Designated Initializers, waste unused and unnecessary memory? how to allocate only the needed part of the array?

Question

Hi guys!

I want to use designated initializers, but what about the others index that aren't initialized?

Are they spend my memory too?

for example:

EDIT PART: {

int array[590] = {[2] = 1};

note: I don't use the other array index, like this example i want to use only one, but i will allocate memory for another index?

(okey I know that this example is vague, i don't want only one int or any other type, I don't know how to explain what I want. But I think that explain it is not necessary because my question is not 'how to accomplish this' my question is 'whats happen when I do it?' or 'how it is implemented?', thanks a lot.)

what about the memory in this? I waste 590 piece of memory, or only one? If the first is correct, how Can I spend only one?

and if I do this:?

int array [] = { [2] = 1, [590] = 2};

I will allocate 590 piece of memory, or only two?

}

Thanks so much!

Answer 1

Yes, the declaration you use will create an array of 5 elements, and only set one of them to the value '1'. The rest will be uninitialised (I believe).

What you want, I think, is a hash table or some other kind of associative container, which C does not have in it's standard library. You'll either have to write one yourself, or find one that someone else has written.

Answer 2

Yes memory is allocated when you declare an array, even though you dont initialize every element. If you want only one value stored you need a single variable, you can also Allocate memory dinamically but it requires a good knolewdge of pointers and memory allocation/management functions (malloc,realloc,free,etc.)

Answer 3

Let's ask Mr. Compiler!

#include <stdio.h>
int main( int argc, char ** argv )
{
    int array[] = {[2] = 1, [590] = 2};
    printf("sizeof(array) is %d bytes\n", sizeof(array));
}

Survey says:

$ gcc initsize.c
$ ./a.out
sizeof(array) is 2364 bytes

Yep! 591 * 4 = 2364.

Here, you've allocated a 591 element integer array on the stack. It has to be allocated; the compiler does not know what you may do with it (pass it to a library function it knows nothing about, for example). You told it the size is 591 elements, and it obeys...

P.S. There are many "sparse matrix" C libraries; just google for "C library sparse matrix". But, for a vector of 591 elements, they are total over-kill. Now, if you have 10,000 of such vectors, that's another story.