通过参考传递和修改2D切片

Question

I'm having difficulty figuring out how to change a multidimensional slice by reference.

func main() {

    matrix := [][]int{
        {1, 0, 0},
        {1, 0, 0},
        {0, 1, 1},
    }
    fmt.Println("Before")
    printMatrix(matrix)
    changeMatrixByReference(&matrix)

    fmt.Println("After")
    printMatrix(matrix)
}

func changeMatrixByReference(matrix *[][]int) {
    //&matrix[0][0] = 3
}

func printMatrix(matrix [][]int) {
    for i := 0; i < len(matrix); i++ {
        for j := 0; j < len(matrix[0]); j++ {
            fmt.Printf("%d", matrix[i][j])
        }
        fmt.Println("")
    }
}

How can I change the matrix 2d slice inside the function changeMatrixByReference? I expect when printMatrix runs the second time matrix[0][0] becomes 3.

Answer 1

To set matrix[0][0] to 3, using pointer dereferencing:

(*matrix)[0][0] = 3

Try this:

package main

import "fmt"

func main() {

    matrix := [][]int{
        {1, 0, 0},
        {1, 0, 0},
        {0, 1, 1},
    }
    fmt.Println("Before")
    printMatrix(matrix)
    changeMatrixByReference(&matrix)

    fmt.Println("After")
    printMatrix(matrix)
}

func changeMatrixByReference(matrix *[][]int) {
    (*matrix)[0][0] = 3
}

func printMatrix(matrix [][]int) {
    for i := 0; i < len(matrix); i++ {
        for j := 0; j < len(matrix[0]); j++ {
            fmt.Printf("%d", matrix[i][j])
        }
        fmt.Println("")
    }
}

---

You don't need a pointer to change an element of an underlying array of a slice:
Try this:

package main

import "fmt"

func main() {

    matrix := [][]int{
        {1, 0, 0},
        {1, 0, 0},
        {0, 1, 1},
    }
    fmt.Println("Before")
    printMatrix(matrix)
    changeMatrixByReference(matrix)

    fmt.Println("After")
    printMatrix(matrix)
}

func changeMatrixByReference(matrix [][]int) {
    matrix[0][0] = 3
}

func printMatrix(matrix [][]int) {
    for i := 0; i < len(matrix); i++ {
        for j := 0; j < len(matrix[0]); j++ {
            fmt.Printf("%d", matrix[i][j])
        }
        fmt.Println("")
    }
}

Output:

Before
100
100
011
After
300
100
011