Initializing a 2D array of pointer of type double through double pointer with structure

Question

I am doing something like following;

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


typedef struct
{
 double **arr;
}communication;

int main()
{
    int r = 3, c = 4, i, j, count;

    communication *comm;
    comm->arr = (double **)malloc(r * sizeof(double *));
    for (i=0; i<r; i++)
         comm->arr[i] = (double *)malloc(c * sizeof(double));

    count = 1;
    for (i = 0; i <  r; i++)
      for (j = 0; j < c; j++)
         comm->arr[i][j] = count*2.0;  
    for (i = 0; i <  r; i++)
      for (j = 0; j < c; j++)
         printf("%lf ", *(comm->arr[i]+j));


   return 0;
}

It is giving me seg fault error. Please tell me how can I define a 2D array through double-pointer method with structure just like I am trying to do in above code?

`comm` is uninitialized. make it `communication comm;` instead as a pointer and access `comm.arr` — TruthSeeker, Feb 20 '20 at 12:58
do something like this communication *comm = new communication; — RaHuL, Feb 20 '20 at 13:03
Was your question asked here https://stackoverflow.com/questions/60317881/initializing-a-2d-array-of-pointer-of-type-double-through-double-pointer resolved ? May be related — Neywat, Feb 20 '20 at 13:03
@SergeBallesta They are using a `typedef` so there's no need for the `struct` keyword. — Lundin, Feb 20 '20 at 13:25

score 1 · Answer 1 · edited Feb 20 '20 at 13:21

1

You have not allocated memory to your comm variable.

Also count is uninitialized.

edited Feb 20 '20 at 13:21

machine_1

4,266
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answered Feb 20 '20 at 13:03

Eraklon

4,206
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Doliprane · Answer 2 · 2020-02-20T14:06:15.593

1

Either you do this :

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


typedef struct
{
 double **arr;
}communication;

int main()
{
    int r = 3, c = 4, i, j, count;

    communication comm;
    comm.arr = malloc(r * sizeof(double *));
    for (i=0; i<r; i++)
         comm.arr[i] = malloc(c * sizeof(double));

    count = 1;
    for (i = 0; i <  r; i++)
      for (j = 0; j < c; j++)
         comm.arr[i][j] = count*2.0;  
    for (i = 0; i <  r; i++)
      for (j = 0; j < c; j++)
         printf("%lf ", *(comm.arr[i]+j));


   return 0;
}

Or this :

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


typedef struct
{
 double **arr;
}communication;

int main()
{
    int r = 3, c = 4, i, j, count;

    communication *comm;
    comm = malloc(sizeof(communication));
    comm->arr = malloc(r * sizeof(double *));
    for (i=0; i<r; i++)
         comm->arr[i] = malloc(c * sizeof(double));

    count = 1;
    for (i = 0; i <  r; i++)
      for (j = 0; j < c; j++)
         comm->arr[i][j] = count*2.0;  
    for (i = 0; i <  r; i++)
      for (j = 0; j < c; j++)
         printf("%lf ", *(comm->arr[i]+j));


   return 0;
}

edited Feb 20 '20 at 14:06

answered Feb 20 '20 at 13:07

Doliprane

151
5

1

Please do not advise to use a C++ syntax for C code. You should at least warn future readers about that point... – Serge Ballesta Feb 20 '20 at 13:12
Can you be clearer please? – Doliprane Feb 20 '20 at 13:30
1

You should not cast malloc in C... – Serge Ballesta Feb 20 '20 at 14:01
You're right I'm gonna edit that – Doliprane Feb 20 '20 at 14:05
Thanks to all. It is working now. The memory allocation was the error. – Hritikesh Semwal Mar 03 '20 at 12:57

Hitokiri · Answer 3 · 2020-02-20T13:27:03.790

1

You have to allocate the memory for comm variable like the allocation of arr. Because comm is pointer, the defaut value may be NULL. So, you cannot point to the array arr inside of the struct communication.

Add this code below, your program will be work well:

 communication *comm = (communication *) malloc (sizeof(communication));

edited Feb 20 '20 at 13:27

answered Feb 20 '20 at 13:09

Hitokiri

3,607
1
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29

1

Nit: since it’s an `auto` variable, `comm` is not initialized with any particular value, `NULL` or otherwise. – John Bode Feb 20 '20 at 13:21
I agree with you. Pointer doesn't have default value. I will correct the answer – Hitokiri Feb 20 '20 at 13:26
Thanks to all for your replies. It is working now. – Hritikesh Semwal Feb 22 '20 at 10:50

Initializing a 2D array of pointer of type double through double pointer with structure

3 Answers3