Why does Swift have default values for dictionaries?

Paul Hudson    @twostraws    May 21st 2020

Whenever you read a value from a dictionary, you might get a value back or you might get back nil – there might be no value for that key. Having no value can cause problems in your code, not least because you need to add extra functionality to handle missing values safely, and that’s where dictionary default values come in: they let you provide a backup value to use for when the key you ask for doesn’t exist.

For example, here’s a dictionary that stores the exam results for a student:

let results = [
    "english": 100,
    "french": 85,
    "geography": 75
]

As you can see, they sat three exams and scored 100%, 85%, and 75% for English, French, and Geography. If we wanted to read their history score, how we do it depends on what we want:

1. If a missing value means the student failed to take the test, then we could use a default value of 0 so that we always get an integer back.
2. If a missing value means the student has yet to take the exam, then we should skip the default value and instead look for a nil value.

So, it’s not like you always need a default value when working with dictionaries, but when you do it’s easy:

let historyResult = results["history", default: 0]

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