# formUnion(_:) Inserts the elements of the given sequence into the set. ``` mutating func formUnion(_ other: S) where Element == S.Element, S : Sequence ``` ## Parameters `other` A sequence of elements. `other` must be finite. ## Discussion If the set already contains one or more elements that are also in `other`, the existing members are kept. If `other` contains multiple instances of equivalent elements, only the first instance is kept. ``` var attendees: Set = ["Alicia", "Bethany", "Diana"] let visitors = ["Diana", "Marcia", "Nathaniel"] attendees.formUnion(visitors) print(attendees) // Prints "["Diana", "Nathaniel", "Bethany", "Alicia", "Marcia"]" ``` --- 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)