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iOS Swift 2.0 Collections and Control Flow Dictionaries Dealing With Non-Existent Data

Vera Hsu
Vera Hsu
997 Points

How to find the key based on the value in a dictionary?

We already know how to find value from key. Can we reverse it, and find the key based on value?

1 Answer

Steven Deutsch
Steven Deutsch
21,046 Points

Hey Vera Hsu,

You have to keep in mind, while keys must be unique - their values can be the same. This means that one value can return multiple keys. There's no simple method that I know of, other than using some higher order function to do the work. However, consider the code below:

// Here we have a dictionary of [String: Int]
let aDict = ["milk": 1, "eggs": 12, "apple": 1, "bacon": 3]

/* We can decompose the dictionary as a tuple of its keys and values with a for loop
We can then use a conditional statement to return the key based on the value */
for (key, value) in aDict {
    if value == 1 {

// We can also check if a key exists by checking if it is equal to nil
let keyExists = aDict["milk"] != nil
print(keyExists) // will print true

Hope this helps. Good Luck!