Thread: How to use Pointer inside a Structure

I hav made a structure in my C program:

Code:

typedef struct species
{ int Trophic;
  int Niche[10000];
} species;

So each species has a Trophic value and a Niche array.
The Niche array can at maximum carry 9999 elements (0 to 9999). But in reality, MOST of the species take JUST 2 or 3 elements in their Niche array. Just 1 or 2 species use a large number of Niche elements. But still I am having to assign a size of 10000 to every species' Niche array. That must be taking a lot of memory.

Now the number of Niche elements that a specie occupies in its niche array (i.e how many elements of the niche array are occupied) is decided as the program runs. But once decided, it remains constant.

So I am trying to use Dynamic Memory Allocation for the Niche array so that the species that take a few Niche elements are assigned a small size to their Niche array and the species that take a larger number of Niche elements get larger size to their Niche array. But I am not able to use a pointer inside structure.

I am very new to pointers etc. But still have studied & practiced a bit of them but not able to use it in structures.

Can someone please guide me.

Regards
Alice

Well, you put a pointer inside a structure and use it.

Code:

typedef struct species {
    int Trophic;
    int n_of_Niches;
    int *Niches;
} species;

You'll have to allocate memory for each "array" when you make a new species thing.

Or malloc for one huge Niche array that guesses at the amount required for all species. The structure can then point to specific index in that array where that species' elements start. If you need to increase that array, use realloc.

Originally Posted by nonoob

Or malloc for one huge Niche array that guesses at the amount required for all species. The structure can then point to specific index in that array where that species' elements start. If you need to increase that array, use realloc.

I don't think that's such a good idea. realloc might give you a pointer to a whole new chunk of memory instead of simply extending what you have. Then you have to go through every instance of a species and update the Niche element to point to the right place in the new array. Not trivial unless you bother keeping track of the absolute offset in the old jumbo array or remember the old and new base address and do some arithmetic. And that's assuming you have a nice collection of species where you can find all instances and update them in one fell swoop.

Thanx a ton tabstop, nonoob, anduril462

I have developed a short code (using calloc). Can u please check it carefully if I have made any mistakes ,like memory leaks etc.. (although I have checked it by printing the outputs, but am not shure about memory leaks)

Code:

typedef struct species
{
    int * niche;
} species;
 
void main()
{
  const int TotalSpecies = 3;
  species s[TotalSpecies];
  int i,j,nichesize;

  for( i = 0; i < TotalSpecies; i++)
   {
	  printf("enter the niche size of specie %d	\n",i);
	  scanf("%d",&nichesize);

	  s[i].niche = calloc(nichesize,sizeof(int));

	  printf("enter the niche value(s) of specie %d\n",i);

  	  for(j=0;j<nichesize;j++)
	   	  scanf("%d",&s[i].niche[j]);
   }


   for( i = 0; i < TotalSpecies;i++)
		 free(s[i].niche);

}

Thanx a lot u all
Regards
Alice

Originally Posted by anduril462

I don't think that's such a good idea. realloc might give you a pointer to a whole new chunk of memory instead of simply extending what you have. Then you have to go through every instance of a species and update the Niche element to point to the right place in the new array. Not trivial unless you bother keeping track of the absolute offset in the old jumbo array or remember the old and new base address and do some arithmetic. And that's assuming you have a nice collection of species where you can find all instances and update them in one fell swoop.

You're right. Then the "pointer" would be better off being an integer index. An offset relative to the beginning of the array, no matter which absolute memory address it's based at.