# RandomAccessCollection A collection that supports efficient random-access index traversal. ``` protocol RandomAccessCollection : BidirectionalCollection where Self.Indices : RandomAccessCollection, Self.SubSequence : RandomAccessCollection ``` ## Overview Random-access collections can move indices any distance and measure the distance between indices in O(1) time. Therefore, the fundamental difference between random-access and bidirectional collections is that operations that depend on index movement or distance measurement offer significantly improved efficiency. For example, a random-access collection’s `count` property is calculated in O(1) instead of requiring iteration of an entire collection. # Conforming to the RandomAccessCollection Protocol The `RandomAccessCollection` protocol adds further constraints on the associated `Indices` and `SubSequence` types, but otherwise imposes no additional requirements over the `BidirectionalCollection` protocol. However, in order to meet the complexity guarantees of a random-access collection, either the index for your custom type must conform to the `Strideable` protocol or you must implement the `index(_:offsetBy:)` and `distance(from:to:)` methods with O(1) efficiency. --- 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)