# prefix(upTo:) Returns a subsequence from the start of the collection up to, but not including, the specified position. ``` func prefix(upTo end: Self.Index) -> Self.SubSequence ``` ## Parameters `end` The “past the end” index of the resulting subsequence. `end` must be a valid index of the collection. ## Return Value A subsequence up to, but not including, the `end` position. ## Discussion The resulting subsequence *does not include* the element at the position `end`. 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, but not including, that index: ``` let numbers = [10, 20, 30, 40, 50, 60] if let i = numbers.firstIndex(of: 40) { print(numbers.prefix(upTo: i)) } // Prints "[10, 20, 30]" ``` Passing the collection’s starting index as the `end` parameter results in an empty subsequence. ``` print(numbers.prefix(upTo: numbers.startIndex)) // Prints "[]" ``` Using the `prefix(upTo:)` method is equivalent to using a partial half-open range as the collection’s subscript. The subscript notation is preferred over `prefix(upTo:)`. ``` if let i = numbers.firstIndex(of: 40) { print(numbers[.. 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)