Swift Array optional Type and subscripting (Beta 3)

Question

I'm following the 2014 WWDC tutorial 408: Swift Playgrounds using XCode Beta 3 (30 minutes in). The Swift syntax has changed since Beta 2.

var data = [27, 46, 96, 79, 56, 85, 45, 34, 2, 57, 29, 66, 99, 65, 66, 40, 40, 58, 87, 64]

func exchange<T>(data: [T], i: Int, j: Int) {
    let temp = data[i]
    data[i] = data[j]  // Fails with error '@lvalue $T8' is not identical to 'T'
    data[j] = temp     // Fails with error '@lvalue $T5' is not identical to 'T'
}

exchange(data, 0 , 2)
data

Why I can't modify a mutable integer array in this way?

Answer 1

Because subroutine parameters are implicitly defined with let hence, non mutable. Try changing the declaration to:

func exchange<T>(inout data: [T], i: Int, j: Int) {

and the invocation to:

exchange(&date, 0, 2)

You can also use var but that would only allow the array to be modified within the subroutine. The big change for beta 3 was to make arrays really pass by value instead of just kind of sorta pass by value some of the time, but not the rest.