initializing arrays of structures in c

dreamgoat · 09-26-2004, 03:42 PM

I'm trying to initialize an array of structures, each with an integer and 4 integer pointers. I have written the following:

main(){
struct lattice_site{
int spin;
int *right_pointer;
int *left_pointer;
int *up_pointer;
int *down_pointer;
};
struct lattice_site mc_array[2];
mc_array[0] = {1,0,0,0,0};
return 0;
}

The compiler gives me the following error:
structure_tester.c: In function `main':
structure_tester.c:15: parse error before '{' token

line 15 refers to the line " mc_array[0] = {1,0,0,0,0}; "

What confuses me is that this syntax works when you aren't using an array of pointers. That is, if I replace mc_array[i] with mc_array in both places, it initializes just fine.

I really need to make a 2-dimensional array of structures like the one above. Will this work?

Thanks!

itsme86 · 09-26-2004, 06:54 PM

You can set a struct equal to another one so you can set up a "dummy" struct instance and then loop...

Code:

#include <stdio.h>

int main(void)
{
  struct lattice_site
  {
    int spin;
    int *right_pointer;
    int *left_pointer;
    int *up_pointer;
    int *down_pointer;
  };
  struct lattice_site dummy = {1,0,0,0,0};
  struct lattice_site mc_array[2];
  int i;

  for(i = 0;i < 2;++i)
    mc_array[i] = dummy;

  return 0;
}

This might be a gcc extension, but I don't think it is.

wapcaplet · 09-26-2004, 07:00 PM

You could also use a sort of "constructor function" that initializes the struct:

Code:

void init_lattice_site(struct lattice_site lat)
{
  lat.spin = 1;
  lat.right_pointer = NULL;
  lat.left_pointer = NULL;
  lat.up_pointer = NULL;
  lat.down_pointer = NULL;
};

int main()
{
  /* stuff */
  for(i = 0;i < 2;++i)
    init_lattice_site(mc_array[i]);
}

That way, all initialization stuff is in one place. The 'dummy' method is probably easier, and may even be a bit quicker, since it has no function calls.

dustu76 · 09-27-2004, 01:12 AM

I tried the original code by initializing the individual elements of the struct, viz.

mc_array[0].spin = 0 ;
mc_array[0].right_pointer = (int *) 1 ;
.
.
.

The above style works without warnings (using the default compiler with Sun OS 5.9).

However, mc_arrary[0] = { 1, 0, ... } or mc_array[0] = {1, (int *)0, ... } does not work ?!

Just what I observed.

grub · 09-27-2004, 01:36 AM

Initialize like this

Code:

main(){
struct lattice_site{
int spin;
int *right_pointer;
int *left_pointer;
int *up_pointer;
int *down_pointer;
};
struct lattice_site mc_array[2]={{1,0,0,0,0}};
// mc_array[0] = {1,0,0,0,0};
return 0;
}

dreamgoat · 09-27-2004, 02:02 AM

Thanks everyone. These are all terrific replies. If anyone's interested, I ended up going with something like this one:

mc_array[0].spin = 0 ;
mc_array[0].right_pointer = (int *) 1 ;

This was easy to loop, though I had an extra dimension to deal with later on, and a lot of relationships between the structures and the pointers to keep track of. In fact, I'm still trying to get the details just right. Still, it's all working ok for a first attempt at a data structure!