This is not as simple as it sounds. The scanf and if statement method is about the only simple way you're gonna do this. You would need these in an infinite loop, constantly prompting the user for input. This is not "monitoring" as you put it, simply prompting the user and waiting. I'm sure I understand what you're asking, that is for the code to monitor the keystrokes in the background, so to speak, while the program does other things.
To do what you want is not as easy as it sounds... sorry.