更改数组元素无效

Question

There is something wrong with the compiler or my understanding of Go. I have a weird bug and have whittled it down to a simple example. For some reason I can't change the array directly within a loop.

    var nums [2]int
    for i, v := range nums {
        if i == 0 {
            nums[i+1]++
        } else {
            fmt.Print(v)
        }
    }

This increments nums[1] then prints it. So it should print 1 but it prints 0.

Answer 1

When you use an array in an expression you get its "value" (ie, a complete new copy of the array). This includes using an array in the range expression of a loop. So when you print v you are printing the copy, but the actual nums array has been changed.

Looping on a slice instead of the array gives you what you expect:

    var nums [2]int
    for i, v := range nums[:] {
        if i == 0 {
            nums[i+1]++
        } else {
            fmt.Print(v)
        }
    }