Question

How to dynamically allocate a 2D array of ints using pointer to a pointer in C

Answer 1

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    int row = 3, col = 4;
    
    int **arr = (int **)malloc(row * sizeof(int *));
    for (int i = 0; i < row; i++)
         arr[i] = (int *)malloc(row * sizeof(int));
 
    for (int i = 0; i < row; i++)
      for (int j = 0; j < col; j++)
            arr[i][j] = i + j;
 
    for (int i = 0; i < row; i++)
    {
       for (int j = 0; j < col; j++)
          printf("%d ", arr[i][j]);
       printf("\n"); 
    }
    
    for (int i = 0; i < row; i++)
        free(arr[i]);
    free(arr);

    return 0;
}
 
   
/*
run:
  
0 1 2 3
1 2 3 4
2 3 4 5

*/

 

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Answer 2

#include <stdio.h>
#include <stdlib.h>
  
int main(void)
{
    int row = 3, col = 4;
  
    int **arr  = (int **)malloc(sizeof(int *) * row);
    arr[0] = (int *)malloc(sizeof(int) * col * row);
 
    for(int i = 0; i < row; i++)
        arr[i] = (*arr + col * i);
 
    for (int i = 0; i < row; i++)
      for (int j = 0; j < col; j++)
            arr[i][j] = i + j;
 
    for (int i = 0; i < row; i++)
    {
       for (int j = 0; j < col; j++)
          printf("%d ", arr[i][j]);
       printf("\n"); 
    }
    
    free(arr[0]);
    free(arr);
  
    return 0;
}
  
    
/*
run:
      
0 1 2 3
1 2 3 4
2 3 4 5
  
*/

 

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