---
title: "formIndex(_:offsetBy:)"
framework: swift
role: symbol
role_heading: Instance Method
path: "swift/substring/utf16view/formindex(_:offsetby:)"
---

# formIndex(_:offsetBy:)

Offsets the given index by the specified distance.

## Declaration

```swift
func formIndex(_ i: inout Self.Index, offsetBy distance: Int)
```

## 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.

## Discussion

Discussion The value passed as distance must not offset i beyond the bounds of the collection. note: O(1) if the collection conforms to RandomAccessCollection; otherwise, O(k), where k is the absolute value of distance.