Golang切片参考混乱

Question

package main

import (
    "fmt"
)

func main() {
    values := make([]int, 0, 100)
    val := make([][]int, 2)
    for i:=0; i<2; i++ {
        values = values[:0]
        for j:=0; j<2; j++ {
            values = append(values, i+j)
        }
        val[i] = values
        fmt.Println(values, val) //
    }
    fmt.Println(val)
}

---

https://play.golang.org/p/5x60VfDXbFw

when appending slice，val is expected to be [[0, 1], [1, 2]], but got [[1,2], [1,2]]

Answer 1

This happens because the slice val contains pointers to its subslices, rather than the subslices themselves. In your code, you initially place a pointer to values in positions val[0]. Then you modify values and then set a pointer to values in val[1]. But both val[0] and val[1] point to the same underlying object (values), which has been modified.

You can fix this by creating a new values slice on each iteration of the outer loop, this way each sub slice of val will be a different slice.

For example:

func main() {
    val := make([][]int, 2)
    for i:=0; i<2; i++ {
        values := make([]int, 0, 100)
        for j:=0; j<2; j++ {
            values = append(values, i+j)
        }
        val[i] = values
        fmt.Println(values, val) //
    }
    fmt.Println(val)
}

Output from fmt.Println:

[0 1] [[0 1] []]    // values, val
[1 2] [[0 1] [1 2]] // values, val
[[0 1] [1 2]]       // val