how to iterate over a number to get every other digit - c

Question

For a homework program we are asked to check if credit card numbers are valid.
While I am fairly sure about how I want to solve this problem I am stuck with iterating over a creditcard number and getting every second digit.
I already know that I need to use modulo and division and I do get the second to last number, but I am lost at how to iterate over the rest of the numbers.
In the code is the code I used to check that I did indeed get the second to last number, and after that some code I tried to use to iterate over the complete number. Hope somebody could show me in the right direction!
long number = 378282246310005;
int digit;
int cc_number;
cc_number = number % 10;
digit = cc_number / 10;
printf("%i\n", digit);
while (number > 0)
{
digit += number % 10;
number /= 10;
printf("%li", number);
}
printf("\n");

Answer 1

Before your while loop you reversed the last digit and the remaining number.
number % 10 is the last digit, number / 10 is the remaining number.
Adding to digit doesn't make sense if the variable is intended to contain a single digit. (You may need to add the digits for a validity check.)
Example code to extract all digits and their positions:
#include <stdio.h>
int main(void)
{
long number = 378282246310005;
int digit;
int position = 0;
printf("%li\n", number);
while (number > 0)
{
digit = number % 10;
number /= 10;
printf("(%i) %i ", position, digit);
position++;
}
printf("\n");
return 0;
}
This prints
378282246310005
(0) 5 (1) 0 (2) 0 (3) 0 (4) 1 (5) 3 (6) 6 (7) 4 (8) 2 (9) 2 (10) 8 (11) 2 (12) 8 (13) 7 (14) 3
To select every other digit you could check position % 2 or toggle a variable between 0 and 1 in every loop cycle.
Edit:
Explanation as requested in ron_g's comment: "Why's it printing out the digits in reverse?"
The loop separates the last digit from the remaining part of the number, so it prints the last digit first. Then it repeats the same with the remaining part as long as the remaining number is not 0.
The answer keeps the original implementation from the question which is based on storing the number as a long int. In this case it is easier to start with the last digit. When the number would be stored as a string it would be easier to start with the first digit.
/* number is initially the whole number or the remaining part */
while (number > 0)
{
/* remainder of division by 10 is the last digit */
digit = number % 10;
/* division by 10 is the remaining part after cutting off the last digit */
number /= 10;
printf("(%i) %i ", position, digit);
position++;
}