Changing a 2D array variable changes the original 2D array variable as well. Why?

Question

Why is matrix change as well? Go variables are passed or copied by value, not reference unless explicitly specified using pointers vs values. Is there something that I don't understand?


import "fmt"

func main() {
    matrix := [][]int{
        {100, 20, 30, 10, 11},
        {15, 100, 16, 4, 2},
        {3, 5, 100, 2, 4},
        {19, 6, 18, 100, 3},
        {16, 4, 7, 16, 100},
    }
    var matrix2 [][]int
    matrix2 = matrix
    matrix2[0][1] = 11111111
    fmt.Println(matrix)
    fmt.Println(matrix2)

    // Output :
    [[100 11111111 30 10 11] [15 100 16 4 2] [3 5 100 2 4] [19 6 18 100 3] [16 4 7 16 100]]
    [[100 11111111 30 10 11] [15 100 16 4 2] [3 5 100 2 4] [19 6 18 100 3] [16 4 7 16 100]]
}

Does this answer your question? [updating one position in a slice of slices results in multiple updates](https://stackoverflow.com/questions/69005086/updating-one-position-in-a-slice-of-slices-results-in-multiple-updates) — , Sep 05 '21 at 22:46
https://stackoverflow.com/questions/39993688/are-slices-passed-by-value/39993797#39993797 — , Sep 05 '21 at 23:05
to check for the underlying array of a slice https://stackoverflow.com/questions/36706843/how-to-get-the-underlying-array-of-a-slice-in-go/36707413#36707413 — , Sep 05 '21 at 23:06

score -1 · Accepted Answer · answered Sep 05 '21 at 23:01

-1

Go slices are references to an underlying array. A slice is like a pointer to a value, not like a direct value. If you use arrays instead of slices, you will see different behavior:

package main
import "fmt"

func main() {
    matrix := [5][5]int{
        {100, 20, 30, 10, 11},
        {15, 100, 16, 4, 2},
        {3, 5, 100, 2, 4},
        {19, 6, 18, 100, 3},
        {16, 4, 7, 16, 100},
    }
    var matrix2 [5][5]int
    matrix2 = matrix
    matrix2[0][1] = 11111111
    fmt.Println(matrix)
    fmt.Println(matrix2)
}

Output:

[[100 20 30 10 11] [15 100 16 4 2] [3 5 100 2 4] [19 6 18 100 3] [16 4 7 16 100]]
[[100 11111111 30 10 11] [15 100 16 4 2] [3 5 100 2 4] [19 6 18 100 3] [16 4 7 16 100]]

You can see that because I used an array [5][5]int of arrays [5]int instead of a slice [][]int of slices []int, the assignment matrix2 = matrix made a copy of the entire matrix.

If you want to use a variable-size matrix, you will have to write your own function to create matrix copies. The above code only works if you know the size of your matrix at compile-time (e.g. 5x5).

answered Sep 05 '21 at 23:01

Dietrich Epp

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the wording is not right. There is no such thing like references in the language. quoting https://go.dev/blog/slices-intro `A slice is a descriptor of an array segment. It consists of a pointer to the array, the length of the segment, and its capacity` and `arrays are values` – Sep 05 '21 at 23:14
Thank you so much. This answers my question. Is slice the only type that is like a pointer to a value or is there other types that acts like this? – Leo Sep 05 '21 at 23:18
@mh-cbon: A pointer is a reference to a location in memory. – Dietrich Epp Sep 05 '21 at 23:19
@mh-cbon: Carefully distinguishing between "pointer" and "reference" is useful in languages like C++, where the terms refer to distinct types. In most other languages, like Java, Python, or Go, it is not so necessary to distinguish "pointer" and "reference". – Dietrich Epp Sep 05 '21 at 23:22
1

yes, but this specific word is ambiguous. This is not justified, the logic of slice internals perfectly fits the language and using its vocabulary to explain their behavior is more adequate. – Sep 05 '21 at 23:22
@mh-cbon: That is incorrect. There is no ambiguity here, because "reference" doesn't have some distinct meaning in Go. I used "reference" because it is clearer in this case. – Dietrich Epp Sep 05 '21 at 23:23
I appreciate any attempts to help make the answer clearer, but this line of discussion doesn't help make the answer clearer. – Dietrich Epp Sep 05 '21 at 23:24
Yes, i understand. For the careful reader this should be clear by now. – Sep 05 '21 at 23:25

Changing a 2D array variable changes the original 2D array variable as well. Why?

1 Answers1