Following are different ways to create a 2D array on heap (or dynamically allocate a 2D array).
In the following examples, we have considered ‘r‘ as number of rows, ‘c‘ as number of columns and we created a 2D array with r = 3, c = 4 and following values
1 2 3 4
5 6 7 8
9 10 11 12
Method 1 :- Using a single pointer
#include <stdio.h>
#include <stdlib.h>
int main()
{
int r = 3, c = 4;
int *arr = (int *)malloc(r * c * sizeof(int));
int i, j, count = 0;
for (i = 0; i < r; i++)
for (j = 0; j < c; j++)
*(arr + i*c + j) = ++count;
for (i = 0; i < r; i++)
for (j = 0; j < c; j++)
printf("%d ", *(arr + i*c + j));
/* Code for further processing and free the
dynamically allocated memory */
return 0;
}
Method 2 :- Using an array of pointers
#include <stdio.h>
#include <stdlib.h>
int main()
{
int r = 3, c = 4, i, j, count;
int *arr[r];
for (i=0; i<r; i++)
arr[i] = (int *)malloc(c * sizeof(int));
// Note that arr[i][j] is same as *(*(arr+i)+j)
count = 0;
for (i = 0; i < r; i++)
for (j = 0; j < c; j++)
arr[i][j] = ++count; // Or *(*(arr+i)+j) = ++count
for (i = 0; i < r; i++)
for (j = 0; j < c; j++)
printf("%d ", arr[i][j]);
/* Code for further processing and free the
dynamically allocated memory */
return 0;
}
Method 3 :- Using pointer to a pointer
#include <stdio.h>
#include <stdlib.h>
int main()
{
int r = 3, c = 4, i, j, count;
int **arr = (int **)malloc(r * sizeof(int *));
for (i=0; i<r; i++)
arr[i] = (int *)malloc(c * sizeof(int));
// Note that arr[i][j] is same as *(*(arr+i)+j)
count = 0;
for (i = 0; i < r; i++)
for (j = 0; j < c; j++)
arr[i][j] = ++count; // OR *(*(arr+i)+j) = ++count
for (i = 0; i < r; i++)
for (j = 0; j < c; j++)
printf("%d ", arr[i][j]);
/* Code for further processing and free the
dynamically allocated memory */
return 0;
}