Are are elements of an empty array NULL valued?

Question

I tried this Armstrong program but found myself entangled in this empty array thing. Working of this program has been bothering me for a while now and still can't seem to figure it what's wrong here. Yea, so just wanted to ask what value are the elements of an empty or incomplete array assigned to? Is it the NULL character i.e. '\0'? I tried checking it on an online C compiler where this assertion seems true but GCC tells us the opposite. I tried this approach to Armstrong problem and here's my code:

#include <stdio.h>
#include <math.h>

int main()
{
    int num,i,cub,j;
    i = cub = 0;
    int sto[20];
    scanf("%d",&num);
    j = num;
    while(num != 0) 
    {
        sto[i] = num%10;
        num = num / 10;
        i++;
    }
    i = 0;
    while(sto[i] != '\0')
    {
       cub += pow(sto[i],3);
       i++;
    }
    num = j;
    printf("cub: %d     num: %d\n\n",cub,num);
    if(j == cub)
      printf("The number is an Armstrong number");
    else 
      printf("The number is not an Armstrong number");

    return 0;
}

I know there is other approach to this problem but what I am looking for is the answer to the above mentioned question.

Answer 1

You have declared an array in a block scope without storage specifier static

int sto[20];

such an array has automatic storage duration. That is it will not be alive after exiting the block where it is defined.

If you would declare the array in the file scope for example before the function main or with the storage specifier static in a block it would have static storage duration.

From the C Standard (6.7.9 Initialization)

10 If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:

— if it has pointer type, it is initialized to a null pointer;

— if it has arithmetic type, it is initialized to (positive or unsigned) zero;

— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

— if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;