Is it possible to dynamically allocate 2-D array in c with using calloc() once?

Question

All the solutions I have seen online has calloc() function used twice, is it possible to do with only using it once? The below code is not printing the correct array elements

int **ptr;

//To allocate the memory 
ptr=(int **)calloc(n,sizeof(int)*m);

printf("\nEnter the elments: ");

//To access the memory
for(i=0;i<n;i++)
{  
 for(j=0;j<m;j++)
 {  
  scanf("%d",ptr[i][j]);
 }
}

The problem with your code is, how does the pointer know the dimensions of your array? It needs those dimensions to calculate the correct memory address. That's why it only works directly with one-dimensional arrays. — Blaze, Oct 05 '18 at 08:51
Possible duplicate: [Correctly allocating multi-dimensional arrays](https://stackoverflow.com/questions/42094465/correctly-allocating-multi-dimensional-arrays) — Lundin, Oct 05 '18 at 09:24
Possible duplicate of [Correctly allocating multi-dimensional arrays](https://stackoverflow.com/questions/42094465/correctly-allocating-multi-dimensional-arrays) @Lundin Hammer time — Stargateur, Oct 05 '18 at 09:26
@Stargateur I try to avoid dupe-hammering to questions/answers posted by myself, unless they are community wikis or such. — Lundin, Oct 05 '18 at 09:28

David Ranieri · Accepted Answer · 2018-10-05T09:28:58.887

4

Since C99 you can use pointers to VLAs (Variable Length Arrays):

int n, m;

scanf("%d %d", &n, &m);

int (*ptr)[m] = malloc(sizeof(int [n][m]));

for (i = 0; i < n; i++)
{  
    for (j = 0; j < m; j++)
    {  
        scanf("%d", &ptr[i][j]); // Notice the address of operator (&) for scanf
    }
}
free(ptr); // Call free only once

edited Oct 05 '18 at 09:28

answered Oct 05 '18 at 09:00

David Ranieri

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What if m,n are defined at runtime? – Pratik Patwari Oct 05 '18 at 09:17
2

That's the purpose of VLAs, you can set it at runtime, edited in order to reflect that. – David Ranieri Oct 05 '18 at 09:17
Since C11 VLA are optionnel ;) – Stargateur Oct 05 '18 at 09:25
4

Notably, this code actually doesn't use a VLA, but a pointer to VLA. The C11 committee were confused and failed to see the benefit of pointers to VLA, which is important in modern C programming. Unlike actual allocated VLAs, which are much less useful. – Lundin Oct 05 '18 at 09:26

score 0 · Answer 2 · answered Oct 06 '18 at 11:54

If it's just about minimising the number of calls to memory allocation functions you can created such a jagged array like this:

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

int ** alloc_jagged_2d_array_of_int(size_t n, size_t m)
{
  int ** result = NULL;
  size_t t = 0;

  t += n * sizeof *result;
  t += n*m * sizeof **result;

  result = calloc(1, t);
  if (NULL != result)
  {
    for (size_t i = 0; i < n; ++i)
    {
      result[i] = ((int*) (result + n)) + i*m;
    }
  }

  return result;
}

Use it like this:

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

int ** alloc_jagged_2d_array_of_int(size_t, size_t);

int main(void)
{
  int result = EXIT_SUCCESS;

  int ** p = alloc_jagged_2d_array_of_int(2, 3);
  if (NULL == p)
  {
    perror("alloc_jagged_2d_array_of_int() failed");
    result = EXIT_FAILURE;
  }
  else
  {
    for (size_t i = 0; i < 2; ++i)
    {
      for (size_t j = 0; j < 3; ++j)
      {
        p[i][j] = (int) (i*j);
      }
    }
  }

  /* Clean up. */

  free(p);

  return result;
}

Is it possible to dynamically allocate 2-D array in c with using calloc() once?

2 Answers2

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