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reduce(_:_:)

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

Declaration

func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Self.Element) 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 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(_:_:).

See Also

Transforming a String’s Characters