go - append to a new slice affect original slice

I have the following code which causes a weird result. I cannot understand why:

func main() {
    var s = []int{2, 3}
    var s1 = append(s, 4)
    var a = append(s1, 5)
    var b = append(s1, 6)
    fmt.Println(s)
    fmt.Println(s1)
    fmt.Println(a)

    fmt.Println(b)
}

This then results in:

[2 3]
[2 3 4]
[2 3 4 6]
[2 3 4 6]

My question is: why a is not [2 3 4 5] but [2 3 4 6]? I know append to b changes a, but how. Is this a bug because I never changed a directly and obviously I don't want this happen?

Answer

Keep in mind that a slice is a structure of 3 fields.

• a pointer to the underlying array


• length of the slice


• capacity of the slice

append() function may either modify its argument in-place or return a copy of its argument with an additional entry, depending on the size and capacity of its input. append() function creates a new slice, if the length the slice is greater than the length of the array pointed by the slice.