Question

I have, essentially, a matrix of data (lets say ints) that I would like to store in a 2D array in which the dimensions are not known until runtime (say x columns and y rows). I want to populate the array in a function, so I assume I need to do something like this:

int main(int argc, char **argv) {
    int y = atoi(argv[1]);
    int x = atoi(argv[2]);
    int **matrix = malloc(x * sizeof(int*));
    populateMatrix(matrix, y, x);
    return 0;
}

void populateMatrix(**matrix, int y, int x) {
    int i, j;
    for (i = 0; i < y; i++) {
        for (j = 0; j < x; j++) {
            matrix[i][j] = i * j; // populated with trivial data to keep it simple
        }
    }
}

Obviously this doesn’t work, but I’m not sure how to do what I’m describing exactly.

Answer 1

What you’re missing is that each of the inner arrays needs to be malloc’ed too.

int **matrix = malloc(x * sizeof(int *));

should be something like:

int **matrix = (int **)malloc(y * sizeof(int *));
for (i = 0; i < y; ++i) {
    matrix[i] = (int *)malloc(x * sizeof(int));
}

That said, most matrix libraries I’m aware of would just use:

int *matrix = (int *)malloc(x * y * sizeof(int));

and then use:

int n = matrix[y * cols + x];

to read the individual elements. For (non-sparse) matrices this is more efficent than having a separately allocated block for each row. It also guarantees that the data is contiguous in memory which can make CPU caches more effective.