CS50 Pset1 credit

Question

after 2 days and half i did it it's tricky at first but this problem made me understand loops and array much deeper , the question is my code is good as style and design ? any ideas should consider

#include<stdio.h>
#include<cs50.h>
#include<math.h>
unsigned long long get_long_long(string prompt);

int main(void)
{
    unsigned long long card = get_long_long("Number:");
    unsigned long long card1 = card ;
    int sum, sum1, counter = 0 ;
    while (card1 != 0)         
    {
        card1 = card1 / 10 ;
        counter++ ;
    }

    if (counter != 15 && counter != 16)    
    {
        printf("INVALID\n") ;
    }

    int cc[counter];
    for (int i = 0; i < counter; i++)        
    {
    cc[i] = card % 10 ;
    card = card / 10 ;
    }

    int cc1[counter];
    for (int i = 0; i < counter; i++)
    {
        cc1[i] = cc[i];
    }

    for (int i = 1; i < counter; i += 2)    
    {
        cc1[i] =  cc1[i] * 2 ;
        if (cc1[i] >= 10)
        {
            for (int j = 0; j < 2; j++) 
            {
                int first_degit = cc1[i] / 10 ;
                int secnd_degit = cc1[i] % 10 ;
                cc1[i] = first_degit + secnd_degit ;
            }
        }
        sum = sum + cc1[i] ;
    }

    for (int i = 0; i < counter; i += 2)        
    {
        sum1 = sum1 + cc1[i] ;
    }

    int sum2 = sum1 + sum ;
    sum2  = sum2 % 10 ;

    if (counter == 15)      
    {
        if (sum2 == 0)
        {
            if (cc[14] == 3 && cc[13] == 7)
            {
                printf("AMEX\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else
        {
            printf("INVALID\n");
        }
    }

    if (counter == 16)      
    {
        if (sum2 == 0)
        {
            if ((cc[15] == 5 && cc[14] == 5) || (cc[15] == 5 && cc[14] == 1) || (cc[15] == 5 && cc[14] == 3))
            {
                printf("MASTERCARD\n");
            }
            else if (cc[15] == 4)
            {
                printf("VISA\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else
        {
            printf("INVALID\n");
        }
    }
}
unsigned long long get_long_long(string prompt)
{
    long long n ;
    do
    {
        n = get_long("%s", prompt) ;
    }
    while (n < 1) ;
    return n ;
}

Answer 1

While your program will likely pass the automated tests of C50, in terms of design, you could think of the following:

1. As per the problem description,

Visa uses 13- and 16-digit numbers

Your program seems to consider a 13-digit number invalid

2. You may not necessarily need to duplicate card into card1 and run them digit-to-digit twice to do the checks. Surely, it is fast anyway, but you asked about design. You get card number as long long int anyway, so you could just say something like:

    if (card < 999999999999 || card > 9999999999999999 ||
         (card > 9999999999999 && card < 99999999999999))
    {
        printf("INVALID\n");
        return 1;
    }

and be done with it in terms of number of digits.

3. In terms of VISA / MC/ AMEX detection, a series of if statements may do the job, but a somewhat more elegant solution (after all other checks) could be a switch statement like this:

    if (last_digit == 4)
    {
        printf("VISA\n");
        return 0;
    }

    int digits = last_digit * 10 + prev_digit;

    switch (digits)
    {
        case 34:
        case 37:
            // Check 15 digits
            // ...
            printf("AMEX\n");
            break;
            // Otherwise invalid
            // ...
        case 51:
        case 52:
        case 53:
        case 54:
        case 55:
            // Check 16 digits
            // ...
            printf("MASTERCARD\n");
            break;
            // Otherwise invalid
            // ...
        default:
            printf("INVALID\n");
            return 1;
    }

This allows you to add more number checks (MC actually uses some of the 2* range).