35 — Todo list: adding, dropping, listing

Every program so far asked once and stopped. A program people actually use keeps taking commands until it is told to stop. One list and one loop that keeps going is all it takes.

#Steps

1. A loop with no count is repeat forever. Getting out needs break:

   repeat forever
       ask command say quit to stop
       if command equals quit
           break
       end
       show got it
   end
   show that is everything
   

   copyrun it →

   Without the break the program never ends.

2. Adding is one line into a list:

   set jobs to an empty list
   append buy milk to jobs
   append read a book to jobs
   show how many jobs
   show jobs joined by comma
   

   copyrun it →

   2, then buy milk, read a book.

3. Dropping something that is not there stops the program, so ask first. Put what you are looking for in a name — something written as several words cannot go straight into a condition:

   set jobs to an empty list
   append buy milk to jobs
   set wanted to buy milk
   if jobs contains wanted
       remove wanted from jobs
   else
       show there is no such job
   end
   show how many jobs
   

   copyrun it →

   You get 0.

4. Four commands means three else ifs. The last else is the way out:

   set command to drop
   if command equals add
       show adding
   else if command equals drop
       show dropping
   else if command equals list
       show showing
   else
       show stopping
   end
   

   copyrun it →

5. The whole thing:

   set jobs to an empty list
   repeat forever
       ask command one of add, drop, list, quit
       if command equals add
           ask job what needs doing
           append job to jobs
       else if command equals drop
           ask job what is done
           if jobs contains job
               remove job from jobs
           else
               show there is no such job
           end
       else if command equals list
           show how many jobs
           show jobs joined by comma
       else
           break
       end
   end
   show that is everything
   

   copyrun it →

#Try it yourself

Make list show one job per line — inside for each job in jobs with place, showing the place and the job gives you a numbered list. Closing the program loses everything; keeping it in a file is what guide 37 is for.

#What you learned

• repeat forever runs until it meets break. Always leave a way out.
• Several commands means else if in a row, with the last else as the exit.
• Asking whether something is there before dropping it is what keeps the program running.
• One list is enough to be a program — it just disappears when you close it.