---
title: "formIndex(_:offsetBy:limitedBy:)"
framework: Swift
role: symbol
role_heading: Instance Method
platforms: [iOS 27.0+, iPadOS 27.0+, Mac Catalyst 27.0+, macOS 27.0+, tvOS 27.0+, visionOS 27.0+, watchOS 27.0+]
path: "swift/uniquearray/formindex(_:offsetby:limitedby:)"
---

# formIndex(_:offsetBy:limitedBy:)

Offsets the given index by the specified distance, but no further than the given limiting index.

## Declaration

```swift
func formIndex(_ index: inout UniqueArray<Element>.Index, offsetBy n: inout Int, limitedBy limit: UniqueArray<Element>.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

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. note: O(1)