CS50 Credit Card Validation

Let's acknowledge the elephant in the room first.

long cc = get_long("Credit Card: " );

...

int x = cc;

The C standard specifies long to be at least 32 bits, whereas int must be at least 16 bits. The actual values are dependent on your system and your library implementation of course. But more often than not, long will be capable of storing more bits than an int. As is the case here. This means "numbers with more than 10 digits", essentially numbers that are too large to be stored into an int, will cause undefined behavior. To know exactly which number is the upper limit for int in your system/environment, you may print the value of INT_MAX, defined in limits.h.

The solution is, of course, to store the long variable in another long variable, not an int. Or, simply pass the value to a function that does the necessary work. Putting everything in main isn't being very organized now is it.

How about we make a function that basically prints all the details about a card given the card's number?

The signature will look like-

void print_card_details(long num)

Now we need a function to put the card through luhn's algorithm. We can also make a function for that-

int is_valid(long num)
{
    int curr_digit, add_digit, prod_sum = 0, sum = 0;
    for (int digit_count = 0; num != 0; num /= 10, digit_count++)
    {
        // Strip each digit from number, starting from the end
        curr_digit = num % 10;
        if (digit_count % 2 != 0)
        {
            // Every 2nd digit from the right goes through this
            // The current digit gets doubled
            // The digits of that result are added to the sum
            add_digit = curr_digit * 2;
            prod_sum += add_digit % 10 + add_digit / 10;
        }
        else
        {
            // The remaining digits go through this
            // They are all summed up
            sum += curr_digit;
        }
    }
    if ((prod_sum + sum) % 10 != 0)
    {
        // If the sum of prod_sum + sum doesn't end in 0
        // It is invalid
        return 0;
    }
    else
    {
        // The card is valid
        return 1;
    }
}

The conventional way to iterate through the digits of a number is not to bruteforcefully divide arbitrary powers of 10 manually, but to iterate through it and divide and modulus by 10. For example, this snippet-

while (x != 0)
{
    printf("Current digit: %dn", x % 10);
    x /= 10;
}

Will print all digits of the number stored in x. This is essentially what we've used in the luhn's algorithm loop. Except we also keep a count of the total digits, because we only want every second digit starting from the end. How do we know the current digit qualifies this criteria? We check if the current digit_count is even (by dividing by 2 and checking the leftover is 0).

The formula that follows-

add_digit = curr_digit * 2;
prod_sum += add_digit % 10 + add_digit / 10;

is basically the implementation of this-

Multiply every other digit by 2, starting with the number’s second-to-last digit, and then add those products’ digits together.

Make sure only the digits of the resulting add_digit is added. So if add_digit ended up being 12. We need to add 1 + 2. That's exactly what add_digit % 10 + add_digit / 10 does. 12 % 10 is, of course, 2. And 12 / 10 is 1.

This function returns 1 if the card is valid, 0 if it's not. You can fit this up in your main function and check the return value to know whether the card is valid.

If it is valid, move on to the next step of checking the number of digits the card has, as well as what it begins with.

We can make a loop to count the number of digits, as well as store the very first and second digit of the number.

int len = 0;
int curr_digit = 0, prev_digit = 0;
while(num != 0)
{
    prev_digit = curr_digit;
    curr_digit = num % 10;
    num /= 10;
    len++;
}

This will give you the length of the card number. Notice, in the last iteration, the value of prev_digit is the second digit and the curr_digit is the first. So curr_digit * 10 + prev_digit will yield the first 2 numbers (together) that the credit card number begins with.

Finally, you just need a bunch of simple if/else clauses to verify which card it is. You're only asked to check for a very small subset as well. So here it is-

// Construct the 2 digit number that this card num begins with
int begins_with = curr_digit * 10 + prev_digit;
if (len == 13 && begins_with / 10 == 4)
{
    // We know only VISA uses 13 digits
    // And it begins with 4 (second digit does not matter)
    printf("VISAn");
}
else if (len == 15 && begins_with == 34 ||)
{
    // We know only AMEX uses 15 digits
    printf("AMEXn");
}
else if (len == 16)
{
    // Both VISA and MASTERCARD use 16 digits
    if (curr_digit == 4)
    {
        // But VISA card number begins with 4
        printf("VISAn");
    }
    else if (curr_digit == 5)
    {
        // MASTERCARD number begins with 5
        printf("MASTERCARDn");
    }
    else
    {
        // Out of context for this problem
        printf("INVALIDn");
    }
}
else
{
    // Out of context for this problem
    printf("INVALIDn");
}

Put that all together, and you should hopefully get

void print_card_details(long num)
{
    if (!is_valid(num))
    {   
        // Card did not pass luhn's algo
        printf("INVALIDn");
        return;
    }
    int len = 0;
    int curr_digit = 0, prev_digit = 0;
    while(num != 0)
    {
        prev_digit = curr_digit;
        curr_digit = num % 10;
        num /= 10;
        len++;
    }
    // Construct the 2 digit number that this card num begins with
    int begins_with = curr_digit * 10 + prev_digit;
    if (len == 13 && curr_digit == 4)
    {
        // We know only VISA uses 13 digits
        // And it begins with 4 (second digit does not matter)
        printf("VISAn");
    }
    else if (len == 15 && (begins_with == 34 || begins_with == 37))
    {
        // We know only AMEX uses 15 digits
        printf("AMEXn");
    }
    else if (len == 16)
    {
        // Both VISA and MASTERCARD use 16 digits
        if (curr_digit == 4)
        {
            // But VISA card number begins with 4
            printf("VISAn");
        }
        else if (begins_with >= 51 && begins_with <= 55)
        {
            // MASTERCARD number begins with 51, 52, 53, 54, or 55
            printf("MASTERCARDn");
        }
        else
        {
            // Out of context for this problem
            printf("INVALIDn");
        }
    }
    else
    {
        // Out of context for this problem
        printf("INVALIDn");
    }
    return;
}