credit.c

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

int main(void)
{
    //enter CC
    long cc=0;
    cc = get_long("Number: ");

    //make copies variables of CC (4)
    //When we process digits, we delete them
    long cc1,cc2,cc3,cc4;
    cc1 = cc;
    cc2 = cc;
    cc3 = cc;
    cc4 = cc;

    int d=0;
    int a=0,b=0;
    int sum=0;
    int total=0;

    //Add alternate digits (pass 1)
    //32452345 8345
    cc1 = cc1 / 10;     //ignore last digit

    while (cc1>0)
    {
        d = cc1 % 10;       //get last digit
        d = d * 2;          //double it
        b = d % 10;         //get 2nd digit
        a = d / 10;         //get 1st digit
        sum = a + b;        //sum the 2 digits
        total = total + sum;    //add sum to total
        cc1 = cc1 / 100;        //chop off 2 digits
    }

    //Add other unused digits (pass 2)
    //8345
    while (cc2>0)
    {
       d = cc2 % 10;
       total = total + d;
       cc2 = cc2 / 100;
    }
    //printf("%i", total);

    //Valid if total ends with a zero
    //mod total by 10
    if (total % 10 != 0)
    {
        printf("INVALID\n");
        exit(0);
    }

    int len=0;

    //Calc length of CC (digits)
    while (cc3>0)
    {
        cc3 = cc3 / 10;         //chop off last digit
        len++;
    }
    //printf("length=%i\n", len);

    //Calc 2 digit prefix
    int prefix=0;

    //Keep chopping until there are only 2 digits left
    while (cc4>99)
    {
        cc4 = cc4 / 10;         //chop off last digit
    }
    prefix = cc4;

    //Check all the stuff (length, prefix, valid)
    //AMEX 15 digits, prefix 34/37
    //MC 16 digits, prefix 51-55
    //Visa 13/16 digits, prefix 40-49
    if (len == 15 && (prefix == 34 || prefix == 37))
    {
        printf("AMEX\n");
    } else if (len == 16 && (prefix >= 51 && prefix <= 55))
    {
        printf("MASTERCARD\n");
    } else if ((len==13 ||len==16) && (prefix >= 40 && prefix <= 49))
    {
        printf("VISA\n");
    } else
    {
        printf("INVALID\n");
    }

}