--- title: BidirectionalCollection framework: swift role: symbol role_heading: Protocol path: swift/bidirectionalcollection --- # BidirectionalCollection A collection that supports backward as well as forward traversal. ## Declaration ```swift protocol BidirectionalCollection : Collection where Self.Indices : BidirectionalCollection, Self.SubSequence : BidirectionalCollection ``` ## Overview 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. ## Topics ### Associated Types - [Element](swift/bidirectionalcollection/element.md) - [Index](swift/bidirectionalcollection/index.md) - [Indices](swift/bidirectionalcollection/indices-swift.associatedtype.md) - [SubSequence](swift/bidirectionalcollection/subsequence.md) ### Instance Properties - [endIndex](swift/bidirectionalcollection/endindex.md) - [indices](swift/bidirectionalcollection/indices-4qv42.md) - [last](swift/bidirectionalcollection/last.md) - [startIndex](swift/bidirectionalcollection/startindex.md) ### Instance Methods - [contains(_:)](swift/bidirectionalcollection/contains(_:)-1l08t.md) - [contains(_:)](swift/bidirectionalcollection/contains(_:)-60wyq.md) - [difference(from:)](swift/bidirectionalcollection/difference(from:).md) - [difference(from:by:)](swift/bidirectionalcollection/difference(from:by:).md) - [distance(from:to:)](swift/bidirectionalcollection/distance(from:to:).md) - [dropLast(_:)](swift/bidirectionalcollection/droplast(_:).md) - [firstMatch(of:)](swift/bidirectionalcollection/firstmatch(of:)-6v8ci.md) - [firstMatch(of:)](swift/bidirectionalcollection/firstmatch(of:)-7m8f4.md) - [firstRange(of:)](swift/bidirectionalcollection/firstrange(of:)-1di7b.md) - [firstRange(of:)](swift/bidirectionalcollection/firstrange(of:)-3jqrg.md) - [firstRange(of:)](swift/bidirectionalcollection/firstrange(of:)-5sum7.md) - [formIndex(after:)](swift/bidirectionalcollection/formindex(after:).md) - [formIndex(before:)](swift/bidirectionalcollection/formindex(before:).md) - [index(_:offsetBy:)](swift/bidirectionalcollection/index(_:offsetby:).md) - [index(_:offsetBy:limitedBy:)](swift/bidirectionalcollection/index(_:offsetby:limitedby:).md) - [index(after:)](swift/bidirectionalcollection/index(after:).md) - [index(before:)](swift/bidirectionalcollection/index(before:).md) - [joined(separator:)](swift/bidirectionalcollection/joined(separator:).md) - [last(where:)](swift/bidirectionalcollection/last(where:).md) - [lastIndex(of:)](swift/bidirectionalcollection/lastindex(of:).md) - [lastIndex(where:)](swift/bidirectionalcollection/lastindex(where:).md) - [matches(of:)](swift/bidirectionalcollection/matches(of:)-5eey9.md) - [matches(of:)](swift/bidirectionalcollection/matches(of:)-5hhx.md) - [popLast()](swift/bidirectionalcollection/poplast().md) - [prefixMatch(of:)](swift/bidirectionalcollection/prefixmatch(of:)-2fwv6.md) - [prefixMatch(of:)](swift/bidirectionalcollection/prefixmatch(of:)-7dq6v.md) - [ranges(of:)](swift/bidirectionalcollection/ranges(of:)-40wx3.md) - [ranges(of:)](swift/bidirectionalcollection/ranges(of:)-9qfdo.md) - [removeLast()](swift/bidirectionalcollection/removelast().md) - [removeLast(_:)](swift/bidirectionalcollection/removelast(_:).md) - [reversed()](swift/bidirectionalcollection/reversed().md) - [split(maxSplits:omittingEmptySubsequences:separator:)](swift/bidirectionalcollection/split(maxsplits:omittingemptysubsequences:separator:).md) - [split(separator:maxSplits:omittingEmptySubsequences:)](swift/bidirectionalcollection/split(separator:maxsplits:omittingemptysubsequences:).md) - [starts(with:)](swift/bidirectionalcollection/starts(with:)-4972u.md) - [starts(with:)](swift/bidirectionalcollection/starts(with:)-97xlm.md) - [suffix(_:)](swift/bidirectionalcollection/suffix(_:).md) - [trimmingPrefix(_:)](swift/bidirectionalcollection/trimmingprefix(_:)-1luge.md) - [trimmingPrefix(_:)](swift/bidirectionalcollection/trimmingprefix(_:)-781ik.md) - [wholeMatch(of:)](swift/bidirectionalcollection/wholematch(of:)-1wbp6.md) - [wholeMatch(of:)](swift/bidirectionalcollection/wholematch(of:)-7741n.md) ### Subscripts - [subscript(_:)](swift/bidirectionalcollection/subscript(_:)-5tj5.md) - [subscript(_:)](swift/bidirectionalcollection/subscript(_:)-7tzd8.md) ## Relationships ### Inherits From - [Collection](swift/collection.md) - [Sequence](swift/sequence.md) ### Inherited By - [RandomAccessCollection](swift/randomaccesscollection.md) - [StringProtocol](swift/stringprotocol.md) ### Conforming Types - [AnyBidirectionalCollection](swift/anybidirectionalcollection.md) - [AnyRandomAccessCollection](swift/anyrandomaccesscollection.md) - [AnyRegexOutput](swift/anyregexoutput.md) - [Array](swift/array.md) - [ArraySlice](swift/arrayslice.md) - [ClosedRange](swift/closedrange.md) - [CollectionOfOne](swift/collectionofone.md) - [ContiguousArray](swift/contiguousarray.md) - [DefaultIndices](swift/defaultindices.md) - [DiscontiguousSlice](swift/discontiguousslice.md) - [EmptyCollection](swift/emptycollection.md) - [EnumeratedSequence](swift/enumeratedsequence.md) - [FlattenSequence](swift/flattensequence.md) - [Int.Words](swift/int/words-swift.struct.md) - [Int16.Words](swift/int16/words-swift.struct.md) - [Int32.Words](swift/int32/words-swift.struct.md) - [Int64.Words](swift/int64/words-swift.struct.md) - [Int8.Words](swift/int8/words-swift.struct.md) - [KeyValuePairs](swift/keyvaluepairs.md) - [LazyDropWhileSequence](swift/lazydropwhilesequence.md) - [LazyFilterSequence](swift/lazyfiltersequence.md) - [LazyMapSequence](swift/lazymapsequence.md) - [LazyPrefixWhileSequence](swift/lazyprefixwhilesequence.md) - [LazySequence](swift/lazysequence.md) - [Range](swift/range.md) - [RangeSet.Ranges](swift/rangeset/ranges-swift.struct.md) - [Repeated](swift/repeated.md) - [ReversedCollection](swift/reversedcollection.md) - [Slice](swift/slice.md) - [String](swift/string.md) - [String.UTF16View](swift/string/utf16view.md) - [String.UTF8View](swift/string/utf8view.md) - [String.UnicodeScalarView](swift/string/unicodescalarview.md) - [Substring](swift/substring.md) - [Substring.UTF16View](swift/substring/utf16view.md) - [Substring.UTF8View](swift/substring/utf8view.md) - [Substring.UnicodeScalarView](swift/substring/unicodescalarview.md) - [UInt.Words](swift/uint/words-swift.struct.md) - [UInt128.Words](swift/uint128/words-swift.struct.md) - [UInt16.Words](swift/uint16/words-swift.struct.md) - [UInt32.Words](swift/uint32/words-swift.struct.md) - [UInt64.Words](swift/uint64/words-swift.struct.md) - [UInt8.Words](swift/uint8/words-swift.struct.md) - [Unicode.Scalar.UTF16View](swift/unicode/scalar/utf16view.md) - [Unicode.Scalar.UTF8View](swift/unicode/scalar/utf8view.md) - [UnsafeBufferPointer](swift/unsafebufferpointer.md) - [UnsafeMutableBufferPointer](swift/unsafemutablebufferpointer.md) - [UnsafeMutableRawBufferPointer](swift/unsafemutablerawbufferpointer.md) - [UnsafeRawBufferPointer](swift/unsaferawbufferpointer.md) ## See Also ### Collection Traversal - [RandomAccessCollection](swift/randomaccesscollection.md)