# prefix(through:) Returns a subsequence from the start of the collection through the specified position. ``` func prefix(through position: Self.Index) -> Self.SubSequence ``` ## Parameters `position` The index of the last element to include in the resulting subsequence. `position` must be a valid index of the collection that is not equal to the `endIndex` property. ## Return Value A subsequence up to, and including, the given position. ## Discussion The resulting subsequence *includes* the element at the position specified by the `through` parameter. The following example searches for the index of the number `40` in an array of integers, and then prints the prefix of the array up to, and including, that index: ``` let numbers = [10, 20, 30, 40, 50, 60] if let i = numbers.firstIndex(of: 40) { print(numbers.prefix(through: i)) } // Prints "[10, 20, 30, 40]" ``` Using the `prefix(through:)` method is equivalent to using a partial closed range as the collection’s subscript. The subscript notation is preferred over `prefix(through:)`. ``` if let i = numbers.firstIndex(of: 40) { print(numbers[...i]) } // Prints "[10, 20, 30, 40]" ``` > Complexity: O(1) --- 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)