Argument Passing Behavior

When arguments are passed to a function, they are copied. Therefore, values that are passed into a function are unchanged in the caller's scope when the function returns. This behavior is known as call-by-value.

// Declare a function that changes the first two elements
// of an array of integers.
//
fun change(_ numbers: [Int]) {
    // Change the elements of the passed in array.
    // The changes are only local, as the array was copied.
    //
    numbers[0] = 1
    numbers[1] = 2
    // `numbers` is `[1, 2]`
}

let numbers = [0, 1]

change(numbers)
// `numbers` is still `[0, 1]`

Parameters are constant, i.e., it is not allowed to assign to them.

fun test(x: Int) {
    // Invalid: cannot assign to a parameter (constant)
    //
    x = 2
}