---
title: "merging(_:uniquingKeysWith:)"
framework: swift
role: symbol
role_heading: Instance Method
path: "swift/dictionary/merging(_:uniquingkeyswith:)-4vrqz"
---

# merging(_:uniquingKeysWith:)

Creates a dictionary by merging the given dictionary into this dictionary, using a combining closure to determine the value for duplicate keys.

## Declaration

```swift
func merging<E>(_ other: [Key : Value], uniquingKeysWith combine: (Value, Value) throws(E) -> Value) throws(E) -> [Key : Value] where E : Error
```

## Parameters

- `other`: A dictionary to merge.
- `combine`: A closure that takes the current and new values for any duplicate keys. The closure returns the desired value for the final dictionary.

## Return Value

Return Value A new dictionary with the combined keys and values of this dictionary and other.

## Discussion

Discussion Use the combine closure to select a value to use in the returned dictionary, or to combine existing and new values. As the key-value pairs in other are merged with this dictionary, the combine closure is called with the current and new values for any duplicate keys that are encountered. This example shows how to choose the current or new values for any duplicate keys: let dictionary = ["a": 1, "b": 2] let otherDictionary = ["a": 3, "b": 4]

let keepingCurrent = dictionary.merging(otherDictionary)       { (current, _) in current } // ["b": 2, "a": 1] let replacingCurrent = dictionary.merging(otherDictionary)       { (_, new) in new } // ["b": 4, "a": 3]