Golang slice append exception

Issue

This is a leet code problem, and when I try to answer with the code blew:

package main

import "fmt"

func main() {
    nums := []int{9, 0, 3, 5, 7}
    fmt.Println(subsets(nums))
}

func subsets(nums []int) [][]int {
    var result [][]int
    result = append(result, []int{})
    for _, v := range(nums) {
        for _, rv := range(result) {
            result = append(result, append(rv, v))
        }
    }
    return result
}

Leetcode told me wrong answer:

Wrong Answer
Details 
Input
[9,0,3,5,7]
Output
[[],[9],[0],[9,0],[3],[9,3],[0,3],[9,0,3],[5],[9,5],[0,5],[9,0,5],[3,5],[9,3,5],[0,3,5],[9,0,3,7],[7],[9,7],[0,7],[9,0,7],[3,7],[9,3,7],[0,3,7],[9,0,3,7],[5,7],[9,5,7],[0,5,7],[9,0,5,7],[3,5,7],[9,3,5,7],[0,3,5,7],[9,0,3,7,7]]
Expected
[[],[9],[0],[0,9],[3],[3,9],[0,3],[0,3,9],[5],[5,9],[0,5],[0,5,9],[3,5],[3,5,9],[0,3,5],[0,3,5,9],[7],[7,9],[0,7],[0,7,9],[3,7],[3,7,9],[0,3,7],[0,3,7,9],[5,7],[5,7,9],[0,5,7],[0,5,7,9],[3,5,7],[3,5,7,9],[0,3,5,7],[0,3,5,7,9]]

The output slice index 15, it should be [9,0,3,5] like the expected, but the result is [9,0,3,7].
So I try to run this code by go playgroud online,the answer is the same wrong, and then I run this code in goland with debug mode,I find when I make the slice append([9,0,3], 7), the output slice index 15 change at the same time.

My local go env: go version go1.17.6 windows/amd64

I’m just a beginner to golang, could anyone explain this situation?
Thank you very much.

ps: I try to use blew code to recover same issue, but I failed.

package main

import "fmt"

func main() {
    a := [][]int{{}, {9}, {0}, {9, 0}, {3}, {9, 3}, {0, 3}, {9, 0, 3}, {5}, {9, 5}, {0, 5}, {9, 0, 5}, {3, 5}, {9, 3, 5}, {0, 3, 5}, {9, 0, 3, 5}}
    i := 7
    for _, v := range a {
        // fmt.Println(a)
        a = append(a, append(v, i))
        // fmt.Println(a)
    }
    fmt.Println(a)
}

result:
[[] [9] [0] [9 0] [3] [9 3] [0 3] [9 0 3] [5] [9 5] [0 5] [9 0 5] [3 5] [9 3 5] [0 3 5] [9 0 3 5] [7] [9 7] [0 7] [9 0 7] [3 7] [9 3 7] [0 3 7] [9 0 3 7] [5 7] [9 5 7] [0 5 7] [9 0 5 7] [3 5 7] [9 3 5 7] [0 3 5 7] [9 0 3 5 7]]

Solution

You are reusing the same backing array in some of your slices, because that’s what append does if there’s capacity remaining. A simple fix is to replace append(rv, v) with append(append([]int{}, rv...), v), creating an entirely new slice. An alternative is to force the append to allocate a fresh backing array by capping the slice to its current length: append(rv[:len(rv):len(rv)], v).

Playground link with working code: https://go.dev/play/p/Gc-yF5KQOAO

Answered By – Paul Hankin

Answer Checked By – Robin (GoLangFix Admin)

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