# formIndex(_:offsetBy:limitedBy:) Offsets the given index by the specified distance, but no further than the given limiting index. ``` func formIndex(_ index: inout UniqueArray.Index, offsetBy n: inout Int, limitedBy limit: UniqueArray.Index) ``` ## Parameters `index` A valid index of the array. On return, `index` is set to the resulting position. `n` The distance to offset `index`. On return, `n` is set to zero if the operation succeeded without hitting the limit; otherwise, `n` reflects the number of steps that couldn’t be taken. `limit` A valid index of the array to use as a limit. If `n > 0`, a limit that is less than `index` has no effect. Likewise, if `n < 0`, a limit that is greater than `index` has no effect. ## Discussion If the operation was able to offset `index` by exactly the requested number of steps without hitting `limit`, then on return `n` is set to `0`, and `index` is set to the adjusted index. If the operation hits the limit before it can take the requested number of steps, then on return `index` is set to `limit`, and `n` is set to the number of steps that couldn’t be taken. The value passed as `n` must not offset `index` beyond the bounds of the container, unless the index passed as `limit` prevents offsetting beyond those bounds. > Note: To improve performance, this method does not validate that the > given index is valid before offseting it. Index validation is > deferred until the resulting index is used to access an element. > This optimization may be removed in future versions; do not rely on it. > 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)