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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Don,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Thanks, once you mentioned it was the threading issue, it all made sense to me.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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?<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Thanks.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> alice-teachers-bounces+schultz_marty=hotmail.com@lists.andrew.cmu.edu [mailto:alice-teachers-bounces+schultz_marty=hotmail.com@lists.andrew.cmu.edu] <b>On Behalf Of </b>Don Slater<br><b>Sent:</b> Monday, April 15, 2013 2:19 PM<br><b>To:</b> Alice educators<br><b>Subject:</b> Re: alice-teacher Passing an object to a method doesn't appear to work completely in Alice 2.3.2<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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.<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>See the attached screen shot, from the method "world.KeepScore":<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>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.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>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.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>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.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I dissolved the doTogether and got the following output after running the program:<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><span class=apple-tab-span><b> </b></span><strong>tiger</strong><o:p></o:p></p></div><div><p class=MsoNormal><span class=apple-tab-span><b> </b></span><strong>none</strong><o:p></o:p></p></div><div><p class=MsoNormal><span class=apple-tab-span><b> </b></span><strong>tiger</strong><o:p></o:p></p></div><div><p class=MsoNormal><span class=apple-tab-span><b> </b></span><strong>none</strong><o:p></o:p></p></div><div><p class=MsoNormal><span class=apple-tab-span><b> </b></span><strong>tiger</strong><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I hope that this helps… <o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>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.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Look at how different your pseudocode looks if you had written it this way<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Method world.keepScore looks similar to:<o:p></o:p></span></p></div></div></blockquote><p class=MsoNormal><span class=apple-tab-span> </span>DoTogether<br><br><o:p></o:p></p><div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> // precondition: Local variable animalThatWasHit (type=OBJECT, initial value set to paintball)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 20 animalThatWasHit set value to world.whoGotHit<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 20 print animalThatWasHit<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 20 world.whenHit target=animalThatWasHit<o:p></o:p></span></p></div></div></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>All the best,<o:p></o:p></p></div><div><p class=MsoNormal>Don Slater<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><img width=661 height=299 id="_x0033_c4e784f-cede-4b8c-bb1a-e0d525ed846d" src="cid:image001.png@01CE39E5.6582AD80"><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Apr 14, 2013, at 9:48 PM, Marty Schultz <<a href="mailto:schultz_marty@hotmail.com">schultz_marty@hotmail.com</a>> wrote:<o:p></o:p></p></div><p class=MsoNormal><br><br><o:p></o:p></p><div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Does anyone have a solution for this (I’ve included pseudo line numbers to make it clear):<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Function world.whoGotHit (returns type OBJECT) ends with:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 10 print “world.whoGotHit”<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 11 print tiger.name<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 12 return tiger<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Method world.keepScore looks similar to:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 20 Local variable animalThatWasHit (type=OBJECT, initial value set to paintball)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 22 animalThatWasHit set value to world.whoGotHit<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 23 print animalThatWasHit<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 24 world.whenHit target=animalThatWasHit<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Method world.whenHit<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 30 parameter target (type=OBJECT)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 32 print “world.whenHit”<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> 33 print target<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>When I PLAY this program, the PRINT statements show:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> world.whoGotHit (from line number 10)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> tiger (from line number 11)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> the value of world.keepScore.animalThatWasHit is tiger (from line 23 – this is expected)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> world.whenHit (from line 32)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> the value of world.whenHit.target is paintball (from line 33- this seems WRONG – it should be “tiger”)<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>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.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><p class=MsoNormal><span style='font-size:13.5pt;font-family:"Helvetica","sans-serif"'>_______________________________________________<br>alice-teachers mailing list<br><a href="mailto:alice-teachers@lists.andrew.cmu.edu"><span style='color:purple'>alice-teachers@lists.andrew.cmu.edu</span></a><br><a href="https://lists.andrew.cmu.edu/mailman/listinfo/alice-teachers"><span style='color:purple'>https://lists.andrew.cmu.edu/mailman/listinfo/alice-teachers</span></a><o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p></div></div></div></body></html>