For loop with the length of array that has values

In C++ arrays, there is no concept of an array element being "empty".
That idea simply doesn't exist. Erase it from your mind.
Each array element always exists.

In C & C++, when part of an array is explicitly initialized, and part is not explicitly initialized, the part that you didn't specify defaults to zero.

So, you effectively wrote:

int abc[100] = {1, 2, 3, 4, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0....};

If you want, you can treat value 0 as a marker for values that aren't set.

int i;
for(i=0; abc[i] != 0; ++i)
{
    cout << "[" <<i<< "] : " << abc[i] <<endl;
}