Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.
SDK
- Xcode 10.2+
Framework
- Swift Standard Library
Declaration
func flatMap<SegmentOfResult>(_ transform: ((key: Key, value: Value)) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence
Parameters
transformA 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 flat 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 is equivalent to Array(s.
Complexity: O(m + n), where n is the length of this sequence and m is the length of the result.