Generic Instance Method

reduce(_:_:)

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

Declaration

func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, UInt8) throws -> Result) rethrows -> Result

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.

Return Value

The final accumulated value. If the sequence has no elements, the result is initialResult.

Discussion

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 initialResult0 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(_:_:).

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

See Also

Combining Sequence Elements

func allSatisfy((UInt8) -> Bool) -> Bool

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

func compactMap<ElementOfResult>((UInt8) -> ElementOfResult?) -> [ElementOfResult]

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

func contains(UInt8) -> Bool

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

func contains(where: (UInt8) -> Bool) -> Bool

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

func distance(from: Int, to: Int) -> Int

Returns the distance between two indexes.

func drop(while: (UInt8) -> Bool) -> Slice<DispatchData>

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

func dropFirst(Int) -> Slice<DispatchData>

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

func dropLast(Int) -> Slice<DispatchData>

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

func elementsEqual<OtherSequence>(OtherSequence) -> Bool

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

func elementsEqual<OtherSequence>(OtherSequence, by: (UInt8, OtherSequence.Element) -> Bool) -> Bool

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.

func enumerated() -> EnumeratedSequence<DispatchData>

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

func filter((UInt8) -> Bool) -> [UInt8]

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

func first(where: (UInt8) -> Bool) -> UInt8?

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

func firstIndex(of: UInt8) -> Int?

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

func firstIndex(where: (UInt8) -> Bool) -> Int?

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

func forEach((UInt8) -> Void)

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

func formIndex(inout Int, offsetBy: Int)

Offsets the given index by the specified distance.

func formIndex(inout Int, offsetBy: Int, limitedBy: Int) -> Bool

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

func formIndex(after: inout Int)

Replaces the given index with its successor.

func formIndex(before: inout Int)

Replaces the given index with its predecessor.

func index(Int, offsetBy: Int) -> Int

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

func index(Int, offsetBy: Int, limitedBy: Int) -> Int?

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

func last(where: (UInt8) -> Bool) -> UInt8?

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

func lastIndex(of: UInt8) -> Int?

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

func lastIndex(where: (UInt8) -> Bool) -> Int?

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

func lexicographicallyPrecedes<OtherSequence>(OtherSequence) -> Bool

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

func lexicographicallyPrecedes<OtherSequence>(OtherSequence, by: (UInt8, UInt8) -> Bool) -> Bool

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

func map<T>((UInt8) -> T) -> [T]

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

func max() -> UInt8?

Returns the maximum element in the sequence.

func max(by: (UInt8, UInt8) -> Bool) -> UInt8?

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

func min() -> UInt8?

Returns the minimum element in the sequence.

func min(by: (UInt8, UInt8) -> Bool) -> UInt8?

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

func prefix(Int) -> Slice<DispatchData>

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

func prefix(through: Int) -> Slice<DispatchData>

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

func prefix(upTo: Int) -> Slice<DispatchData>

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

func prefix(while: (UInt8) -> Bool) -> Slice<DispatchData>

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

func randomElement() -> UInt8?

Returns a random element of the collection.

func randomElement<T>(using: inout T) -> UInt8?

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

func reversed() -> ReversedCollection<DispatchData>

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

func shuffled() -> [UInt8]

Returns the elements of the sequence, shuffled.

func shuffled<T>(using: inout T) -> [UInt8]

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

func sorted() -> [UInt8]

Returns the elements of the sequence, sorted.

func sorted(by: (UInt8, UInt8) -> Bool) -> [UInt8]

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

func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator: (UInt8) -> Bool) -> [Slice<DispatchData>]

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

func split(separator: UInt8, maxSplits: Int, omittingEmptySubsequences: Bool) -> [Slice<DispatchData>]

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

func starts<PossiblePrefix>(with: PossiblePrefix) -> Bool

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

func starts<PossiblePrefix>(with: PossiblePrefix, by: (UInt8, PossiblePrefix.Element) -> Bool) -> Bool

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.

func suffix(Int) -> Slice<DispatchData>

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

func suffix(from: Int) -> Slice<DispatchData>

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