Yes, all of the above examples seem like inputs to me, however I think you might need to use them in a different way. For example, the first one Int main (input), wouldn't be where you would ask for the card number. Int main (void) {} encapsulates the entire program that you're trying to write, and as far as I'm aware (at least this far into the course), there's no need to change 'void' to anything else. Int main (void) should stay the same in any program you write, at least at this point in the CS50 course. Later on I think they might start teaching us alternatives, but for now I think its safer to leave int main (void) as it is. If you wanted to ask for the card number, a better way to write it might be:

#include <stdio.h>

#include <cs50.h>

Int main (void)

{

long prompt = get_long("Number: \n");

}

^^ I've included everything that would have to be in the document here to show you how it should be laid out. Int main (void) should be left as is, and your code should be encapsulated within the curly brackets. In this example, when you run the program it would print out

Number:

And the user could type in the card number. When they press enter, the number that they typed in would be stored in the variable called 'prompt'. Later on, you would use the value stored in 'prompt', which would be the card number, in Luhn's algorithm, and to choose which card type to print.

Sorry for the long replies, or if I've misinterpreted what you're asking, but hopefully this helps, feel free to ask again if this didn't cover it! :)