# Collection Implementations ## Topics ### Structures [`struct Index`](/documentation/Swift/LazyPrefixWhileSequence/Index) A position in a `LazyPrefixWhileCollection` or `LazyPrefixWhileBidirectionalCollection` instance. ### Instance Properties [`var count: Int`](/documentation/Swift/LazyPrefixWhileSequence/count) The number of elements in the collection. [`var endIndex: LazyPrefixWhileSequence.Index`](/documentation/Swift/LazyPrefixWhileSequence/endIndex) The collection’s “past the end” position—that is, the position one greater than the last valid subscript argument. [`var indices: DefaultIndices`](/documentation/Swift/LazyPrefixWhileSequence/indices-swift.property) The indices that are valid for subscripting the collection, in ascending order. [`var isEmpty: Bool`](/documentation/Swift/LazyPrefixWhileSequence/isEmpty) A Boolean value indicating whether the collection is empty. [`var startIndex: LazyPrefixWhileSequence.Index`](/documentation/Swift/LazyPrefixWhileSequence/startIndex) The position of the first element in a nonempty collection. ### Instance Methods [`func distance(from: Self.Index, to: Self.Index) -> Int`](/documentation/Swift/LazyPrefixWhileSequence/distance(from:to:)-5ytzv) Returns the distance between two indices. [`func firstIndex(of: Self.Element) -> Self.Index?`](/documentation/Swift/LazyPrefixWhileSequence/firstIndex(of:)) Returns the first index where the specified value appears in the collection. [`func firstIndex(where: (Self.Element) throws -> Bool) rethrows -> Self.Index?`](/documentation/Swift/LazyPrefixWhileSequence/firstIndex(where:)) Returns the first index in which an element of the collection satisfies the given predicate. [`func formIndex(inout Self.Index, offsetBy: Int)`](/documentation/Swift/LazyPrefixWhileSequence/formIndex(_:offsetBy:)) Offsets the given index by the specified distance. [`func formIndex(inout Self.Index, offsetBy: Int, limitedBy: Self.Index) -> Bool`](/documentation/Swift/LazyPrefixWhileSequence/formIndex(_:offsetBy:limitedBy:)) Offsets the given index by the specified distance, or so that it equals the given limiting index. [`func formIndex(after: inout Self.Index)`](/documentation/Swift/LazyPrefixWhileSequence/formIndex(after:)) Replaces the given index with its successor. [`func index(Self.Index, offsetBy: Int) -> Self.Index`](/documentation/Swift/LazyPrefixWhileSequence/index(_:offsetBy:)-2y8r8) Returns an index that is the specified distance from the given index. [`func index(Self.Index, offsetBy: Int, limitedBy: Self.Index) -> Self.Index?`](/documentation/Swift/LazyPrefixWhileSequence/index(_:offsetBy:limitedBy:)-279cj) Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index. [`func index(after: LazyPrefixWhileSequence.Index) -> LazyPrefixWhileSequence.Index`](/documentation/Swift/LazyPrefixWhileSequence/index(after:)) Returns the position immediately after the given index. [`func index(of: Self.Element) -> Self.Index?`](/documentation/Swift/LazyPrefixWhileSequence/index(of:)) Returns the first index where the specified value appears in the collection. [`func indices(of: Self.Element) -> RangeSet`](/documentation/Swift/LazyPrefixWhileSequence/indices(of:)) Returns the indices of all the elements that are equal to the given element. [`func indices(where: (Self.Element) throws -> Bool) rethrows -> RangeSet`](/documentation/Swift/LazyPrefixWhileSequence/indices(where:)) Returns the indices of all the elements that match the given predicate. [`func map((Self.Element) throws(E) -> T) throws(E) -> [T]`](/documentation/Swift/LazyPrefixWhileSequence/map(_:)-69szl) Returns an array containing the results of mapping the given closure over the sequence’s elements. [`func prefix(through: Self.Index) -> Self.SubSequence`](/documentation/Swift/LazyPrefixWhileSequence/prefix(through:)) Returns a subsequence from the start of the collection through the specified position. [`func prefix(upTo: Self.Index) -> Self.SubSequence`](/documentation/Swift/LazyPrefixWhileSequence/prefix(upTo:)) Returns a subsequence from the start of the collection up to, but not including, the specified position. [`func randomElement() -> Self.Element?`](/documentation/Swift/LazyPrefixWhileSequence/randomElement()) Returns a random element of the collection. [`func randomElement(using: inout T) -> Self.Element?`](/documentation/Swift/LazyPrefixWhileSequence/randomElement(using:)) Returns a random element of the collection, using the given generator as a source for randomness. [`func removingSubranges(RangeSet) -> DiscontiguousSlice`](/documentation/Swift/LazyPrefixWhileSequence/removingSubranges(_:)) Returns a collection of the elements in this collection that are not represented by the given range set. [`func split(separator: Self.Element, maxSplits: Int, omittingEmptySubsequences: Bool) -> [Self.SubSequence]`](/documentation/Swift/LazyPrefixWhileSequence/split(separator:maxSplits:omittingEmptySubsequences:)-1na6p) Returns the longest possible subsequences of the collection, in order, around elements equal to the given element. [`func suffix(from: Self.Index) -> Self.SubSequence`](/documentation/Swift/LazyPrefixWhileSequence/suffix(from:)) Returns a subsequence from the specified position to the end of the collection. ### Subscripts [`subscript(LazyPrefixWhileSequence.Index) -> LazyPrefixWhileSequence.Element`](/documentation/Swift/LazyPrefixWhileSequence/subscript(_:)) Accesses the element at the specified position. [`subscript(UnboundedRange) -> Self.SubSequence`](/documentation/Swift/LazyPrefixWhileSequence/subscript(_:)-1oo1v) [`subscript(Range) -> Slice`](/documentation/Swift/LazyPrefixWhileSequence/subscript(_:)-59k49) Accesses a contiguous subrange of the collection’s elements. [`subscript(RangeSet) -> DiscontiguousSlice`](/documentation/Swift/LazyPrefixWhileSequence/subscript(_:)-75yeo) Accesses a view of this collection with the elements at the given indices. [`subscript(R) -> Self.SubSequence`](/documentation/Swift/LazyPrefixWhileSequence/subscript(_:)-9tkrc) Accesses the contiguous subrange of the collection’s elements specified by a range expression. ### Type Aliases [`typealias Indices`](/documentation/Swift/LazyPrefixWhileSequence/Indices) A type that represents the indices that are valid for subscripting the collection, in ascending order. [`typealias SubSequence`](/documentation/Swift/LazyPrefixWhileSequence/SubSequence) A collection representing a contiguous subrange of this collection’s elements. The subsequence shares indices with the original 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)