# Collection Implementations ## Topics ### Structures [`struct Index`](/documentation/Swift/EnumeratedSequence/Index) A type that represents a position in the collection. ### Instance Properties [`var count: Int`](/documentation/Swift/EnumeratedSequence/count) The number of elements in the collection. [`var count: Int`](/documentation/Swift/EnumeratedSequence/count-3o2fx) The number of elements in the collection. [`var endIndex: EnumeratedSequence.Index`](/documentation/Swift/EnumeratedSequence/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/EnumeratedSequence/indices-swift.property) The indices that are valid for subscripting the collection, in ascending order. [`var isEmpty: Bool`](/documentation/Swift/EnumeratedSequence/isEmpty) A Boolean value indicating whether the collection is empty. [`var isEmpty: Bool`](/documentation/Swift/EnumeratedSequence/isEmpty-1yllc) A Boolean value indicating whether the collection is empty. [`var startIndex: EnumeratedSequence.Index`](/documentation/Swift/EnumeratedSequence/startIndex) The position of the first element in a nonempty collection. ### Instance Methods [`func distance(from: EnumeratedSequence.Index, to: EnumeratedSequence.Index) -> Int`](/documentation/Swift/EnumeratedSequence/distance(from:to:)) Returns the distance between two indices. [`func distance(from: Self.Index, to: Self.Index) -> Int`](/documentation/Swift/EnumeratedSequence/distance(from:to:)-6kmqj) Returns the distance between two indices. [`func firstIndex(where: (Self.Element) throws -> Bool) rethrows -> Self.Index?`](/documentation/Swift/EnumeratedSequence/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/EnumeratedSequence/formIndex(_:offsetBy:)) Offsets the given index by the specified distance. [`func formIndex(inout Self.Index, offsetBy: Int, limitedBy: Self.Index) -> Bool`](/documentation/Swift/EnumeratedSequence/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/EnumeratedSequence/formIndex(after:)) Replaces the given index with its successor. [`func index(EnumeratedSequence.Index, offsetBy: Int) -> EnumeratedSequence.Index`](/documentation/Swift/EnumeratedSequence/index(_:offsetBy:)) Returns an index that is the specified distance from the given index. [`func index(Self.Index, offsetBy: Int) -> Self.Index`](/documentation/Swift/EnumeratedSequence/index(_:offsetBy:)-4bcg9) Returns an index that is the specified distance from the given index. [`func index(EnumeratedSequence.Index, offsetBy: Int, limitedBy: EnumeratedSequence.Index) -> EnumeratedSequence.Index?`](/documentation/Swift/EnumeratedSequence/index(_:offsetBy:limitedBy:)) Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index. [`func index(Self.Index, offsetBy: Int, limitedBy: Self.Index) -> Self.Index?`](/documentation/Swift/EnumeratedSequence/index(_:offsetBy:limitedBy:)-4rme3) Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index. [`func index(after: EnumeratedSequence.Index) -> EnumeratedSequence.Index`](/documentation/Swift/EnumeratedSequence/index(after:)) Returns the position immediately after the given index. [`func indices(where: (Self.Element) throws -> Bool) rethrows -> RangeSet`](/documentation/Swift/EnumeratedSequence/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/EnumeratedSequence/map(_:)-9le0) 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/EnumeratedSequence/prefix(through:)) Returns a subsequence from the start of the collection through the specified position. [`func prefix(upTo: Self.Index) -> Self.SubSequence`](/documentation/Swift/EnumeratedSequence/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/EnumeratedSequence/randomElement()) Returns a random element of the collection. [`func randomElement(using: inout T) -> Self.Element?`](/documentation/Swift/EnumeratedSequence/randomElement(using:)) Returns a random element of the collection, using the given generator as a source for randomness. [`func removingSubranges(RangeSet) -> DiscontiguousSlice`](/documentation/Swift/EnumeratedSequence/removingSubranges(_:)) Returns a collection of the elements in this collection that are not represented by the given range set. [`func suffix(from: Self.Index) -> Self.SubSequence`](/documentation/Swift/EnumeratedSequence/suffix(from:)) Returns a subsequence from the specified position to the end of the collection. ### Subscripts [`subscript(EnumeratedSequence.Index) -> EnumeratedSequence.Element`](/documentation/Swift/EnumeratedSequence/subscript(_:)) Accesses the element at the specified position. [`subscript(Range) -> Slice`](/documentation/Swift/EnumeratedSequence/subscript(_:)-5gng6) Accesses a contiguous subrange of the collection’s elements. [`subscript(UnboundedRange) -> Self.SubSequence`](/documentation/Swift/EnumeratedSequence/subscript(_:)-7y5my) [`subscript(R) -> Self.SubSequence`](/documentation/Swift/EnumeratedSequence/subscript(_:)-8ubal) Accesses the contiguous subrange of the collection’s elements specified by a range expression. [`subscript(RangeSet) -> DiscontiguousSlice`](/documentation/Swift/EnumeratedSequence/subscript(_:)-91gqg) Accesses a view of this collection with the elements at the given indices. ### Type Aliases [`typealias Indices`](/documentation/Swift/EnumeratedSequence/Indices) A type that represents the indices that are valid for subscripting the collection, in ascending order. [`typealias SubSequence`](/documentation/Swift/EnumeratedSequence/SubSequence) A collection representing a contiguous subrange of this collection’s elements. The subsequence shares indices with the original collection. --- Copyright © 2026 Apple Inc. 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