How to initialize a 2D array in which the size is determined by argc and argv?

Question

I am working on code which will keep track of each time a specific element in an array is accessed. The array itself will be allocated dynamically based off of the inputs by the user, and so no functions that I have seen are what I'm looking for. To be more specific, how do I dynamically allocate the rows and columns of an array, and then initialize every element to 0? Ex.
./SIM A B

int* array_columns = malloc(atoi(argv[1]) * sizeof(int));
int* array_rows = malloc(atoi(argv[2]) * sizeof(int)); 
int array[*array_rows][*array_columns];

everything that I have seen requires knowing beforehand the number of elements in each row/column. Can anyone give any pointers on how to initialize this array to 0? edit: I have added the line where I attempt to establish the array

Answer 1

This program allocates memory using command line parameters and creates a variable that can be accessed using array syntax. It uses calloc to initialize values to zero:

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


int main(int argc, char *argv[])
{
    int **array;

    int length = atoi(argv[1]);
    int width = atoi(argv[2]);

    array = calloc(length, sizeof(int *));
    for (int i = 0; i < length; i++) {
        array[i] = calloc(width, sizeof(int));
    }

    for (int i = 0; i < length; i++) {
        for (int j = 0; j < width; j++) {
            printf("array[%d][%d] = %d\n", i, j, array[i][j]);
        }
    }

    for (int i = 0; i < length; i++) {
        free(array[i]);
    }
    free(array);

    return 0;
}

Compiled With

gcc -Wall -Werror -o scratch scratch.c

Output

[user@machine]: ./scratch 3 5
array[0][0] = 0
array[0][1] = 0
array[0][2] = 0
array[0][3] = 0
array[0][4] = 0
array[1][0] = 0
array[1][1] = 0
array[1][2] = 0
array[1][3] = 0
array[1][4] = 0
array[2][0] = 0
array[2][1] = 0
array[2][2] = 0
array[2][3] = 0
array[2][4] = 0

Note

I left out input validation and error checking to keep the example small.

Compiler Options

I use -Wall -Werror to turn on all warnings and treat them as errors. This means the compiler won't produce an executable unless all the causes of warnings are fixed. The -o scratch tells the compiler what to name the output file.