[FIXED] Why append slice in Go change the original slice?

Issue

I have this code:

// The input param is A := []int{3, 4, 5, 3, 7}
func someFunc(A []int) int {   
...
    ways := 0       
    i := 0
    for i < len(A) {
        if i+1 == len(A) || i == len(A) {
            fmt.Println("break")
            break
        }
        tempA := A // copy the slice by value

        fmt.Println("A: ", A)
        fmt.Println("tempA: ", A)
        fmt.Println()

        newArr = remove(tempA, i)

        if isAesthetic(newArr) {
            ways++
        }
        i++
    }
...
}

func remove(slice []int, s int) []int {
    return append(slice[:s], slice[s+1:]...)
}

Cosole output:

A:  [3 4 5 3 7]
tempA:  [3 4 5 3 7]

A:  [4 5 3 7 7]
tempA:  [4 5 3 7 7]

A:  [4 3 7 7 7]
tempA:  [4 3 7 7 7]

A:  [4 3 7 7 7]
tempA:  [4 3 7 7 7]

The variable A also changes while I just copy it by value into tempA. And also it is append() function, why the slice that used to append tempA also changed?

Solution

You need to utilize the copy function:

package main
import "fmt"

func main() {
   a := []int{3, 4, 5, 3, 7}
   // bad
   b := a
   // good
   c := make([]int, len(a))
   copy(c, a)
   // CHANGES b and a!
   b = append(b[:1], b[2:]...)
   // c stays the same, a is messed up
   // [3 5 3 7 7] [3 5 3 7] [3 4 5 3 7]
   fmt.Println(a, b, c)
}

https://golang.org/pkg/builtin#copy

Answered By - Zombo

Answer Checked By - Timothy Miller (PHPFixing Admin)