Matrix Question

[CJM]

Hello to the wonderful world of programmers
I am having a great deal trouble with a matrix/arrays.

I need to set up a user defined array n by m
and then get the use to enter values into it, but i cant make any progress, so far i have (taken mostly for other threads on this board)

#include <stdio.h>

int main()
{
  int nSIZE = 1;
  double *ptr;
  int i,j;
  while(nSIZE != 0)
    {
      printf("enter size : \n");
      scanf("%d", &nSIZE);

            ptr = (double *)malloc(sizeof(double)*nSIZE*nSIZE);

	
printf("please enter each value of the matrix: \n");
      for(i=0;i<nSIZE;i++)
	{
	  for(j=0;j<nSIZE;j++)
	    {
			
		scanf( "%d", &ptr[i] );
		(ptr[i]) = j;
		printf("%d", (ptr[i]));
	    }
	  printf("\n");
	}
	  for(i=0;i<nSIZE;i++)
		  	{
	  for(j=0;j<nSIZE;j++)
	  {
	  printf("%d", ptr[i][j]);
	  }
      free(ptr);
    }
}

Thanks for any help!!

[claudiu]

You need ptr to be of type double** . Look at it like a pointer to an array of pointers. The array of pointers are the rows in your matrix.

So you need to allocate memory for n double* and then for each of these m doubles.

Then you free in reverse order, you free each of the row pointers and at the end the pointer to the array of rows.

[nonoob]

The way you are doing it is valid. You've got a single dimension array of nxn elements and you are placing values in it sequentially.

To print it out you'd need to access pnt[i * nSize + j].

[BillyTKid]

#include <stdio.h>

int main()
{
  int nSIZE = 1;
  double **ptr;
  int i,j;
  while(nSIZE != 0)
    {
      printf("enter size : \n");
      scanf("%d", &nSIZE);

      ptr = malloc(nSIZE*sizeof*ptr);
      for(i=0;i<nSIZE;++i) ptr[i]=malloc(nSIZE*sizeof**ptr);

	
printf("please enter each value of the matrix: \n");
      for(i=0;i<nSIZE;i++)
	{
	  for(j=0;j<nSIZE;j++)
	    {			
		scanf( "%lf", &ptr[i][j] );
	    }
	  printf("\n");
	}
	  for(i=0;i<nSIZE;i++)
	  for(j=0;j<nSIZE;j++)
	  {
	  printf("%f", ptr[i][j]);
	  }
      for(i=0;i<nSIZE;++i) free(ptr[i]);
      free(ptr);
    }
}

[nimitzhunter]

#include <stdio.h>



      ptr = malloc(nSIZE*sizeof*ptr);
      for(i=0;i<nSIZE;++i) ptr[i]=malloc(nSIZE*sizeof**ptr);


}

You have to type cast malloc because it's of type void.

 ptr = (double **) malloc(nSIZE*sizeof(double *));
 for(i=0;i<nSIZE;++i) ptr[i]= (double *) malloc(nSIZE*sizeof(double) );

Also, you will have to include "stdlib.h" because that header file contain prototype for malloc.

[claudiu]

You have to type cast malloc because it's of type void.

No, you don't and you shouldn't.

[msh]

You have to type cast malloc because it's of type void.

Only reason you would want to do that in C is to make the code compatible with C++, which does require explicit typecast from void*. On the other hand, you shouldn't be using malloc() and friends in C++; new and delete is the C++ Way.