pset1 - problem with credit ..help plz

Question

I spent so many hours and still can't figure the problem, any guidance would be appreciated.. I checked against all the paypal test numbers and got same response as the staff's execution, though I got a score of 4 in coding.. What's wrong?!

#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <math.h>

int main(void)
{
    // get user’s input
    long long number = get_long("Number: ");

    // loop through number till second-to-last digit
    // multiply each of the underlined digits by 2
    int digit; 
    int length = 0; 
    int lastDigit; 
    int count = 0; 
    int sumMultiplied = 0; 
    int sumNonMultiplied = 0; 
    int all = 0;
    int fstNum = 0;
    int scndNum = 0;
    bool Visa = false;
    bool expressNum = false;
    bool masterCard = false;

    long long number1 = number;
    long long number2 = number;
    long long number3 = number;

    while (number1 != 0)
    {
        number1 = number1 / 10;
        length++;
    }


    lastDigit = number3 % 10;

    while (number3 > 0) // stop when there's no more digits
    {
        // integer division by 10 removes the last digit, so you're ready to read the next one
        number3 = number3 / 10;
        // modulo 10 effectively takes the last (right-most) digit and  ignores the others
        digit = (int)(number3 % 10);        
        count++;
        // get fst number from the left
        if (count == length - 1)
        {
            fstNum = digit;
        }
        // get scnd number from the left
        if (count == length - 2)
        {
            scndNum = digit;
        }
        if (count % 2 != 0)
        {
            digit = digit * 2;
            sumMultiplied = sumMultiplied + digit / 10 + digit % 10;
        }  
        if (count % 2 == 0) 
        {
            sumNonMultiplied = sumNonMultiplied + digit / 10 + digit % 10;
        }
    }    


    if (fstNum == 4)
    {
        Visa = true;
    }
    else if ((fstNum == 3) && ((scndNum == 4) || (scndNum == 7)))
    {
        expressNum = true;
    }
    else if ((fstNum == 5) && ((scndNum >= 1) && (scndNum <= 5)))
    {
        masterCard = true;
    }


    all = lastDigit + sumMultiplied +  sumNonMultiplied;
    // check if Visa
    if ((all % 10 == 0) && (Visa == true))
    {
        printf("VISA\n");
    } 
    // check if American Express or AMEX
    else if ((all % 10 == 0) && (expressNum == true))
    {
        printf("AMEX\n");
    } 
    // check if Master Card
    else if ((all % 10 == 0) && (masterCard == true))
    {
        printf("MASTERCARD\n");
    } 
    // if non of the above then INVALID
    else
    {
        printf("INVALID\n");
    }

}

Answer 1

There seems to be an important component of credit card number validation missing. From the spec:

American Express uses 15-digit numbers, MasterCard uses 16-digit numbers, and Visa uses 13- and 16-digit numbers.

This program will return VISA for card number 4062901840. It starts with a 4, it has a valid checksum, but it is only 10 digits in length.