title: Menu using while-loops
body: <p>How would you implement a menu where the user has to pick the first option before continuing? I'm thinking of using a while loop where:</p>
  
  <pre><code>    menu= '''0 - enter number
      1 - do something with number
      2 - do something else
      3 - do something else
      4 - quit'''
  
      user_option = ()
      while user_option!=4:
          elif user_option==0:
              num = int(input("What is your number? "))
          elif user_option == 1: 
                do something
          elif user_option==2: 
                do something
          elif user_option==3: 
                do something
  </code></pre>
  
  <p>basically I want to figure out a way to have a way where if user picks option 1 2 or 3, then it would say "pick option zero first" and if it picks option zero first then the program would work.</p>