Why does Swift use underscores with loops?

Paul Hudson    @twostraws    October 25th 2021

Updated for Xcode 15

If you want to loop over items in an array, you might write code such as this:

let names = ["Sterling", "Cyril", "Lana", "Ray", "Pam"]

for name in names {
    print("\(name) is a secret agent")
}

Every time the loop goes around, Swift will take one item from the names array, put it into the name constant, then execute the body of our loop – that’s the print() method.

However, sometimes you don’t actually need the value that is currently being read, which is where the underscore comes in: Swift will recognize you don’t actually need the variable, and won’t make the temporary constant for you.

Of course, Swift can really see that anyway – it can see whether or not you’re using name inside the loop, so it can do the same job without the underscore. However, using an underscore does something very similar for our brain: we can look at the code and immediately see the loop variable isn’t being used, no matter how many lines of code are inside the loop body.

So, if you don’t use a loop variable inside the body, Swift will warn you to rewrite it like this:

let names = ["Sterling", "Cyril", "Lana", "Ray", "Pam"]

for _ in names {
    print("[CENSORED] is a secret agent!")
}

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