filling 2d array with mutiplicatio table

Question

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

int** createMatrix(int n)
{
   int i, a, **tab,x;
   tab=(int**)malloc(n*sizeof(int*));
   if(tab==0)
   {
      return NULL;
      free(tab);
   }
   for(i=0;i<n;i++)
   {
      tab[i]=(int*)malloc(n*sizeof(int));
      if(tab[i]==NULL)
      {
         for(x=0;x<i;x++)
         {
            free(tab[x]);
         }
         free(tab[i]);
         return NULL;

      }
   }
}

void fillMatrix(int*** tab, int n)
{
   int i, a;
   for(i=0;i<n;i++)
   {    
      for(a=0;a<n;a++)
      {
         *tab[i][a]=(a*i);
      }
   }
}

int main()
{
   int roz, **tab,i,x;
   printf("size of the array: \n");
   scanf("%d",&roz);
   tab=createMatrix(roz);
   if(tab==NULL)
   {
      printf("error");
      return -1;
   }
   fillMatrix(&tab, roz);
   for(i=0;i<roz;i++)
   {
      printf("\n");
      for(x=0;x<roz;x++)
         printf("%d",tab[i][x]);
   }
   return 0;
}

Hi! I need to write a program that makes 2d arrays and I want to fill them with multiplication table. Program compiles without single warning or error, but after puttintan input it crashes. And by the way, could you tell me why I have to put 3x* in fillMatrix?

Answer 1

int** createMatrix(int n)

You should be returning the double pointer from the function which I see you are not doing.

int** createMatrix(int n)
{
int i, a, **tab,x;
tab=(int**)malloc(n*sizeof(int*));
// Do your allocations and other stuff

return tab;
}

Take care of accessing the elements using triple pointer. Like

(*tab)[i][a] = (a*i);

You can get the job done using doule pointers itself.

Answer 2

And by the way, could you tell me why I have to put *** in fillMatrix?

That is an excellent question. Incidentally, it provides the key to answering the "why does my program crash" question. The reason the program crashes is that you are using the matrix incorrectly: you treat it like a 2D array of pointers, rather than a pointer to a 2D array. If you add parentheses, your program would stop crashing:

(*tab)[i][a]=(a*i);

Better yet, change the program to take ** that it needs:

void fillMatrix(int** tab, int n) {
    ...
    tab[i][a]=(a*i); // <<== No asterisk
}
...
fillMatrix(tab, roz); // <<== No ampersand

Note: when you compile your program, you should see the "control reaches the end of non-void function without returning a value". This is because you forgot to add return tab at the end of the function that creates your matrix.

Demo.

Answer 3

You have several problems

1. Pointeless free(tab) in your createMatrix() function, it's after the return statement, it will never be executed.

2. You free the tab[i] element which is NULL in createMatrix() inside the loop where you malloc the pointers of the array.

   What you should do is

    free(tab); 

   instead.

3. You never return the malloc ed tab .

4. Your fillMatrix() function is unecessarily taking a int *** triple pointer, you don't need that, if you pass the pointer you directly modify the data.

5. You have an operator precedence issue in fillMatrix()

    *tab[i][a] = (a*i); 

   this doesn't mean what you think, first [] is applied, and then you dereference it with * which is equivalent to

    *(tab[i][a]) = (a * i); -> *(tab[i][a]) -> tab[i][a][0] 

   what you want is

    (*tab)[i][a] = a * i; 

6. You don't free the pointers after printing them.

This is your code with all this issues fixed.

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

int **createMatrix(int n)
{
    int i, **tab, x;

    tab = malloc(n*sizeof(int*));
    if (tab == 0)
        return NULL;

    for (i = 0 ; i < n ; i++)
    {
        tab[i] = malloc(n * sizeof(int));
        if (tab[i] == NULL)
        {
            for (x = 0 ; x < i ; x++)
                free(tab[x]);
            free(tab);

            return NULL;
        }
    }

    return tab;
}

void fillMatrix(int **tab, int n)
{
    int i, a;
    for (i = 0 ; i < n ; i++)
    {
        for (a = 0 ; a < n ; a++)
        {
            tab[i][a] = (a*i);
        }
    }
}

int main()
{
    int roz, **tab, i, x;

    printf("size of the array: \n");
    scanf("%d", &roz);
    tab = createMatrix(roz);
    if (tab == NULL)
    {
        printf("error");
        return -1;
    }
    fillMatrix(tab, roz);

    for (i = 0 ; i < roz ; i++)
    {
        printf("\n");
        for (x = 0 ; x < roz ; x++)
            printf("%4d ", tab[i][x]);
        printf("\n");
        free(tab[i]);
    }
    free(tab);

    return 0;
}

Answer 4

You asked:

And by the way, could you tell me why I have to put 3x* in fillMatrix?

That is not necessary. You could use:

void fillMatrix(int** tab, int n)
{
   int i, a;
   for(i=0;i<n;i++)
   {    
      for(a=0;a<n;a++)
      {
         tab[i][a]=(a*i);
      }
   }
}