# 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. ## Relationships ### Conforming Types [`UnsafeMutableRawBufferPointer`](/documentation/Swift/UnsafeMutableRawBufferPointer) [`ContiguousArray`](/documentation/Swift/ContiguousArray) [`Words-swift.struct`](/documentation/Swift/UInt128/Words-swift.struct) [`EnumeratedSequence`](/documentation/Swift/EnumeratedSequence) [`CollectionOfOne`](/documentation/Swift/CollectionOfOne) [`UTF8View`](/documentation/Swift/Unicode/Scalar/UTF8View) [`ReversedCollection`](/documentation/Swift/ReversedCollection) [`DefaultIndices`](/documentation/Swift/DefaultIndices) [`Slice`](/documentation/Swift/Slice) [`UTF16View`](/documentation/Swift/Unicode/Scalar/UTF16View) [`Words-swift.struct`](/documentation/Swift/Int16/Words-swift.struct) [`Range`](/documentation/Swift/Range) [`AnyRegexOutput`](/documentation/Swift/AnyRegexOutput) [`Ranges-swift.struct`](/documentation/Swift/RangeSet/Ranges-swift.struct) [`Words-swift.struct`](/documentation/Swift/Int/Words-swift.struct) [`UnsafeBufferPointer`](/documentation/Swift/UnsafeBufferPointer) [`ClosedRange`](/documentation/Swift/ClosedRange) [`AnyRandomAccessCollection`](/documentation/Swift/AnyRandomAccessCollection) [`Words-swift.struct`](/documentation/Swift/UInt/Words-swift.struct) [`Repeated`](/documentation/Swift/Repeated) [`Words-swift.struct`](/documentation/Swift/UInt32/Words-swift.struct) [`Words-swift.struct`](/documentation/Swift/UInt64/Words-swift.struct) [`UnsafeRawBufferPointer`](/documentation/Swift/UnsafeRawBufferPointer) [`Words-swift.struct`](/documentation/Swift/Int8/Words-swift.struct) [`Words-swift.struct`](/documentation/Swift/UInt16/Words-swift.struct) [`Words-swift.struct`](/documentation/Swift/UInt8/Words-swift.struct) [`EmptyCollection`](/documentation/Swift/EmptyCollection) [`LazyMapSequence`](/documentation/Swift/LazyMapSequence) [`KeyValuePairs`](/documentation/Swift/KeyValuePairs) [`ArraySlice`](/documentation/Swift/ArraySlice) [`Words-swift.struct`](/documentation/Swift/Int32/Words-swift.struct) [`Array`](/documentation/Swift/Array) [`Words-swift.struct`](/documentation/Swift/Int64/Words-swift.struct) [`UnsafeMutableBufferPointer`](/documentation/Swift/UnsafeMutableBufferPointer) [`LazySequence`](/documentation/Swift/LazySequence) ### Inherits From [`BidirectionalCollection`](/documentation/Swift/BidirectionalCollection) [`Sequence`](/documentation/Swift/Sequence) [`Collection`](/documentation/Swift/Collection) --- 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)