# collection\[safe: index]

{% tabs %}
{% tab title="code" %}

```swift
// avoid index out-of-range
extension Collection { 
    /// return the element at the index if it is within range,
    /// otherwise nil.
    ///
    /// Example:
    /// `collection[safe: 20]`
    subscript(safe index: Index) -> Element? { 
        return indices.contains(index) ? self[index] : nil 
    } 
}
```

{% endtab %}

{% tab title="📗 參考" %}

* Test-Driven Development in Swift (2021), p. 59
  {% endtab %}

{% tab title="👥 相關" %}

* [subscripts](https://lochiwei.gitbook.io/ios/swift/subscripts "mention")
  {% endtab %}
  {% endtabs %}


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