Find the key for a given value of a Dictionary element in Swift

Question

My code looks like this:

var dict = ["a": 1, "b": 2]

var valueInDict = 1

My question is whether it's possible to access the key, in this case "a", using only the value.

When you have a problem like this one, the first step is to check the available tools, e.g. public API, which would show you the `allKeys` property of `NSDictionary`. This isn't an optimal way to solve the problem in general, for a better result you might want to consider implementing a storage that will better suit your needs. — A-Live, Nov 15 '15 at 12:32

score 10 · Answer 1 · answered Nov 15 '15 at 12:32

10

Use the fact that Dictionary values and keys properties are documented to have the same order:

if let index = dict.values.indexOf(valueInDict) {
    let key = dict.keys[index]
    print(key)
}

answered Nov 15 '15 at 12:32

Gwendal Roué

score 8 · Accepted Answer · answered Nov 15 '15 at 12:41

It is not possible to get the key by its value, because multiple keys can have the same value. For example, if you make a dictionary like this

let dict = [
    "a" : 7
,   "b" : 3
,   "c" : 11
,   "d" : 7
,   "e" : 3
,   "f" : 11
]

and try to find the key of value 7, there would be two such keys - "a" and "d".

If you would like to find all keys that map to a specific value, you can iterate the dictionary like this:

let search = 7
let keys = dict // This is a [String:int] dictionary
    .filter { (k, v) -> Bool in v == search }
    .map { (k, v) -> String in k }

This produces keys of all entries that have the search value, or an empty array when the search value is not present in the dictionary.

Thank you! This was very helpful. – Shiv Bhatia Nov 15 '15 at 12:52 — Shiv Bhatia, Nov 15 '15 at 12:52

2 Answers2