# init(_:uniquingKeysWith:) Creates a new dictionary from the key-value pairs in the given sequence, using a combining closure to determine the value for any duplicate keys. ``` init(_ keysAndValues: S, uniquingKeysWith combine: (Value, Value) throws(E) -> Value) throws(E) where S : Sequence, E : Error, S.Element == (Key, Value) ``` ## Parameters `keysAndValues` A sequence of key-value pairs to use for the new dictionary. `combine` A closure that is called with the values for any duplicate keys that are encountered. The closure returns the desired value for the final dictionary. ## Discussion You use this initializer to create a dictionary when you have a sequence of key-value tuples that might have duplicate keys. As the dictionary is built, the initializer calls the `combine` closure with the current and new values for any duplicate keys. Pass a closure as `combine` that returns the value to use in the resulting dictionary: The closure can choose between the two values, combine them to produce a new value, or even throw an error. The following example shows how to choose the first and last values for any duplicate keys: ``` let pairsWithDuplicateKeys = [("a", 1), ("b", 2), ("a", 3), ("b", 4)] let firstValues = Dictionary(pairsWithDuplicateKeys, uniquingKeysWith: { (first, _) in first }) // ["b": 2, "a": 1] let lastValues = Dictionary(pairsWithDuplicateKeys, uniquingKeysWith: { (_, last) in last }) // ["b": 4, "a": 3] ``` --- 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)