Returns the result of combining the elements of the sequence using the given closure.
SDK
- Xcode 8.0+
Framework
- Swift Standard Library
Declaration
func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, (key: Key, value: Value)) throws -> Result) rethrows -> Result
Parameters
initialResultThe value to use as the initial accumulating value.
initialis passed toResult nextthe first time the closure is executed.Partial Result nextPartialResultA 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
nextclosure or returned to the caller.Partial Result
Return Value
The final accumulated value. If the sequence has no elements, the result is initial.
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 next closure is called sequentially with an accumulating value initialized to initial 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 is called, the following steps occur:
The
nextclosure is called withPartial Result initial—Result 0in this case—and the first element ofnumbers, returning the sum:1.The closure is called again repeatedly with the previous call’s return value and each element of the sequence.
When the sequence is exhausted, the last value returned from the closure is returned to the caller.
If the sequence has no elements, next is never executed and initial is the result of the call to reduce(_:.
Complexity: O(n), where n is the length of the sequence.