title: do something special when user input empty line
body: <p>I'm just starting to study C. I have a program that prints a menu and let users choose what to do step by step. Now I would like to return to the main menu whenever the user enters an empty line, but how can I do that?<br>
  I think I can make a function that return the program to the main menu, but when to call that function? I know it's not good to put an if-else whenever I <code>scanf</code> something...</p>
  
  <p>I am used to the OO world, so this is a bit unfamiliar to me, please help :)</p>