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How to make a variadic function

Paul Hudson       @twostraws

Variadic functions are functions that accept any number of parameters. The most common one in Swift is print() – most people use it to print a single value, but you can actually pass as many as you want, like this:

print(1, 2, 3, 4, 5)

To make a variadic function of your own, just add ... after any parameter. For example, we could write a sum() function that accepts any number of integers and adds them together, like this:

func sum(_ numbers: Int...) -> Int {
    var total = 0

    for number in numbers {
        total += number
    }

    return total
}

Or if you wanted to write that functionally, you would use reduce():

func sum(_ numbers: Int...) -> Int {
    return numbers.reduce(0, +)
}

Notice how we specify Int... rather than Int – that means this function can be called using no integers, one integer, or even a hundred integers, and Swift will automatically convert them to be an array of integers inside the function.

So, it would be called like this:

let total = sum(1, 2, 3, 4, 5)

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

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