credit.c

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

int main(void)
{
    long long cc_num;
    //Get User input
    do
    {
        printf("Please enter your credit card number: ");
        cc_num = get_long_long();
        
    } while (cc_num < 0);
    
    //Determine the length of the credit card number
    int length = 0;
    long long chk_length = cc_num;
    while (chk_length >0)
    {
        chk_length /= 10;
        length++;
    }
    
    //Separate credit card number into an array
    int number[length];
    for (int i =0; i < length; i++)
    {
        number[i] = cc_num % 10;
        cc_num /= 10;
    }

    //Calculate sum of every other digit from the last.
    int added_sum = 0;
    for (int i =0; i < length; i+=2)
    {
        added_sum += number[i];
    }
    
    //Calculate the sum of every other digit from 2nd to last times 2
    int x2_sum = 0;
    for (int i = 1; i<length; i+=2)
    {
        if (number[i]*2 > 9)
        {
            x2_sum += (number[i] * 2) / 10;
            x2_sum += (number[i] * 2) % 10;
        }
        else
        {
            x2_sum += number[i] * 2;
        }
    } 
    
    //Check to see if card is valid and what type of card it is
    int checksum = added_sum + x2_sum;
    if (checksum % 10 == 0 && (length == 13 || length ==15 || length == 16))
    {
        if (number[length-1] == 3 && (number[length-2] == 4 || number[length-2] == 7))
        {
            printf("AMEX\n");
        }
        else if (number[length-1] == 5 && (number[length-2] == 1 || number[length-2] == 2 || number[length-2] == 3 || number[length-2] == 4 || number[length-2] == 5))
        {
            printf("MASTERCARD\n");
        }
        else if (number[length-1 == 4])
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}