cs50: pset1: credit: Stuck, all cards entered invalid - c

Question

Been working on this problem set for the past few hours and I thought I was doing pretty well, but when I ran it, doesn't really work as expected.
Kinda stuck at the moment, can anyone offer some insight?
EDIT
Sorry guys, I'll provide a little more insight on the problem because too little info is provided for it in my post. So basically I have to implement the Luhn's algorithm. According to Luhn’s algorithm, you can determine if a credit card number is (syntactically) valid as follows:
Multiply every other digit by 2, starting with the number’s second-to-last digit, and then add those products’ digits together.
Add the sum to the sum of the digits that weren’t multiplied by 2.
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!
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
printf("type a credit card number: ");
char ccnum[17];
fgets(ccnum, 17, stdin);
int ccnumber[15];
int n = strlen(ccnum);
for (int i = 0; i<16; i++)
{
ccnumber[i] = ccnum[i] -'0';
}
int ccnumber_m[7];
for (int i = 0; i < 16; i+=2)
{
ccnumber_m[i] = ccnumber[i]*2;
}
int sum = 0;
for (int i = 0; i < 8; i++)
{
if (ccnumber_m[i] > 9)
{
sum += (ccnumber_m[i] % 10) + 1;
}
else
{
sum += ccnumber_m[i];
}
printf("%d\n", sum);
}
for (int i = 1; i < 16; i+=2)
{
sum += ccnumber[i];
}
if (sum%10 == 0)
{
if ( (n == 15) && (ccnumber[0] == 34 || ccnumber[0] == 37))
{
printf("AMEX\n");
}
else if ((n == 16) && (ccnumber[0] == 51 || ccnumber[0] == 52 ||ccnumber[0] == 53 || ccnumber[0] == 54 || ccnumber[0] == 55))
{
printf("MASTERCARD\n");
}
else if ((n == 13 || n == 16) && ccnumber[0] == 4)
{
printf("VISA\n");
}
}
else
{
printf("INVALID\n");
}
}
I've added a printf to my sum variable to see where the program is failing and that's where things seem to be screwing up. Every time I run it, different values of sum appear?!
Thanks in advance.

Answer 1

Okay guys, found the solution. Yes I messed up most of my loops and the array ranges and the conditions were all over the place. Did some tweaks to my code overall and after a few more hours, the code works as expected. Here's the updated code.
Pretty sure the code could be more succinct and could be solved much faster. Will take a look at it tomorrow again in order to brainstorm more ideas and find ways to solve it faster.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
printf("type a credit card number: ");
char ccnum[17];
fgets(ccnum, 17, stdin);
int ccnumber[15];
int n = strlen(ccnum);
int counter = 0;
for (int i = 0; i < n; i++)
{
ccnumber[i] = ccnum[i] -'0';
if (ccnumber[i] >= 0)
{
counter ++;
}
}
int sum = 0;
if (counter == 16)
{
int ccnumber_m[7];
int index = 0;
for (int i = counter - 2; i > -1 ; i-=2, index++)
{
ccnumber_m[index] = ccnumber[i]*2;
}
for (int i = 0; i < 8; i++)
{
if (ccnumber_m[i] > 9)
{
sum += (ccnumber_m[i] % 10) + 1;
}
else
{
sum += ccnumber_m[i];
}
}
for (int i = 1; i < counter + 1; i+=2)
{
sum += ccnumber[i];
}
}
else if (counter == 15)
{
int ccnumber_m[6];
int index = 0;
for (int i = counter-2; i > 0; i-=2, index++)
{
ccnumber_m[index] = ccnumber[i]*2;
}
for (int i = 0; i < 7; i++)
{
if (ccnumber_m[i] > 9)
{
sum += (ccnumber_m[i] % 10) + 1;
}
else
{
sum += ccnumber_m[i];
}
}
for (int i = 0; i < counter + 1; i+=2)
{
sum += ccnumber[i];
}
}
else if(counter == 13)
{
int ccnumber_m[5];
int index = 0;
for (int i = counter-2; i > 0; i-=2, index++)
{
ccnumber_m[index] = ccnumber[i]*2;
}
for (int i = 0; i < 6; i++)
{
if (ccnumber_m[i] > 9)
{
sum += (ccnumber_m[i] % 10) + 1;
}
else
{
sum += ccnumber_m[i];
}
}
for (int i = 0; i < counter + 1; i+=2)
{
sum += ccnumber[i];
}
}
else
{
printf("INVALID\n");
return 0;
}
if (sum%10 == 0)
{
if ((counter == 15) && ((ccnumber[0] == 3 && ccnumber[1] == 4) || (ccnumber[0] == 3 && ccnumber[1] == 7)))
{
printf("AMEX\n");
}
else if ((counter == 16) && ((ccnumber[0] == 5 && ccnumber[1] == 1) || (ccnumber[0] == 5 && ccnumber[1] == 2) || (ccnumber[0] == 5 && ccnumber[1] == 3) || (ccnumber[0] == 5 && ccnumber[1] == 4) || (ccnumber[0] == 5 && ccnumber[1] == 5)))
{
printf("MASTERCARD\n");
}
else if ((counter == 13 || counter == 16) && (ccnumber[0] == 4))
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
return 0;
}
}
else
{
printf("INVALID\n");
return 0;
}
}