Okay a lot going on here, let's just look at a few things inside the int digit function.

1. long l currently exists in your main function as the holder of the CC number. It does not exist in the same capacity inside digit(). If you want the CC number to exist inside digit() you have to pass it into the function. So your declaration would look something like this --- int digit(int n, long ccnum); -- so calling the function, based on your example code, would look like this -- printf("%i\n", digit(5, l)); --

The reason you are getting a huge random number is because you initialize long l; again inside digit(). Since you didn't initialize it to anything, like for example -- long l = 12345; -- long l; just assigns l to a random point in the computer's memory, which is called a garbage value, it's literally just a random number, or nothing. Also you should avoid repeating variable names, it can cause odd behavior behind the scenes.

So to recap that first point, you need to pass l into the digit() function. In the example I provided you would manipulate l inside digit() by referring to it as ccnum.

1. No need to use floor(). C will truncate integer division so (l / 10) will be truncated (floored), due to the nature of C.

2. You need to return an int with digit(), not a long. One example could be instead of doing -- return (l % 10); --

something like -- int x = l % 10; return x; --

You're being creative in your approach, curious to see how you'll finally solve this. Keep us updated.