# BidirectionalCollection A collection that supports backward as well as forward traversal. ``` protocol BidirectionalCollection : Collection where Self.Indices : BidirectionalCollection, Self.SubSequence : BidirectionalCollection ``` ## Overview Bidirectional collections offer traversal backward from any valid index, not including a collection’s `startIndex`. Bidirectional collections can therefore offer additional operations, such as a `last` property that provides efficient access to the last element and a `reversed()` method that presents the elements in reverse order. In addition, bidirectional collections have more efficient implementations of some sequence and collection methods, such as `suffix(_:)`. # Conforming to the BidirectionalCollection Protocol To add `BidirectionalProtocol` conformance to your custom types, implement the `index(before:)` method in addition to the requirements of the `Collection` protocol. Indices that are moved forward and backward in a bidirectional collection move by the same amount in each direction. That is, for any valid index `i` into a bidirectional collection `c`: - If `i >= c.startIndex && i < c.endIndex`, then `c.index(before: c.index(after: i)) == i`. - If `i > c.startIndex && i <= c.endIndex`, then `c.index(after: c.index(before: i)) == i`. Valid indices are exactly those indices that are reachable from the collection’s `startIndex` by repeated applications of `index(after:)`, up to, and including, the `endIndex`. --- 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)