The program is stuck in an infinite loop. Look at the while loop setup:
while(credit_number >= 0) // runs the loop until the credit card number is l
i = credit_number % 10; credit_number = credit_number - i;
The first pass subtracts the least significant digit from the cc number, so 371449635398431 becomes 371449635398430. From there on, i is ...
You make a new file by clicking the green plus sign at the top, next to where it says the name of the file you are currently on. On your keyboard, then do command shift S. It will give you options for where to save the file and what the name should be. Name it pseudocode.txt and put it in your mario folder. If this helps, click the check mark or upvote it.
That example doesn't say what data type x and y are, but generally you can assume that numerical variables are real numbers in programming, unless specified otherwise.
There is a way to work with complex numbers in C, but you'd only use it for applications specific to complex numbers. Also, if x and y were complex, x < y isn't defined in the standard ...