# flatMap(_:) Returns an array containing the concatenated results of calling the given transformation with each element of this sequence. ``` func flatMap(_ transform: (Self.Element) 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. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)