# relative(to:) Returns the range of indices described by this range expression within the given collection. ``` func relative(to collection: C) -> Range where C : Collection, Self.Bound == C.Index ``` ## Parameters `collection` The collection to evaluate this range expression in relation to. ## Return Value A range suitable for slicing `collection`. The returned range is *not* guaranteed to be inside the bounds of `collection`. Callers should apply the same preconditions to the return value as they would to a range provided directly by the user. ## Discussion You can use the `relative(to:)` method to convert a range expression, which could be missing one or both of its endpoints, into a concrete range that is bounded on both sides. The following example uses this method to convert a partial range up to `4` into a half-open range, using an array instance to add the range’s lower bound. ``` let numbers = [10, 20, 30, 40, 50, 60, 70] let upToFour = ..<4 let r1 = upToFour.relative(to: numbers) // r1 == 0..<4 ``` The `r1` range is bounded on the lower end by `0` because that is the starting index of the `numbers` array. When the collection passed to `relative(to:)` starts with a different index, that index is used as the lower bound instead. The next example creates a slice of `numbers` starting at index `2`, and then uses the slice with `relative(to:)` to convert `upToFour` to a concrete range. ``` let numbersSuffix = numbers[2...] // numbersSuffix == [30, 40, 50, 60, 70] let r2 = upToFour.relative(to: numbersSuffix) // r2 == 2..<4 ``` Use this method only if you need the concrete range it produces. To access a slice of a collection using a range expression, use the collection’s generic subscript that uses a range expression as its parameter. ``` let numbersPrefix = numbers[upToFour] // numbersPrefix == [10, 20, 30, 40] ``` --- 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)