LOGO-L> UNDO -- third try

["Charles T. Smith" <mrchuc@math.fan.org>]

Hello all,

{Sorry if this is the third time you have seen this post. I keep getting an
"unable to deliver" message. Apparently, there are two messages floating
out there waiting to be delivered. I am afraid they will all come to the
list in a bunch. If so, my apologies, but I am sending this again since I
am still receiving list messages from other people.}

At 04:01 AM -0700 4/13/98, bh@anarres.CS.Berkeley.EDU wrote:
>The trouble I have with
>undo is that I'm not sure how much to undo.  George is assuming,
>as I would, that the person who asked for undo wants to undo a
>line drawn on the screen.  But that's very different from "unto
>the last Logo instruction."
>

Brian brings up a good point. I know of two versions of Logo, Turtle Math
and PowerMath Logo, that have an undo procedure built in. Both implement it
differently.

Turtle Math from LCSI has a button that erases the last line in the command
center and then redraws the screen. Thus if the last line was a graphic
command, the effect is to erase the last line, turn, etc. If the last
command was an assignment, then the effect is to "unassign" the variable.
If the last command was a user defined procedure, the procedure is treated
the same as a primitive. The best use of the undo button is when kids are
experimenting to find the right angle or distance. It works very
cleanly--especially since the command center stays current. It makes it
very simple for kids to fine tune their procedure in the command center and
then copy the commands to the editor.

PowerMath Logo takes a different approach. It provides a "rewind" and a
"forward" button (which can be enabled or disabled.) Clicking the rewind
button only has an effect on the graphic screen, however. It has no effect
on the command center. It undoes the last graphic command given. If the
last command was a user defined procedure it treats each command of the
procedure separately.  Like Turtle Math, it is great for kids who want to
fine tune their drawings. I have also found it useful for demonstrating
things. Using the rewind and forward buttons, I can draw and undraw shapes
quickly, pausing where I need to.

With both of these, there is some overhead. Turtle Math has a slight pause
(very slight on my 040 machines) while the screen redraws. PowerMath does
not have any slowdown at all, but there is a memory hit. Enabling the
feature requires some extra memory, but not much. I run it on some old Mac
SE's with only 2.5 megs of memory without any trouble. I have run PowerMath
on a plus with only 1 meg, but I don't remember if I was able to use the
rewind feature.

I discussed this feature a couple of years ago (before I had had a chance
to use it much) with Michael Tempel. He pointed out then that it could be
implemented in Logo easily by keeping all the commands in a Logo list and
then butlast-ing the list and running it again.

I really like this feature. I would really like to see it in other versions
of Logo. I think it relieves a lot of frustration for kids, especially for
beginners. (Hint, hint!) :-)

Repeat forever [learn]

Chuc



Charles T. Smith                Math Teacher and Logo Maniac
535 Cap Au Gris                 Wright City Middle School
Troy, MO 63379                  #402 North Service Road, P.O. Box 198
314-528-7074                    Wright City, MO 63390
mailto:MrChuc@math.fan.org      http://math.fan.org/~turtle_family


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