how do I make the computer think a button is pressed?

[DanielG]

can someone tell me how to make the computer think a button or the mouse is pressed. So for example, have it so that while the programming is running, if you press on the button 'n' it might think you pressed 'r' at the time. Can someone tell me how to do this in preferably C++ or Turing. Though if it's possible and easy to do it with another language then thats fine.
Thanks.