RE: [Dev-C++] dynamic allocation and structs

[Ioannis V. <no...@ho...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of Per Westermark
> Sent: Thursday, October 24, 2002 4:13 PM
> To: Scott Williams
> Cc: dev...@li...
> Subject: Re: [Dev-C++] dynamic allocation and structs
> 
> 
> 
> // Alternative 1, array of entries.
> mystruct *myarray;
> void func(int count) {
>     int i;
>     myarray = (mystruct*)malloc(count*sizeof(mystruct));
>     for (i = 0; i < count; i++) {
>         myarray[i].xx = yy;
>     }
> }
> 
> // Alternative 2, array of pointers to entries.
> mystruct **myarray;
> void func(int count) {
>     int i;
>     myarray = (mystruct**)malloc(count*sizeof(*mystruct));
>     for (i = 0; i < count; i++) {
>         myarray[i] = (mystruct*)malloc(sizeof(mystruct));
>         myarray[i]->xx = yy;
>     }
> }


The Alternative 2 is wrong. You use a ** pointer and then you assign
values as if it is a *.


A working example of what you are trying to do:

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

int main()
{
 struct mystruct
 {
 	int x;
 };

 struct mystruct **myarray;
 
 int i, j;
 
/* sizeof(*myarray)==sizeof(struct mystruct *) */
 myarray=malloc(100*sizeof(*myarray));

 /* sizeof(**myarray)==sizeof(struct mystruct).
 A 200x100 array on the free store is created */
 for(i=0; i<100; i++)
   myarray[i]=malloc(200*sizeof(**myarray));

 /* Do something */
 for(i=0; i<100; i++)
   for(j=0; j<200; j++)
   {
     myarray[i][j].x=i+j;
     
     printf("%d\n", myarray[i][j].x);
   }


 for(i=0; i<100; i++)
   free(myarray[i]);
   
 free(myarray);

 return 0;
}




Alternative 3 (which must be prefered if possible instead of A-2):

#include <stdlib.h>

int main()
{
 struct mystruct
 {
 	int x;
 };

 struct mystruct (*myarray)[100];

 /* sizeof(*myarray)==sizeof(struct mystruct).
 A 200x100 array on the free store is created */
 myarray=malloc(200*sizeof(*myarray));

 /* Do something */
 
 
 free(myarray);

 return 0;
}



Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.freeurl.com
* Alternative URL: http://run.to/noicys