curious language

[mwforum@xxxxxxxxxxxxxxxxxx (Jeff Knope)]

Hi Jim,

I would say Mr. Papert's procedure is somewhat curious.  But we can use it to
answer your question of how the program stops.

First, there are some sytax errors that have to be fixed:

There is a missing opening bracket in the "ifelse" statement. An "ifelse" must
be followed by two lists. One that tells it what to do if the condition (:n
= 0) is true; the other what to do if the condition is false.  It should
read:

        ifelse :n = 0 [fd :x][repeat 4 [ el :x / 3 (:n - 1) rt 90]]

Note also the added spaces in the (:n-1) expression.  These spaces are
required.

Now, to help make visible what is happening, I suggest the following:

    A. Add to this line instructions to print the values of :x and :n in the
control console at each iteration, as follows:
        
        ifelse :n = 0 [fd :x][repeat 4 [ el :x / 3 (:n - 1) show :x show :n
rt 90]]

    B. Go to the Processes Tab, and click the yellow "speed" button.  This
will slow down the running so you can watch the turtle's behavior.

    C. Issue the commands, cg st pd <ret>.  This clears the graphics screen,
shows the turtle, and puts the pen down.

Now issue el 500 3 <ret> and watch the procedure run in slow-motion.  Review
the outputs in the control console.

Basically, the procedure is in the "repeat" statement (the "else" of the
"ifelse") until :n = 0.  The procedure stops when :n = 0 because at that
point the instruction is to simply fd :x and the entire "ifelse" is satisfied.

I hope this helps.
Jeff




----- Original Message ----- 
From: <jbonnes@xxxxxxxxxxxxxxxx>
To: <mwforum@xxxxxxxxxxxxxxxxxx>
Sent: Wednesday, March 16, 2005 8:18 AM
Subject: [MWForum]curious language


> 
> Thanks Everyone,
> I also just learned that Logo programs can call themselves, Seymour Papert
just
> gave me this program and now I'm trying to understand it.
> to el :x :n
> ifelse :n = 0 [fd :x]
> repeat 4 [ el :x / 3 (:n-1) rt 90]]
> end
> It doesn't do what I want it to, but to me this is intriguing because coming
> from C a program can't call itself. I'v been trying to figure what's going
on.
> The program draws a square and then fills it up with four more squares, if
you
> input :n as 2. How does the program stop.
> Jim Bonnes
> 
> 
> _______________________________________________
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> http://mia.openworldlearning.org/mwforum/
> Attachments archived at:
> http://www.mathcats.com/mwforum/attachments.html
> To unsubscribe or for administrative questions contact 
> mailto:mwforum-admin@xxxxxxxxxxxxxxxxxx
>

Hi Jim,

 

I would say Mr. Papert's procedure is somewhat curious.  But we can use it to answer your question of how the program stops.

 

First, there are some sytax errors that have to be fixed:

 

There is a missing opening bracket in the "ifelse" statement. An "ifelse" must be followed by two lists. One that tells it what to do if the condition (:n = 0) is true; the other what to do if the condition is false.  It should read:

 

        ifelse :n = 0 [fd :x][repeat 4 [ el :x / 3 (:n - 1) rt 90]]

 

Note also the added spaces in the (:n-1) _expression_.  These spaces are required.

 

Now, to help make visible what is happening, I suggest the following:

 

    A. Add to this line instructions to print the values of :x and :n in the control console at each iteration, as follows:

       

        ifelse :n = 0 [fd :x][repeat 4 [ el :x / 3 (:n - 1) show :x show :n rt 90]]

 

    B. Go to the Processes Tab, and click the yellow "speed" button.  This will slow down the running so you can watch the turtle's behavior.

 

    C. Issue the commands, cg st pd <ret>.  This clears the graphics screen, shows the turtle, and puts the pen down.

 

Now issue el 500 3 <ret> and watch the procedure run in slow-motion.  Review the outputs in the control console.

 

Basically, the procedure is in the "repeat" statement (the "else" of the "ifelse") until :n = 0.  The procedure stops when :n = 0 because at that point the instruction is to simply fd :x and the entire "ifelse" is satisfied.

 

I hope this helps.

Jeff

 

 

----- Original Message -----

From: <jbonnes@xxxxxxxxxxxxxxxx>

To: <mwforum@xxxxxxxxxxxxxxxxxx>

Sent: Wednesday, March 16, 2005 8:18 AM

Subject: [MWForum]curious language

> 
> Thanks Everyone,
> I also just learned that
Logo 
programs can call themselves, Seymour Papert just
> gave me this
program 
and now I'm trying to understand it.
> to el :x :n
> ifelse :n =
0 
[fd :x]
> repeat 4 [ el :x / 3 (:n-1) rt 90]]
> end
> It 
doesn't do what I want it to, but to me this is intriguing because 
coming
> from C a program can't call itself. I'v been trying to figure 
what's going on.
> The program draws a square and then fills it up
with 
four more squares, if you
> input :n as 2. How does the program 
stop.
> Jim Bonnes
> 
> 
> 
_______________________________________________
> MWForum mailing 
list
> MWForum@xxxxxxxxxxxxxxxxxx
> 
http://mia.openworldlearning.org/mwforum/
> Attachments archived at:
> http://www.mathcats.com/mwforum/attachments.html
> To unsubscribe or for administrative questions
contact 
> mailto:mwforum-admin@xxxxxxxxxxxxxxxxxx
>