alice-teacher Passing an object to a method doesn't appear to work completely in Alice 2.3.2

[Marty Schultz]

Don,

 

Thanks, once you mentioned it was the threading issue, it all made sense to
me.

 

There's no easy way to explain that to students at their level of
programming understanding, and it's probably nontrivial for Alice to detect
that condition on-the-fly (unstable value within a variable).  Is there a
forum or other repository for things like this - maybe a BEST PRACTICES or
knowledge base?

 

Thanks.

 

From: alice-teachers-bounces+schultz_marty=hotmail.com at lists.andrew.cmu.edu
[mailto:alice-teachers-bounces+schultz_marty=hotmail.com at lists.andrew.cmu.ed
u] On Behalf Of Don Slater
Sent: Monday, April 15, 2013 2:19 PM
To: Alice educators
Subject: Re: alice-teacher Passing an object to a method doesn't appear to
work completely in Alice 2.3.2

 

I asked Marty to send me a copy of his code. This is my reply him, but it
demonstrates a problem using DoTogether which happens frequently, and is
very difficult to overlook and track down.

 

See the attached screen shot, from the method "world.KeepScore":

 

The code of concern is this doTogether, in which you are (1) setting the
value of the variable 'animalThatWasHit',  (2) printing that value, and (3)
sending that value as a parameter, AT THE SAME TIME.

 

A do together generates separate, independent threads for each statement in
the doTogether body, and so what is happening in one statement may or may
not, depending on the timing, have any impact on the other threads,
particularly if they are trying to use the same variable.

 

So it is not so much that the value reverts to paintball, but that you print
as the value is being set. If you had printed it again, after the
doTogether, you would have seen the correct result.

 

I dissolved the doTogether and got the following output after running the
program:

 

            tiger

            none

            tiger

            none

            tiger

 

I hope that this helps. 

 

This is a very difficult bug to track down, one that even experienced Alice
programmers. will make. It is easy to overlook (you did not even think to
include the doTogether's in your pseudocode, and yet they are critical) and
not think about.

 

Look at how different your pseudocode looks if you had written it this way

 

Method world.keepScore looks similar to:

                        DoTogether



                // precondition: Local variable animalThatWasHit
(type=OBJECT, initial value set to paintball)

 

                20 animalThatWasHit set value to world.whoGotHit

                20 print animalThatWasHit

                20 world.whenHit target=animalThatWasHit

 

All the best,

Don Slater

 



 

 

On Apr 14, 2013, at 9:48 PM, Marty Schultz <schultz_marty at hotmail.com>
wrote:





Does anyone have a solution for this (I've included pseudo line numbers to
make it clear):

 

Function world.whoGotHit (returns type OBJECT) ends with:

                10 print "world.whoGotHit"

                11 print tiger.name

                12 return tiger

 

 

Method world.keepScore looks similar to:

                20 Local variable animalThatWasHit (type=OBJECT, initial
value set to paintball)

 

                22 animalThatWasHit set value to world.whoGotHit

                23 print animalThatWasHit

                24 world.whenHit target=animalThatWasHit

 

Method world.whenHit

                30 parameter target (type=OBJECT)

                32 print "world.whenHit"

                33 print target

 

 

When I PLAY this program, the PRINT statements show:

                world.whoGotHit (from line number 10)

                tiger (from line number 11)

                the value of world.keepScore.animalThatWasHit is tiger (from
line 23 - this is expected)

                world.whenHit (from line 32)

                the value of world.whenHit.target is paintball (from line
33- this seems WRONG - it should be "tiger")

 

What appears to be happening is that even though the value of
animalThatWasHit is "tiger" in the world.keepScore method, when the
world.whenHit method is called, the value reverts back to the initial value,
not the value shown in the parent (world.keepScore) method.

 

 

 

               

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