Coding with Ken-Part 2

A MOO Programming Tutorial
for Absolute Beginners
Part 2

Hi!  I'm Ken. I'm here to help you learn to program in MOO!
This tutorial is for Absolute Beginners, no previous
programming experience in any language is assumed!"

Last time we met we built a Box of Donuts.

This time we will elaborate the code for this object and learn
to use the MOO Editor.  Let's look again at our original
program:

@create $thing named Box of Donuts, donuts
@property donuts.kinds {"tasty apple turnover", "yummy
   mooberry croissant", "shiny glazed donut"}

@verb donuts:eat this

@program donuts:eat
donut_choice = this.kinds[random(3)];
player.location:announce_all(player.name," gobbles up a
   ",donut_choice,".");

Before we continue please check your inventory (inv) to be
sure you have already created a Box of Donuts.  If you
haven't please review the first tutorial tape where you
will find directions for creating it.

Today we will make our donut object more dynamic.  We started
out with 3 donuts, but in our original program we could eat
and eat and never empty the box. Lets fix that rather
uncharacteristic behavior.

We will change the first line of the program to
read:
donut_choice = this.kinds[random(length(this.kinds))];

Note that we have replaced the number 3 with
'length(this.kinds)'.  This is because the number of donuts
in the box will vary.  'length(this.kinds)' gives the number
left.

In order to effect this change we will use the MOO Editor. We
are going to invoke the editor, list our  original program,
make a text substitution, compile the new code, and exit the
Editor. Whew!

I can't help you once you are 'in' the Editor so write down
these commands:
 @edit donuts:eat
 list
 s/3/length(this.kinds)/1
 compile
 q        (stands for 'quit editor')

The second command means 'substitute length(this.kinds) for 3
in line #1'.   And 'q' quits the editor.

Once you are in the editor you can see the available editor
commands by typing 'look'.  If you type 'help <command name>'
you can get help on a specific command.

To see if you were successful, type:
   @list donuts:eat
Do you see the changes?

If you were unsuccessful just keep mucking with the editor
until you get it to work.  The editor is NOT easy to learn
and mistakes are inevitable.. keep trying.

It is worth while spending some time mucking around with the
MOO Editor.  It's a line editor and a bit quirky but if you
program a lot you'll find yourself using it frequently.  

Our next step is to build some logic into the program:  IF
(there are donuts left in the box) THEN (let the player take
one and eat it) ELSE (tell the player they're all gone).

In MOO code the general form for  such an 
IF/THEN/ELSE statement is:
if (x is true) (do y) else (do z)  endif
Study the pattern in the following code:

if  (this.kinds)
donut_choice = this.kinds[random(length(this.kinds))];
player.location:announce_all(player.name," gobbles up a
   ",donut_choice,".") ;
else
player.location:announce_all(player.name," reaches for a
   tasty donut, but the box is empty!");
endif

In MOO code the expression (this.kinds) is considered to be
TRUE if there are 1 or more items in the list, FALSE if the
list is empty. 

Now we need to use the editor again to enter the new lines.
When you renter the editor type 'list' and  you will see the
current program.  We want to insert the line 'if (this.kinds)'
at the very beginning.

To do this we must set the 'insertion point' of the editor to
be above line #1.  You must type:
   ins  ^1
to do this.  Then enter the correct line prefixed by a quotation
mark " , as if you were 'speaking' to the editor."

Once again, if you had troubles try again.  The Editor can be
exasperating at times until you get used to it.

Now that you have a feel for the editor figure out where to
insert the rest of the new lines.  If you want to set the
insertion point after a line, type 'ins _6'. Don't forget to
'compile' and q(uit).

If you have tried out the new verb you may have noticed that
we can STILL eat all the donuts we want, the box never seems
to get empty!  Let's fix that now.

We need to add the following line just after getting the
donut_choice:

this.kinds = setremove(this.kinds, donut_choice);

This new line removes the eaten donut from the list of
donut kinds - that donut is no longer in the box  and
the length of 'donuts.kinds' decreases by 1.

Finally, we need a way to refill the box of donuts when
it gets empty.  Lets make a verb BAKE DONUTS.

I will give you the complete code:  Use the skills you
have learned to practice entering the program:

@verb donuts:bake this
@program donuts:bake
this.kinds = {"tasty apple turnover","yummy mooberry
   croissant", "shiny glazed donut"};
player.location:announce_all(player.name, " bakes up a new
   batch of fresh donuts!");
.

If everything worked out ok, you should now have a refillable
box of donuts.  Try it out. Eat donuts until they're all
gone.  Then try baking up some more!   Oooh don't get sick!

Remember to save one of those donuts for me!

Next time we will introduce some more fun features of the
MOO language. Until then, HAPPY MOOING!"