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How to specify default values for dictionary keys

Written by Paul Hudson    @twostraws

Reading a dictionary key returns an option value by default, because the key you asked for might not exist. However, there’s a slightly different subscript you can call that eliminates optionality: when you read a key, you can also provide a default value to use if the key doesn’t exist.

For example, here’s a dictionary representing the high scores in a game:

var scores = ["Taylor Swift": 25, "Ed Sheeran": 20]

If we wanted to read the score of Adele Adkins, we’d get back nil because she doesn’t have a score. And if we tried to read any of the values that do have keys we’d still get back an optional integer.

Fortunately, if you provide a default value while reading a key you can be guaranteed you’ll always get a value back, so the return value for our scores dictionary will always be a real integer rather than an optional.

So, here we can read Adele’s score from the dictionary, or give it a default value of 0 if the key doesn’t exist:

var adeleScore = scores["Adele Adkins", default: 0]

Available from iOS 8.0 – learn more in my book Pro Swift

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