在保持切片的同时生长切片

Question

I have been experimenting with slices. Here is an example program

for n := 1; n <= 10; n++ {
    a := make([]int, 0)
    for j := 0; j < n; j++ {
        a = append(a, 0)
    }
    b := a[:1]
    a = append(a, 0)
    a[0] = 1
    fmt.Println(n, b[0])
}

The output is

1 0
2 0
3 1
4 0
5 1
6 1
7 1
8 0
9 1
10 1

I understand what is going on here. The line a = append(a, 0) allocates a new array if the old array has insufficient length, and the new array has twice the length as the original. Therefore if n is a power of 2, the line

a[0] = 1

will not change the array backed by b as a new array will have been allocated in the line before.

However I could not find an explicit statement in the documentation that the newly allocated array always has twice the length. Does this mean that my code is implementation dependent? Is it bad practice to store a slice of another slice in a variable in this way, or should I just do a[j:k] every time a subslice is needed?

Answer 1

You already know the answer.

• It is implementation dependent
• It is bad practice to sub-slice to a var if you are changing an underlying array/slice
• You better off to use sub-slicing explicitly whenever needed - it's cheap.

Good question though. Thanks for sharing.