# Collection Implementations ## Topics ### Instance Properties [`var first: Self.Element?`](/documentation/Swift/UInt/Words-swift.struct/first) The first element of the collection. [`var isEmpty: Bool`](/documentation/Swift/UInt/Words-swift.struct/isEmpty) A Boolean value indicating whether the collection is empty. [`var underestimatedCount: Int`](/documentation/Swift/UInt/Words-swift.struct/underestimatedCount) A value less than or equal to the number of elements in the collection. ### Instance Methods [`func drop(while: (Self.Element) throws -> Bool) rethrows -> Self.SubSequence`](/documentation/Swift/UInt/Words-swift.struct/drop(while:)) Returns a subsequence by skipping elements while `predicate` returns `true` and returning the remaining elements. [`func dropFirst(Int) -> Self.SubSequence`](/documentation/Swift/UInt/Words-swift.struct/dropFirst(_:)) Returns a subsequence containing all but the given number of initial elements. [`func firstIndex(of: Self.Element) -> Self.Index?`](/documentation/Swift/UInt/Words-swift.struct/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/UInt/Words-swift.struct/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/UInt/Words-swift.struct/formIndex(_:offsetBy:)) Offsets the given index by the specified distance. [`func formIndex(inout Self.Index, offsetBy: Int, limitedBy: Self.Index) -> Bool`](/documentation/Swift/UInt/Words-swift.struct/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/UInt/Words-swift.struct/formIndex(after:)) Replaces the given index with its successor. [`func index(of: Self.Element) -> Self.Index?`](/documentation/Swift/UInt/Words-swift.struct/index(of:)) Returns the first index where the specified value appears in the collection. [`func indices(of: Self.Element) -> RangeSet`](/documentation/Swift/UInt/Words-swift.struct/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/UInt/Words-swift.struct/indices(where:)) Returns the indices of all the elements that match the given predicate. [`func makeIterator() -> IndexingIterator`](/documentation/Swift/UInt/Words-swift.struct/makeIterator()) Returns an iterator over the elements of the collection. [`func map((Self.Element) throws(E) -> T) throws(E) -> [T]`](/documentation/Swift/UInt/Words-swift.struct/map(_:)-2w7k9) Returns an array containing the results of mapping the given closure over the sequence’s elements. [`func prefix(Int) -> Self.SubSequence`](/documentation/Swift/UInt/Words-swift.struct/prefix(_:)) Returns a subsequence, up to the specified maximum length, containing the initial elements of the collection. [`func prefix(through: Self.Index) -> Self.SubSequence`](/documentation/Swift/UInt/Words-swift.struct/prefix(through:)) Returns a subsequence from the start of the collection through the specified position. [`func prefix(upTo: Self.Index) -> Self.SubSequence`](/documentation/Swift/UInt/Words-swift.struct/prefix(upTo:)) Returns a subsequence from the start of the collection up to, but not including, the specified position. [`func prefix(while: (Self.Element) throws -> Bool) rethrows -> Self.SubSequence`](/documentation/Swift/UInt/Words-swift.struct/prefix(while:)) Returns a subsequence containing the initial elements until `predicate` returns `false` and skipping the remaining elements. [`func randomElement() -> Self.Element?`](/documentation/Swift/UInt/Words-swift.struct/randomElement()) Returns a random element of the collection. [`func randomElement(using: inout T) -> Self.Element?`](/documentation/Swift/UInt/Words-swift.struct/randomElement(using:)) Returns a random element of the collection, using the given generator as a source for randomness. [`func removingSubranges(RangeSet) -> DiscontiguousSlice`](/documentation/Swift/UInt/Words-swift.struct/removingSubranges(_:)) Returns a collection of the elements in this collection that are not represented by the given range set. [`func split(maxSplits: Int, omittingEmptySubsequences: Bool, whereSeparator: (Self.Element) throws -> Bool) rethrows -> [Self.SubSequence]`](/documentation/Swift/UInt/Words-swift.struct/split(maxSplits:omittingEmptySubsequences:whereSeparator:)) Returns the longest possible subsequences of the collection, in order, that don’t contain elements satisfying the given predicate. [`func split(separator: Self.Element, maxSplits: Int, omittingEmptySubsequences: Bool) -> [Self.SubSequence]`](/documentation/Swift/UInt/Words-swift.struct/split(separator:maxSplits:omittingEmptySubsequences:)) 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/UInt/Words-swift.struct/suffix(from:)) Returns a subsequence from the specified position to the end of the collection. --- Copyright © 2026 Apple Inc. 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