CS50 Credit: Always getting INVALID return - cs50

Question

Was attempting Credit from CS50 and kept getting INVALID return from my code. I approached this problem by using arrays even though it may not have been the best method. Code compiles with no issues.
My pseudocode logic was:
1) obtain card number
2) use loop to find number of digits
3) check if card contains 13, 15 or 16 digits
4) if so, write digits from long into array
5) have a copy of original array to multiply every other number by 2
6) add the digits of the product
7) check for card length and starting digits
Here is my code:
#include <stdio.h>
#include <cs50.h>
int main(void)
{
// Get credit card number
long num = get_long("Number: ");
// Find number of digits
int digits = 0;
while (num > 0)
{
num /= 10;
digits++;
}
// Check if number of digits is within possible range
if (digits != 13 && digits != 15 && digits != 16)
{
printf("INVALID\n");
}
int originalnumber[digits];
// Write each digit of credit card number into an array
for (int i = digits - 1; i >= 0; i--)
{
originalnumber[i] = num % 10;
num = num / 10;
}
// Multiply alternate digits by 2
int number[digits];
for (int i = 0; i < digits; i++)
{
number[i] = originalnumber[i];
}
for (int i = 1; i < digits; i+=2)
{
number[i] = number[i] * 2;
}
// Add product digits
int sum = 0;
int temp;
for (int i = 0; i < digits; i++)
{
temp = (number[i] % 10) + ((number[i] / 10) % 10);
sum = sum + temp;
}
// Check for card length and starting digits
// AMEX
if (digits == 15)
{
if (originalnumber[14] == 3 && sum % 10 == 0 && (originalnumber[13] == 4 || originalnumber[13] == 7))
{
printf("AMEX\n");
return 0;
}
}
// MasterCard
if (digits == 16)
{
if (originalnumber[15] == 5 && sum % 10 == 0 && (originalnumber[14] == 1 || originalnumber[14] == 2 || originalnumber[14] == 3 || originalnumber[14] == 4 || originalnumber[14] == 5))
{
printf("MASTERCARD\n");
return 0;
}
}
// Visa
if (digits == 13)
{
if (originalnumber[12] == 4 && sum % 10 == 0)
{
printf("VISA\n");
return 0;
}
}
if (digits == 16)
{
if (originalnumber[15] == 4 && sum % 10 == 0)
{
printf("VISA\n");
return 0;
}
}
printf("INVALID\n");
return 1;
}
I tried debug50 and it seems that when I try to sum the digits together using temp and sum, the loop completes with sum still being 0. May I know what is wrong here? Is the flow of my pseudocode wrong or are there any glaring mistakes that I may have overlooked? (stared at this for way too long..)
Thank you in advance!

Answer 1

If sum is always 0, regardless of whether that is what you expect, sum % 10 would always be 0, so that is not the "false" that is failing the tests.
Which should direct your attention to originalnumber.
What is the value of num after this loop?
while (num > 0)
{
num /= 10;
digits++;
}