credit.c

// This program will check if the credit or debit card is valid using the Luhn's algorithm. It can check the American Express, Master Card and VISA cards

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

// Functions needed to calculate part of the Luhn's algorithm
int odd(long number);
int even(long number);

int main(void)
{
    // Declaration of variable "number" (for input from user) and card to check the card type
    long number = 0;
    int card = 0;
    // Loop to get valid input from the user
    do
    {
        // Data type "long" so there will be enough memory for a valid card number
        number = get_long("Number: ");
        // Checking the card type and storing it to variable "card", if wrong type, invalid number or string, prompting the number again
        // 0 = INVALID
        // 1 and 2 = AMEX (15 digits)
        // 3 = MASTERCARD (16 digits)
        // 4 and 5 = VISA (13 and 16 digits)
        if (number > 339999999999999.9 && number < 349999999999999.9)
        {
            // AMEX 1
            card = 1;
        }
        else if (number > 369999999999999.9 && number < 379999999999999.9)
        {
            // AMEX 2
            card = 2;
        }
        else if (number > 5099999999999999.9 && number < 5599999999999999.9)
        {
            // MASTERCARD
            card = 3;
        }
        else if (number > 3999999999999.9 && number < 4999999999999.9)
        {
            // VISA 1
            card = 4;
        }
        else if (number > 3999999999999999.9 && number < 4999999999999999.9)
        {
            // VISA 2
            card = 5;
        }
        else
        {
            // INVALID
            printf("INVALID\n");
            exit(0);
        }
    }
    while (card != 1 && card != 2 && card != 3 && card != 4 && card != 5);
    // Checking validity of the card
    int a = even(number);
    int b = odd(number);
    int check = ((a + b) % 10);
    if (check == 0)
    {
        if (card == 1 || card == 2)
        {
            printf("AMEX\n");
        }
        else if (card == 3)
        {
            printf("MASTERCARD\n");
        }
        else if (card == 4 || card == 5)
        {
            printf("VISA\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}

// Function for calculating the odd numbers total
int odd(long number)
{
    int n1 = 0;
    int n2 = 0;
    for (long i = 0, j = 1000, k = 100; i < 7; i++)
    {
        n1 = ((number % j) / k);
        n2 = n2 + n1;
        j = j * 100;
        k = k * 100;
    }
    int n0 = (number % 10);
    int odd = n2 + n0;
    return odd;
}

// Function for calculating the even numbers total
int even(long number)
{
    int n1 = 0;
    int even = 0;
    for (long i = 0, j = 100, k = 10; i < 8; i++)
    {
        n1 = (((number % j) / k) * 2);
        if (n1 > 9)
        {
            n1 = (n1 - 9);
        }
        even = (even + n1);
        j = j * 100;
        k = k * 100;
    }
    return even;
}