CS50 credit doesn't run for some numbers - c

Question

My code seems to work on some credit card numbers but for others it doesn't even run. I've tried to use long long for the credit card number but it didn't worked. I've spent like 2 hours trying to solve this issue but I can't figure it out. All help is welcomed :)
int main(void)
{
// Get credit card number
long num = get_cc_number();
// Check if the credit card number is valid
valid = check_sum(num);
// Check the length of the credit card number
length = check_length(num);
// Get the first two digits of the credit card number and first number of VISA
digits = get_first_digits(num);
digit_visa = digits / 10;
// Check if the card is American Express, Mastercard, Visa or Invalid
if (valid == 1)
{
if (length == 16)
{
if (digits <= 55 && digits >= 51)
{
printf("MASTERCARD\n");
}
else if (digit_visa == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
else if (length == 15)
{
if (digits == 34 || digits == 37)
{
printf("AMEX\n");
}
else
{
printf("INVALID\n");
}
}
else if (length == 13)
{
if (digit_visa == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
}
else
{
printf("INVALID\n");
}
}
long get_cc_number(void)
{
long cc_number;
cc_number = get_long("Credit Card Number: ");
return cc_number;
}
int check_sum(int num)
{
int num1 = num;
while (num1 >= 10)
{
sec_to_last = num1 % 100;
double_sec_to_last = sec_to_last * 2;
if (double_sec_to_last >= 10)
{
first_dig = double_sec_to_last / 10;
sec_dig = double_sec_to_last % 10;
first_sum += first_dig;
first_sum += sec_dig;
}
else
{
first_sum += double_sec_to_last;
}
num1 = num1 / 100;
}
int num2 = num;
while (num2 >= 10)
{
last = num2 % 10;
second_sum += last;
}
second_sum += first_sum;
if (second_sum % 10 == 0)
{
return 1;
}
else
{
return 0;
}
}
long check_length(long num)
{
long num_length = floor(log10(labs(num))) + 1;
return num_length;
}
long get_first_digits(long num)
{
long i = num;
while (i >= 100)
{
i /= 10;
}
return i;
}

Answer 1

A while back, I reviewed an issue like this where the user was getting tripped up on acquiring a credit card check digit. With that, I wrote a small proof-of-principle test program that allows validation of credit card numbers using the Luhn algorithm. Following, is that code snippet.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int check_valid(char * num)
{
int sum = 0;
int work = 0;
char card[20];
if ((strlen(num) %2 == 0)) /* Even numbers - do not need a leading zero */
{
strcpy(card, num);
}
else /* Odd numbers - add a leading zero to evaluate */
{
strcpy(card, "0");
strcat(card, num);
}
printf("Length of number is: %d\n", (int)strlen(num));
for (int i = 0; i < strlen(card); i++)
{
work = card[i] - '0';
if ((i %2) == 0)
{
work = (card[i] - '0') * 2;
if (work > 9)
{
work = work - 9;
}
}
sum = sum + work;
printf("Digit is: %d Value is: %d Sum is %d\n", (card[i]- '0'), work, sum);
}
return ((sum % 10) == 0);
}
int main()
{
char number[20];
int x = -1;
printf("Enter a number: ");
x = scanf("%s", number);
x = check_valid(number);
if (x == 0)
printf("Invalid\n");
else
printf("Valid\n");
return 0;
}
It doesn't identify the card issuer, just verifies that the number is valid.
As noted in the comments, one probably would want to utilize a string entry as this code snippet does rather than trying to utilize a very large integer. You might try going that route with a string as well. Give this a try and see if it allows you to progress.