删除切片中的元素时分配给新变量会产生意外结果

Question

I am seeing some unintended behaviour when trying to delete an element within a slice. Below is my code:

package main

import "fmt"

func main() {
    x := []int{1,2,3,4,5,6,7,8}
    y := append(x[:3],x[4:]...)
    fmt.Println(x)
    fmt.Println(y)
}

playground

the output is:

[1 2 3 5 6 7 8 8]
[1 2 3 5 6 7 8]

I would expect the output to be:

[1 2 3 4 5 6 7 8]
[1 2 3 5 6 7 8]

Why is the result not what I expected?

In other words since there is no assignment to change the value x I would expect it to have the same initialized value but for some reason it doesn't and has the same value as y with the last element duplicated. Is this a bug?

Answer 1

The append function operates in-place when enough space is available. Slice away the capacity to avoid this behaviour:

y := append(x[:3:3],x[4:]...)