and another sequence contain equivalent elements, 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.`

See Also: `elementsEqual(_:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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".characters.enumerated() {
print("\(n): '\(c)'")
}
// Prints "0: 'S'"
// Prints "1: 'w'"
// Prints "2: 'i'"
// Prints "3: 'f'"
// Prints "4: 't'"``````

When enumerating a collection, the integer part of each pair is a counter for the enumeration, not necessarily the index of the paired value. These counters can only be used as indices 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 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.characters.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<RandomAccessSlice<Base>>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func filter(_:)

Returns an array containing, in order, the elements of the sequence that satisfy the given predicate.

In this example, `filter` is used to include only names shorter than five characters.

``````let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let shortNames = cast.filter { \$0.characters.count < 5 }
print(shortNames)
// Prints "["Kim", "Karl"]"``````

`isIncluded`: 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 returned array. Returns: An array of the elements that `includeElement` allowed.

#### Declaration

`func filter(_ isIncluded: (Base._Element) throws -> Bool) rethrows -> [Base._Element]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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: (Base._Element) throws -> Bool) rethrows -> Base._Element?`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func flatMap<ElementOfResult>(_: (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: (Base._Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func flatMap<SegmentOfResult where SegmentOfResult : Sequence>(_: (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(count: \$0, repeatedValue: \$0) }
// [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]

let flatMapped = numbers.flatMap { Array(count: \$0, repeatedValue: \$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. See Also: `joined()`, `map(_:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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: (Base._Element) throws -> Swift.Void) rethrows`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func formIndex(after:)

Replaces the given index with its successor.

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

See Also: `index(after:)`

#### Declaration

`func formIndex(after i: inout RandomAccessSlice.Index)`
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 RandomAccessSlice.Index)`
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:)`.

See Also: `index(_:offsetBy:limitedBy:)`, `formIndex(_:offsetBy:)` Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: RandomAccessSlice.Index, offsetBy n: RandomAccessSlice.IndexDistance) -> RandomAccessSlice.Index`
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`.

See Also: `index(_:offsetBy:)`, `formIndex(_:offsetBy:limitedBy:)` Complexity: O(1) if the collection conforms to `RandomAccessCollection`; otherwise, O(n), where n is the absolute value of `n`.

#### Declaration

`func index(_ i: RandomAccessSlice.Index, offsetBy n: RandomAccessSlice.IndexDistance, limitedBy limit: RandomAccessSlice.Index) -> RandomAccessSlice.Index?`
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: RandomAccessSlice.Index) -> RandomAccessSlice.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: RandomAccessSlice.Index) -> RandomAccessSlice.Index`
func index(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.index(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`.

See Also: `index(of:)`

#### Declaration

`func index(where predicate: (Base._Element) throws -> Bool) rethrows -> Base.Index?`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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. See Also: `lexicographicallyPrecedes(_:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func makeIterator()

Returns an iterator over the elements of the collection.

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
func map(_:)

Returns an array containing the results of mapping the given closure over the sequence's elements.

In this example, `map` is used first to convert the names in the array to lowercase strings and then to count their characters.

``````let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let lowercaseNames = cast.map { \$0.lowercaseString }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { \$0.characters.count }
// 'letterCounts' == [6, 6, 3, 4]``````

`transform`: A mapping closure. `transform` accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type. Returns: An array containing the transformed elements of this sequence.

#### Declaration

`func map<T>(_ transform: (Base._Element) throws -> T) rethrows -> [T]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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`.

See Also: `max()`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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`.

See Also: `min()`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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) -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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.index(of: 40) {
print(numbers.prefix(through: 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) See Also: `prefix(upTo:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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.index(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 "[]"``````

`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) See Also: `prefix(through:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
func prefix(while:)

Returns a subsequence containing the initial elements until `predicate` returns `false` and skipping the remaining elements.

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

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

#### Declaration

`func prefix(while predicate: (Base._Element) throws -> Bool) rethrows -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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, Base._Element) throws -> Result) rethrows -> Result`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func reversed()

Returns a view presenting the elements of the collection in reverse order.

You can reverse a collection without allocating new space for its elements by calling this `reversed()` method. A `ReversedRandomAccessCollection` instance wraps an underlying collection and provides access to its elements in reverse order. This example prints the elements of an array in reverse order:

``````let numbers = [3, 5, 7]
for number in numbers.reversed() {
print(number)
}
// Prints "7"
// Prints "5"
// Prints "3"``````

If you need a reversed collection of the same type, you may be able to use the collection's sequence-based or collection-based initializer. For example, to get the reversed version of an array, initialize a new `Array` instance from the result of this `reversed()` method.

``````let reversedNumbers = Array(numbers.reversed())
print(reversedNumbers)
// Prints "[7, 5, 3]"``````

Complexity: O(1)

#### Declaration

`func reversed() -> ReversedRandomAccessCollection<RandomAccessSlice<Base>>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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.

See Also: `sorted()`

#### Declaration

`func sorted(by areInIncreasingOrder: (Base._Element, Base._Element) -> Bool) -> [Base._Element]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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.characters.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.characters.split(
maxSplits: 1, whereSeparator: { \$0 == " " }
).map(String.init))
// 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.characters.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 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: (Base._Element) throws -> Bool) rethrows -> [RandomAccessSlice<Base>]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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`.

See Also: `starts(with:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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 the entire 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. `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 equal to `maxLength`.

#### Declaration

`func suffix(_ maxLength: Int) -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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.index(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 "[]"``````

`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: Base.Index) -> RandomAccessSlice<Base>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`

### 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 Index : Strideable, IndexDistance == Index.Stride, Indices == CountableRange

var indices: CountableRange<Base.Index>

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

#### Declaration

`var indices: CountableRange<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: Base.Index, to end: Base.Index) -> 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: Base.Index, offsetBy n: Base.Index.Stride) -> 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: Base.Index) -> 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: Base.Index) -> Base.Index`

#### Declared In

`RandomAccessCollection`

#### Where Indices == DefaultBidirectionalIndices

var indices: DefaultBidirectionalIndices<RandomAccessSlice<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: DefaultBidirectionalIndices<RandomAccessSlice<Base>> { get }`

#### Declared In

`RandomAccessCollection` , `BidirectionalCollection`

#### Where Indices == DefaultIndices

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

#### Declared In

`RandomAccessCollection` , `BidirectionalCollection` , `Collection`

#### Where Iterator.Element : BidirectionalCollection

func joined()

Returns the elements of this collection of collections, 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 collection of collections.

See Also: `flatMap(_:)`, `joined(separator:)`

#### Declaration

`func joined() -> FlattenBidirectionalCollection<RandomAccessSlice<Base>>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`

#### Where Iterator.Element : Collection

func joined()

Returns the elements of this collection of collections, 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 collection of collections.

See Also: `flatMap(_:)`, `joined(separator:)`

#### Declaration

`func joined() -> FlattenCollection<RandomAccessSlice<Base>>`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`

#### Where Iterator.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. See Also: `lexicographicallyPrecedes(_:by:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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`.

See Also: `max(by:)`

#### Declaration

```@warn_unqualified_access func max() -> Base._Element?```

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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`.

See Also: `min(by:)`

#### Declaration

```@warn_unqualified_access func min() -> Base._Element?```

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
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.

See Also: `sorted(by:)`

#### Declaration

`func sorted() -> [Base._Element]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`

#### Where Iterator.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: Base._Element) -> Bool`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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.

See Also: `elementsEqual(_:by:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`
func index(of:)

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

After using `index(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.index(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`.

See Also: `index(where:)`

#### Declaration

`func index(of element: Base._Element) -> Base.Index?`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`
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.characters.split(separator: " ")
.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.characters.split(separator: " ", maxSplits: 1)
.map(String.init))
// 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.characters.split(separator: " ", omittingEmptySubsequences: false)
.map(String.init))
// 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: Base._Element, maxSplits: Int = default, omittingEmptySubsequences: Bool = default) -> [RandomAccessSlice<Base>]`

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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`.

See Also: `starts(with:by:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`

#### Where Iterator.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.

See Also: `flatMap(_:)`, `joined(separator:)`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `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.

See Also: `joined()`

#### Declaration

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

#### Declared In

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`

#### Where Iterator.Element == String

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

`RandomAccessCollection`, `BidirectionalCollection`, `Collection`, `Sequence`