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# formIndex(after:)

Replaces the given index with its successor.

```
func formIndex(after index: inout Int)
```

## Parameters

`index`

A valid index of the array. `i` must be less
than `endIndex`.

## Discussion> Note: To improve performance, this method does not validate that the
> given index is valid before incrementing 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)

---

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