RE: Credit in PSET1

Question

I have come up with, what seems to me, to be a logical walkthrough of the CREDIT problem (after banging my head against a wall for too long) - but the code does not make sense to the compiler. And I grant that some things need to be rewritten. Sorry to add this question to the millions already posed on this problem, but if someone has some hints - I'd be eternally grateful.

Here is my code:

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

int main(void)

/*this first loop requires user input of a long_long
with a length of at least 13 digits (10^12) and a
maximum of 16 digits (10^16)*/

{
    long long n = 0;
    long long o = 0;
    long long l = 0;
    int g = 10;
    int suma = 0;
    int sumb = 0;
    int sumfin = 0;
    int count = 0;
    int counta = 0;
    int countb = 0;

    do
    {
        printf("Enter credit card number: ");
        n = get_long_long();
    }
    while (n >= (10^12) || n <= (10^16));

// Code to find first digit to prove Credit Card Company
// Found https://codeforwin.org/2015/06/how-to-find-first-and-last-digit-of-any-number.html

        long long first = n;

// Remove last digit from number till only one digit is left
    while(first >= 10)
    {
        first = first / 10;
    }

// This code will compute Luhn's Algorithm:

    for (o = n; o > 99; o / 100)
    {
        l = (o / g) %10;
        suma = (l * 2) + suma;
    }

    for (long long p = n; p > 99; p / 100)
    {
        int q = p % 10;

        sumb = q + sumb;
    }

    sumfin = suma + sumb;

    if (sumfin % 10 == 0)
    {

// The following code will give the number of digits in long long n
// Code was learned from https://www.programiz.com/c-programming/examples/digits-count

while(n != 0)
    {
        // n = n/10
        n /= 10;
        ++count;
    }

// The following will require number of digits to verify that it
// is a real credit card number 13, 15 or 16 digits. Credit for code:
// https://stackoverflow.com/questions/9721042/count-number-of-digits-which-method-is-most-efficient

    if (count == 12 || 15 && first == 4)
    {
        Printf("VISA\n");
    }
    else (count == 14 && first == 3)
    {
        Printf("AMEX\n");
    }
    else (count == 15 && first == 5)
    {
        Printf("MC\n");
    }

}
    else
    {
        printf("Credit Card number NOT valid\n");
    }

return 0;
}

I seem to be getting my first error at the for loops for Luhn's Algorithm. And then in the If Loops to prove form what company the card is issued.

Thanks!

Tufnel

Answer 1

The ^ operator is bitwise XOR, so you're saying while (n >= 6 || n <= 26), which makes your do..while loop infinite. Based on the credit specs, I don't see why you would need that loop at all.

o / 100 divides, but otherwise doesn't do anything. You probably meant o /= 100, which assigns the result to the variable divided. Same for p / 100, another infinite loop. And I'd check for > 0 rather than > 99.

It might be o = n / 10 to start at the second-to-last digit. And the doubling seems done wrong. If the doubling yields a two-digit number, you'd add the digits.

Also, count == 12 || 15 && first == 4 is wrong in multiple ways: First, it would have to be count == 12 || count == 15. Then, && has higher precedence than ||, so you need parentheses, like (count == 12 || count == 15) && first == 4.

Umm... and I think it should be count == 13 and count == 16.

Is every number starting with 3 AMEX? Like 380000000000000? Or 5900000000000000 a valid MASTERCARD? I just made them up, so hope I got the checksum right.

And the output strings should be MASTERCARD and INVALID, not MC (Hammer?) and Credit Card number NOT valid. And you forgot the case of a valid checksum, but no known card type. Read the specs carefully. That's a very important thing. Sometimes even copy&paste strings from the specs to not include any typos.

PS: Unless you're into ASM or Basic, you probably never form a loop with if or equivalent. So don't talk about "If Loops".