如何在循环中将切片用作堆栈

Question

I am using a slice as a stack.

When I do this

stack := make([]int, 0)
stack = append(stack, 1, 2, 3, 4, 5)
for len(stack) != 0 {
    var a int
    a, stack = stack[len(stack) - 1], stack[:len(stack) - 1]
    fmt.Print(a)
}

I get the output

54321

as expected.

When I do this

stack := make([]int, 0)
stack = append(stack, 1, 2, 3, 4, 5)
a, stack := stack[len(stack) - 1], stack[:len(stack) - 1]
fmt.Println(a, len(stack))

I get the output

5 4

as expected. However, when I try to combine these by doing this

stack := make([]int, 0)
stack = append(stack, 1, 2, 3, 4, 5)
for len(stack) != 0 {
    a, stack := stack[len(stack) - 1], stack[:len(stack) - 1]
    fmt.Print(a)
}

the result is a compile error

unused variable 'stack'

This confuses me. I thought I could use := as long as at least one of the variables is being declared (in this case a), and that stack would just get reassigned. However I am clearly wrong. Can anybody explain why the line

a, stack := stack[len(stack) - 1], stack[:len(stack) - 1]

doesn't work in the loop, and how I can do "pop" as a one-liner in a loop?

Answer 1

:= only assigns a new value to an existing variable if the variable was declared earlier in the same block and has the correct type. Otherwise, it creates a new variable. Since your loop is a new block, with a new scope, you get a new stack, which is then never used for anything. There's no way to "halfway" do a :=, so you simply can't have a one-liner here. You'll have to

var a int
a, stack = stack[len(stack)-1], stack[:len(stack)-1]

or else create a type and a method (with the stack as a pointer receiver) so that you can

a := stack.Pop()