When the slice element is removed, the original slice is broken - go

Question

package main
import (
"fmt"
)
func main() {
// test data
var a = [][]int{
[]int{1,2},
[]int{3,4},
[]int{5,6},
[]int{7,8},
}
for i := range a {
fmt.Println(a) // print out
_ = append(a[:i], a[i+1:]...) // remove element
}
}
expected result
[[1 2] [3 4] [5 6] [7 8]]
[[1 2] [3 4] [5 6] [7 8]]
[[1 2] [3 4] [5 6] [7 8]]
[[1 2] [3 4] [5 6] [7 8]]
actual result
[[1 2] [3 4] [5 6] [7 8]]
[[3 4] [5 6] [7 8] [7 8]]
[[3 4] [7 8] [7 8] [7 8]]
[[3 4] [7 8] [7 8] [7 8]]
https://play.golang.org/p/KahcX1MmDHO
why does this result? the original slice is broken. :(

Answer 1

When you re-slice you don't change the underlying array. So the first time through your loop (append(a[:0], a[1:]...)) you create a[:0] which is a slice of len 0 (cap 4) pointing to the element 0 and a[1:] is a slice of len 3 (cap 3) pointing to element 1.
As #icza pointed out using append() in this way just moved the underlying array elements around. For each loop iteration this happens:
0: elements 1-3 are moved to places 0-2
1: elements 2-3 are moved to places 1-2
2: element 3 is moved to place 2
3: nothing is moved since a[4:] has zero length
This gives the changed values that you see.
BTW Please post the simplest code which shows the problem. The nested slice is irrelevant.
func main() {
var a = []int{1,2,3,4}
for i := range a {
fmt.Println(a)
_ = append(a[:i], a[i+1:]...)
}
}