C multidimensional array not accepting modifications to its members

Question

I am very new to C. I was trying to write a very basic matrix program for practice.

The way the matrix works is that it is created with a given number of rows and columns, and then it callocs a single one dimensional array with enough slots (rows * cols slots... You get the idea). Then to access a slot, you call a matrix_getcell on the matrix with the cell and it returns a pointer to the cell.

Here is matrix.h:

#ifndef MATRIX_H
#define MATRIX_H

#include <stdlib.h>
#include <stdio.h>

typedef unsigned int uint;

typedef struct matrix matrix;
struct matrix {
    uint rows;
    uint cols;
    double *data;
};

matrix *matrix_new(uint rows, uint cols) {
    matrix *n = malloc(sizeof(matrix));
    if (n == NULL) exit(1);

    n->data = calloc(rows * cols, sizeof(*(n->data)));
    if (n->data == NULL) exit(1);

    n->rows = rows;
    n->cols = cols;

    return n;
}

void matrix_del(matrix *m) {
    if (m == NULL) return;

    free(m->data);
    free(m);
}

double *matrix_getcell(matrix *m, uint row, uint col) {
    if (row >= m->rows) {
        fprintf(stderr, "Invalid row: %d\n", row);
        exit(1);
    }

    if (col >= m->cols) {
        fprintf(stderr, "Invalid col: %d\n", col);
        exit(1);
    }

    uint pos = (m->rows * row) + col;

    return &(m->data[pos]);
}

#endif

and here is main.c:

#include <stdio.h>

#include "matrix.h"

int main(int argc, char **argv) {
    matrix *m = matrix_new(3, 3);

            /* I know that a 3x3 will have 9 cells, so
             * fill them up with successive numbers
             */
    for (int i = 0; i < 9; i++) {
        m->data[i] = i;
    }

            /* Now, run through each cell, row by column
             * and print out the coords and the contents.
             */
    for (uint r = 0; r < 3; r++) {
        for (uint c = 0; c < 3; c++) {
            double *cur = matrix_getcell(m, r, c);
            printf("(%d, %d): %.3d\n", r, c, *cur);
        }
    }

    matrix_del(m);

    return 0;
}

What I tried to do with this was to initialize EACH separate cell to a successive number, such that when I for-looped over it the second time it would hopefully have outputedd:

(0, 0): 0
(0, 1): 1
(0, 2): 2
(1, 0): 3
(1, 1): 4
(1, 2): 5
(2, 0): 6
(2, 1): 7
(2, 2): 8

But instead, it outputs

(0, 0): 0
(0, 1): 0
(0, 2): 0
(1, 0): 1
(1, 1): 1
(1, 2): 1
(2, 0): 2
(2, 1): 2
(2, 2): 2

I have added (and then removed) code to test whether matric_getcell was returning incorrect results (it doesn't seem to be). I have changed the datatype, I have tried casting... I don't know what else to try.

Why does it seem to be setting each column to the same number?

Answer 1

You matrix_getcell method has a bug inside when you calculate the position of the cell.

double *matrix_getcell(matrix *m, uint row, uint col) {
    ...
    // Should be (m_cols * row) + col.
    uint pos = (m->rows * row) + col;

    return &(m->data[pos]);
}

Another bug is there when you are printing the doubles. You should be using %f instead of %d to print doubles.

//                   v--- "%d" is the problem here
printf("(%d, %d): %.3d\n", r, c, *cur);