Change a dictionary's key in Swift

Question

How can I change a dictionary's key for a particular value? I can't just change dict[i] to dict[i+1] because that changes the value for that particular key. And there's no dict.updateKeyForValue() like there is a dict.updateValueForKey().

Because my keys are Int's and all out of order, I can't modify the entire key-value pair by looping through because I may override a pair that the loop hasn't reached yet. Is there a simpler way? Feel like I'm missing something obvious.

Answer 1

Swift 3

func switchKey<T, U>(_ myDict: inout [T:U], fromKey: T, toKey: T) {
    if let entry = myDict.removeValue(forKey: fromKey) {
        myDict[toKey] = entry
    }
}  

var dict = [Int:String]()

dict[1] = "World"
dict[2] = "Hello"

switchKey(&dict, fromKey: 1, toKey: 3)
print(dict) /* 2: "Hello"
               3: "World" */

Swift 2

func switchKey<T, U>(inout myDict: [T:U], fromKey: T, toKey: T) {
    if let entry = myDict.removeValueForKey(fromKey) {
        myDict[toKey] = entry
    }
}    

var dict = [Int:String]()

dict[1] = "World"
dict[2] = "Hello"

switchKey(&dict, fromKey: 1, toKey: 3)
print(dict) /* 2: "Hello"
               3: "World" */

Answer 2

I'm personally using an extension which imo makes it easier :D

extension Dictionary {
    mutating func switchKey(fromKey: Key, toKey: Key) {
        if let entry = removeValue(forKey: fromKey) {
            self[toKey] = entry
        }
    }
}

Then to use it:

var dict = [Int:String]()

dict[1] = "World"
dict[2] = "Hello"

dict.switchKey(fromKey: 1, toKey: 3)
print(dict) /* 2: "Hello"
               3: "World" */