# prefix(while:) Returns a sequence containing the initial, consecutive elements that satisfy the given predicate. ``` func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> [Self.Element] ``` ## Parameters `predicate` A closure that takes an element of the sequence as its argument and returns a Boolean value indicating whether the element should be included in the result. ## Return Value A sequence of the initial, consecutive elements that satisfy `predicate`. ## Discussion The following example uses the `prefix(while:)` method to find the positive numbers at the beginning of the `numbers` array. Every element of `numbers` up to, but not including, the first negative value is included in the result. ``` let numbers = [3, 7, 4, -2, 9, -6, 10, 1] let positivePrefix = numbers.prefix(while: { $0 > 0 }) // positivePrefix == [3, 7, 4] ``` If `predicate` matches every element in the sequence, the resulting sequence contains every element of the sequence. > Complexity: O(*k*), where *k* 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)