credit.c

/**
 * credit.c
 *
 * Jody W Moore
 *
 * this program takes in a credit card number
 * and returns VALID or INVALID number.
 *
 **/
#include <cs50.h>
#include <math.h>
#include <stdio.h>

int main(void)
{

    long long num = 0;
    int digit = 0;
    int odd = 0;
    int ctr = 0;
    int prod = 0;
    int prod2 = 0;
    int cheksum = 0;
    int validater = 0;
    int even = 0;
    int id1 = 0;

    // promt user for credit card # using GetLongLong
    do
    {
        printf("Number: ");
        num = GetLongLong();
    }
    while(num <= 0);

    while (num > 0 )
    {
        digit = num % 10;
        num = num / 10;
        ctr++;

        if ( ctr != 0 )
        {
            odd = digit + odd;
        }
        if ( ctr == 0 )
        {
            prod = digit * 2 ;

            if (prod <= 9)
            {
                even = even + prod;
            }
            else
            {
                int temp_num = prod % 10;
                prod2 = temp_num + 1;
                even = even + prod2 ;
            }
         }
    }

    id1 = digit;
    cheksum = even + odd;
    validater = cheksum % 10;

    if ( validater != 0)
    {
        printf("INVALID\n");
    }
    else
    {
        if ( id1 == 3 )
        {
            printf("AMEX\n");
        }
        else if ( id1 == 4 )
        {
            printf("VISA\n");
        }
        else if (id1 == 5 )
        {
            printf("MASTERCARD\n");
        }
        else if ( id1 == 6)
        {
            printf("Discover\n");
        }
    }
}