Validate integer length for serial number

Question

I had been given homework for making an app library, and I am stuck with validation. How do I validate integer length? For example, I am inputting the book serial number but the length serial number must be between 13-15 digit. I know how to do it with string, but I dont know how to do it with integer/number.

Below is the validation for the book name

if(strlen(name)>=3 && strlen(name)<=25 && validasiName(name)){
    break;
}else{
    system("cls");
}

and this one for the serial number

if(ISBN>=13 && ISBN<=16){
    break;
}else{
    system("cls");
}

Answer 1

When you're asking for the number of digits, you are really asking "how many times can I divide this number by 10 before it's less than 10?"

A simple recursive implementation would look something like this:

#include <stdio.h>
#include <limits.h>

int numLen(unsigned long num){
    if(num < 10) 
        return 1;
    return 1 + numLen(num/10);
}


int main()
{
    printf("%d\n", numLen(1));
    printf("%d\n", numLen(12));
    printf("%d\n", numLen(ULONG_MAX));
}

Just make sure you're not putting in negative numbers