CODEWITHSUNDEEP
go
alif
alif

Reputation: 33

Pass and modify 2d slice by reference

I'm trying to figure 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.

Upvotes: 1

Views: 103

Answers (1)

wasmup
wasmup

Reputation: 16233

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("")
    }
}

For as long as you don't modify the slice header (like when adding element), you don't need a pointer, elements accessed by their index are stored in a backing array for which the slice header holds a pointer for you:
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

Upvotes: 1

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