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How to break out of multiple loop levels using labeled statements

Swift version: 5.4

Paul Hudson    @twostraws   

Swift has a built-in break keyword that escapes the current loop you’re in, but what happens if you’re in two loops or more and want to break out of them all?

Swift’s labeled statements are designed to solve this problem: they let you exit any number of loops or conditions, so execution picks up directly after the block you labeled.

For example, consider this pair of loops that will find the first number that, when squared, makes 144:

let numbers = 1...100

for number1 in numbers {
    for number2 in numbers {
        if number1 == number2 && number1 * number2 == 144 {
            print("Square found: \(number1)")
        }
    }
}

As soon as we’ve found that square, we can stop looking. The problem is, a regular break won’t work here because it will exit only the inner loop – the outer loop will keep counting 13, 14, 15, and so on up to 100. However, if we add a label to the outer loop we can break out of both loops at once, like this:

outerLoop: for number1 in numbers {
    for number2 in numbers {
        if number1 == number2 && number1 * number2 == 144 {
            print("Square found: \(number1)")
            break outerLoop
        }
    }
}

Notice the outerLoop: before the for number1 loop, and also the matching break outerLoop – that will cause both loops to exit as soon as the correct number is found.

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