cash.c

Harvard CS50 - Problem Set 1

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

int main(void)
{
    // get creditcard number
    long card_number;
    card_number = get_long("Number: ");

    // get length of creditcard number
    long l = card_number;
    int length = 0;
    while (l > 0)
    {
        l /= 10;
        length++;
    }

    // check length
    if (length != 13 && length != 15 && length != 16)
    {
        printf("INVALID\n");
        return 0;
    }

    // calculate checksum
    int last_digit, second_last_digit;
    int Sum1 = 0;
    int Sum2 = 0;
    int Total = 0;
    long x = card_number;
    do
    {
        // get last digit
        last_digit = x % 10;
        x /= 10;
        Sum1 += last_digit;

        // get second last digit
        second_last_digit = x % 10;
        x /= 10;

        // multiply * 2
        second_last_digit *= 2;

        // add up d1 + d2
        int d1 = second_last_digit % 10;
        int d2 = second_last_digit / 10;
        Sum2 += d1 + d2;
    }

    while (x > 0);
    Total = Sum1 + Sum2;

    // check algorithm result
    if (Total % 10 != 0)
    {
        printf("INVALID\n");
        return 0;
    }

    // get the first 2 digits
    long num;
    while (card_number >= 100)
    {
        card_number /= 10;
        num = card_number % 100;
    }

    // check which card
    if ((length == 15) && (num == 34 || num == 37))
    {
        printf("AMEX\n");
    }
    else if ((length == 16) && (num >= 51 && num <= 55))
    {
        printf("MASTERCARD\n");
    }
    else if ((length == 13 || length == 16) && (num >= 40 && num <= 49))
    {
        printf("VISA\n");
    }
    else
    {
        printf("INVALID\n");
    }
}