Generic Instance Method

flatMap(_:)

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

Declaration

func flatMap<SegmentOfResult>(_ transform: (CollectionDifference<ChangeElement>.Change) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence

Parameters

transform: A closure that accepts an element of this sequence as its argument and returns a sequence or collection.

Return Value

The resulting flattened array.

Discussion

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()).

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