Matrix contamination in C

Question

I am new to C and I don't understand why I am not able to easily populate a 2d array.

Here is my code: one function populates the matrix and the other displays it.

#include <stdio.h>

int matrix[5][5];
int size = 5;

void fill_matrix() {
    for(int k=0; k<size; k++) {
        for(int l=0; l<size; l++) {
            matrix[k][l] = k + l;
        }
    }
}

void print_matrix() {
    for(int k=0; k<size; k++) {
        for(int l=0; l<size; l++) {
            printf("%d ", *matrix[k,l]);
        }
        printf("\n");
    }
}

int main() {
    fill_matrix();
    print_matrix();
}

And the result is this

Instead of this

I know that the answer is probably extremely simple, but I was't able to find a solution after few hours so here I am.

`*matrix[k,l]` -> `matrix[k][l]`. Bonus: [Comma operator](https://en.wikipedia.org/wiki/Comma_operator) — Eugene Sh., Sep 24 '19 at 21:07
@EugeneSh. Don't forget the star at the beginning of the "before" to make it clear it doesn't belong in the "after" — Christian Gibbons, Sep 24 '19 at 21:07
Note that `*matrix[k,l]` is the same as `matrix[l][0]`, which is why each output row you printed was `0 1 2 3 4` - the value of the 0th elements of the individual rows. (OK, not quite, but the explanation is a bit much for a comment...) — Andrew Henle, Sep 24 '19 at 21:13
@AndrewHenle Maybe this question deserves an answer after all... — Eugene Sh., Sep 24 '19 at 21:14
@EugeneSh. Go ahead. I'm not sure I could explain it in a way that's easily understood. — Andrew Henle, Sep 24 '19 at 21:15
Well, this is related: https://stackoverflow.com/questions/1083488/dereferencing-pointer-to-integer-array — Andrew Henle, Sep 24 '19 at 21:17

score 0 · Accepted Answer · answered Sep 24 '19 at 21:27

What you meant to do was matrix[k][l] to address the row and column of the matrix.

Instead, what you have, *matrix[k,l]. What is happening here is the comma operator is being used in the expression to index the array. The result of the expression is equal to that of the right-side of the comma operator, so effectively what you have is *matrix[l]. matrix[l], using only one index for a two dimensional array, points you to the lth row of the array, so it is essentially of type int[5]. Then when you dereference it with the asterisk, you are grabbing the contents of the first element of that row.

Indeed, I was confused by the fact that no warning was coming from the compiler about the comma syntax (that is valid in some languages). I thought it was some kind of Pythonic copy error and that the first row was replicated throughout the whole matrix. Thanks — Kipr, Sep 25 '19 at 08:42

Matrix contamination in C

1 Answers1