# partition(by:) Reorders the elements of the collection such that all the elements that match the given predicate are after all the elements that don’t match. ``` mutating func partition(by belongsInSecondPartition: (Self.Element) throws -> Bool) rethrows -> Self.Index ``` ## Parameters `belongsInSecondPartition` A predicate used to partition the collection. All elements satisfying this predicate are ordered after all elements not satisfying it. ## Return Value The index of the first element in the reordered collection that matches `belongsInSecondPartition`. If no elements in the collection match `belongsInSecondPartition`, the returned index is equal to the collection’s `endIndex`. ## Discussion After partitioning a collection, there is a pivot index `p` where no element before `p` satisfies the `belongsInSecondPartition` predicate and every element at or after `p` satisfies `belongsInSecondPartition`. This operation isn’t guaranteed to be stable, so the relative ordering of elements within the partitions might change. In the following example, an array of numbers is partitioned by a predicate that matches elements greater than 30. ``` var numbers = [30, 40, 20, 30, 30, 60, 10] let p = numbers.partition(by: { $0 > 30 }) // p == 5 // numbers == [30, 10, 20, 30, 30, 60, 40] ``` The `numbers` array is now arranged in two partitions. The first partition, `numbers[.. Complexity: O(*n*), where *n* is the length of the collection. --- 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)