How do I separate residue from long long number?

Question

I'm taking the CS50 course on edx.org; it's called Introduction to Computer Science.

I'm trying to solve 1st week problem set. So user inputs credit card number and I have to develop some sort of algorithm to check if it's number is valid. To do so I need to separate whole 16-digit number to digits. And I'm stuck here. I guess I need to do this in loop, at getting at each step digit by digit and to do so I wanted to divide user's input by 10 each step and somehow get the residue.

I can't convert to type int because of int's restrictions on number of digits it can hold. How can I implement this kind of function? I tried this, but then realized it leads to nothing... At first glance at least. cre_num stays for credit number.

long long check(long long cre_num)
{
    double part, i;

    for (i = 0.1; i <= 1; i = i+0.1)
    {
        if (cre_num/10 == i)
        {
            part = i;
        }
    }
    return part;
}

Answer 1

You need to put in a vector, or kind of it, 16 digits of a number (long int)?

When you % a number by 10, you get the last digit of it, like this:

13%10 = 3
3%10 = 3 (03 = 3)
523%10 = 3

So, if you %10 you get the last digit and put in your vector, and than you /10, because /10 will remove the last digit, the one that you have already saved.

If you want, I can try to code it, but I think that you've already that kind of done.