# formIndex(_:offsetBy:limitedBy:) Offsets the given index by the specified distance, or so that it equals the given limiting index. ``` func formIndex(_ i: inout Self.Index, offsetBy distance: Int, limitedBy limit: Self.Index) -> Bool ``` ## Parameters `i` A valid index of the collection. `distance` The distance to offset `i`. `distance` must not be negative unless the collection conforms to the `BidirectionalCollection` protocol. `limit` A valid index of the collection to use as a limit. If `distance > 0`, a limit that is less than `i` has no effect. Likewise, if `distance < 0`, a limit that is greater than `i` has no effect. ## Return Value `true` if `i` has been offset by exactly `distance` steps without going beyond `limit`; otherwise, `false`. When the return value is `false`, the value of `i` is equal to `limit`. ## Discussion The value passed as `distance` must not offset `i` beyond the bounds of the collection, unless the index passed as `limit` prevents offsetting beyond those bounds. > Complexity: O(1) if the collection conforms to > `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute > value of `distance`. --- 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)