defining a 2D array with malloc and modifying it

Question

How do i define a 2D array using malloc ? (lets say 10X20).

second, can i increase number of rows or cols without creating a new increased array and copying all data

Answer 1

Instead of using int[row][col] you should better "wrap your matrix" into a one-dimensional array int[row*col].

Here is sample code:

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

#define DET_MALLOC_FAIL -1
#define DET_READ_FAIL -2
#define DET_WRITE_FAIL -3

typedef struct det_matrix_s {
    double * vect;
    size_t order;
} det_matrix;

void *det_sf_malloc(size_t dsize);
det_matrix * det_matrix_new(size_t order);
#define DET_MAT_ELEM(matr, i, j) \
    ((matr)->vect[((matr)->order * (i)) + (j)])
int det_matrix_read(det_matrix * matr, FILE * src);
int det_matrix_write(det_matrix * matr, FILE * dest);
void det_matrix_delete(det_matrix * matr);

/**
* Malloc wrapper
*/
void * det_sf_malloc(size_t dsize)
{
    void *data = malloc(dsize);
    if(NULL == data){
        exit(DET_MALLOC_FAIL);
    }
    return (data);
}

/**
* Allocates memory for a new matrix
*/
det_matrix * det_matrix_new(size_t order)
{
    det_matrix * res = det_sf_malloc(1 * sizeof(*res));
    double * vect = det_sf_malloc(order * order * sizeof(*vect));
    res->order = order;
    res->vect = vect;
    return (res);
}

/**
* Reads matrix 
*/
int det_matrix_read(det_matrix * matr, FILE * src)
{
    size_t i, j;
    if(NULL == matr || NULL == src){
        return (DET_READ_FAIL);
    }
    for(i = 0; i < matr->order; ++i){
        for(j = 0; j < matr->order; ++j){
            if(stdin == src){
                fprintf(stdout, "mat[%d][%d] = ", i, j);
            }
            fscanf(src, "%lf", &DET_MAT_ELEM(matr, i, j));
        }
    }
    return (EXIT_SUCCESS);
}

/**
* Writes matrix
**/
int det_matrix_write(det_matrix * matr, FILE * dest)
{
    size_t i, j;
    if(NULL == matr || NULL == dest){
        return (DET_WRITE_FAIL);
    }
    for(i = 0; i < matr->order; ++i){
        for(j = 0; j < matr->order; ++j){
            fprintf(dest, "%5.2lf ", DET_MAT_ELEM(matr, i, j));
        }
        fprintf(dest, "\n");    
    }
    return (EXIT_SUCCESS);
}

/**
* Free memory for matrix
*/
void det_matrix_delete(det_matrix * matr)
{
    free(matr->vect);
    free(matr);
}

/**
* Main
*/ 
int main(int argc, char * argv[])
{
    det_matrix * mat = det_matrix_new(3);
    det_matrix_read(mat, stdin);
    det_matrix_write(mat, stdout);
    det_matrix_delete(mat);
    return (EXIT_SUCCESS);
}

If re-allocation is an issue, than problems may appear. And you should use the bi dimensional version.