A collection that supports backward as well as forward traversal.

protocol BidirectionalCollection : Collection where Self.Indices : BidirectionalCollection, Self.SubSequence : BidirectionalCollection


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 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.



Associated Types

Instance Subscripts

Instance Properties

Instance Methods


Instance Properties

Instance Methods


Conforming Types