# 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`. ## Relationships ### Conforming Types [`Words-swift.struct`](/documentation/Swift/UInt8/Words-swift.struct) [`Ranges-swift.struct`](/documentation/Swift/RangeSet/Ranges-swift.struct) [`LazySequence`](/documentation/Swift/LazySequence) [`UTF8View`](/documentation/Swift/Unicode/Scalar/UTF8View) [`ReversedCollection`](/documentation/Swift/ReversedCollection) [`Words-swift.struct`](/documentation/Swift/Int64/Words-swift.struct) [`ArraySlice`](/documentation/Swift/ArraySlice) [`ClosedRange`](/documentation/Swift/ClosedRange) [`Words-swift.struct`](/documentation/Swift/UInt16/Words-swift.struct) [`LazyDropWhileSequence`](/documentation/Swift/LazyDropWhileSequence) [`UTF16View`](/documentation/Swift/Substring/UTF16View) [`UnsafeBufferPointer`](/documentation/Swift/UnsafeBufferPointer) [`FlattenSequence`](/documentation/Swift/FlattenSequence) [`Repeated`](/documentation/Swift/Repeated) [`UnsafeMutableBufferPointer`](/documentation/Swift/UnsafeMutableBufferPointer) [`Words-swift.struct`](/documentation/Swift/Int16/Words-swift.struct) [`LazyFilterSequence`](/documentation/Swift/LazyFilterSequence) [`Words-swift.struct`](/documentation/Swift/Int32/Words-swift.struct) [`Range`](/documentation/Swift/Range) [`EnumeratedSequence`](/documentation/Swift/EnumeratedSequence) [`UnicodeScalarView`](/documentation/Swift/String/UnicodeScalarView) [`Words-swift.struct`](/documentation/Swift/UInt/Words-swift.struct) [`CollectionOfOne`](/documentation/Swift/CollectionOfOne) [`LazyPrefixWhileSequence`](/documentation/Swift/LazyPrefixWhileSequence) [`AnyRegexOutput`](/documentation/Swift/AnyRegexOutput) [`Words-swift.struct`](/documentation/Swift/Int8/Words-swift.struct) [`LazyMapSequence`](/documentation/Swift/LazyMapSequence) [`Slice`](/documentation/Swift/Slice) [`Words-swift.struct`](/documentation/Swift/UInt64/Words-swift.struct) [`UTF16View`](/documentation/Swift/String/UTF16View) [`KeyValuePairs`](/documentation/Swift/KeyValuePairs) [`UnsafeMutableRawBufferPointer`](/documentation/Swift/UnsafeMutableRawBufferPointer) [`AnyBidirectionalCollection`](/documentation/Swift/AnyBidirectionalCollection) [`EmptyCollection`](/documentation/Swift/EmptyCollection) [`UTF8View`](/documentation/Swift/String/UTF8View) [`Array`](/documentation/Swift/Array) [`DefaultIndices`](/documentation/Swift/DefaultIndices) [`DiscontiguousSlice`](/documentation/Swift/DiscontiguousSlice) [`UTF16View`](/documentation/Swift/Unicode/Scalar/UTF16View) [`AnyRandomAccessCollection`](/documentation/Swift/AnyRandomAccessCollection) [`UnicodeScalarView`](/documentation/Swift/Substring/UnicodeScalarView) [`Substring`](/documentation/Swift/Substring) [`UnsafeRawBufferPointer`](/documentation/Swift/UnsafeRawBufferPointer) [`String`](/documentation/Swift/String) [`Words-swift.struct`](/documentation/Swift/UInt32/Words-swift.struct) [`ContiguousArray`](/documentation/Swift/ContiguousArray) [`Words-swift.struct`](/documentation/Swift/Int/Words-swift.struct) [`Words-swift.struct`](/documentation/Swift/UInt128/Words-swift.struct) [`UTF8View`](/documentation/Swift/Substring/UTF8View) ### Inherited By [`StringProtocol`](/documentation/Swift/StringProtocol) [`RandomAccessCollection`](/documentation/Swift/RandomAccessCollection) ### Inherits From [`Sequence`](/documentation/Swift/Sequence) [`Collection`](/documentation/Swift/Collection) --- Copyright © 2026 Apple Inc. 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