CS50 Problem Set 1 (Fall 2020) - Credit

credit.c

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

// Function Declaration
int creditCardType(long cardNumber);


int main(void)
{
    long c;

    do
    {
        // Get Credit Card number
        c = get_long("Number: ");
    }
    // do while to not accept less than 12 digit numbers
    while (c < 12);

    // calculate length of entered card
    int length = 0;
    long calcuLength = c;

    while (calcuLength != 0)
    {
        calcuLength /= 10;
        length++;
    }



    long myNum = c;
    int digit;
    int position;
    int multiply = 0;
    int sum = 0;
    int output;



// checksum if the card is valid
    for (position = 1; position <= length; position ++)
    {
        // get last credit card number
        digit = myNum % 10;
        // reverses the credit card number for running Luhn’s Algorithm
        myNum = myNum / 10;

        // First Step of Luhn's Algorithm
        // Multiply every other digit by 2, starting with the number’s second-to-last digit,
        // and then add those products’ digits together.

        // Below it gets every other digit
        if (position % 2 == 0)
        {
            // Multiplying digit by 2
            multiply = digit * 2;
            //  adding those digit together
            sum += multiply % 10;
            sum += multiply / 10;

        }
        // According to Luhn's Algorithm than:
        // Add the sum to the sum of the digits that weren’t multiplied by 2

        // This gets digits which were not multiplied by 2
        if (position % 2 != 0)
        {
            // Adding the sum of digit
            sum = sum + digit;

        }


    }
    //   If the total’s last digit is 0
    // (or, put more formally, if the total modulo 10 is congruent to 0), the number is valid!
    if (sum % 10 == 0)
    {
        // function runs to tell which card company
        output = creditCardType(c);
        return 0;
    }
    else
    {
        printf("INVALID\n");
        return 0;

    }

}

// function for card company
int creditCardType(long cardNumber)
{
    // mastercard first two digit is 34 or 37
    // dividing by 10000000000000 gets first two digit
    if (cardNumber / 10000000000000 == 34 || cardNumber / 10000000000000 == 37)
    {
        printf("AMEX\n");
        return 0;
    }
    // Mastercard uses 16 digit numbers and start with 51....55
    else if (cardNumber / 100000000000000 >= 51 && cardNumber / 100000000000000 <= 55)
    {
        printf("MASTERCARD\n");
        return 0;
    }
    // visa uses 13 & 16 and starts with 4
    else if (cardNumber / 1000000000000000 == 4 || cardNumber / 1000000000000 == 4)
    {
        printf("VISA\n");
        return 0;
    }
    else
    {
        printf("INVALID\n");
        return 0;
    }
    printf("\n");
    return 0;

}

hi, why would c < 12 mean c cannot be fewer than 12 digits? wouldn't it mean c must be more than number 12?

Because credit card minimum number starts with 13(VISA) so it should not accept fewer than 12 digits. I could also write c < 13 to be more precise.

On line 19, "while c < 12" literally means it should accept any values from 13 upward rather than values of length 13. You should first get the length of c before using that condition.

Clever solution and easy to follow your documentation..thanks for sharing!