How can this possibly be null?

Question

     mySquare = startSquare;
     if(mySquare == null) print("WHAT?");
     
     while(mySquare != myGoal)
     {
         if(mySquare == null) print("Square Null");
     }

mySquare is not null before the While ("WHAT?" never gets printed), however, somehow, mySquare becomes null right after the while ("Square Null" always gets printed).

Can anyone explain to me how is that even possible?

Answer 1

Answer by Jeff-Kesselman · May 30, 2014 at 11:07 PM

Is this really ALL the code? Because it looks like an idiot loop as presented. If not your problem is probably in the code you didn't bother to show us.

Assuming that this code is really the code you are running, then the answer has got to be that the variable is being changed by another thread. Make sure the variable is a local, not a global, to make sure something else isn't setting it.

Otherwise its probably a lot more likely that either the code you are showing us is not the same code, or that its not really the code that is generating your output -- there is either another copy of the code out there in another script OR you have attached this script to more then one object OR you have attached on an object more the once.

When you see something "impossible" in programming it ALWAYS means one of your assumptions is wrong. So go through all your assumptions one by one and start proving them right. You can start by showing us what you assumed was unimportant in the above code.

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rkGio · May 30, 2014 at 11:14 PM 0

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The script is only attached to the player, I just checked and I only attached it once to the player, and the whole script is as follows:

  function Awake(){
     
     squareArray = GetComponent(Character).squareArray;
     }

 function MakePath(startSquare : Square, endSquare : Square)
 {
     var mySquare : Square;
     var myGoal : Square = endSquare;

     mySquare = startSquare;
     if(mySquare == null) print("WHAT?");
     
     while(mySquare != myGoal)
     {
         if(mySquare == null) print("Square Null");
     }
 }

rkGio · May 30, 2014 at 11:30 PM 0

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It is indeed the whole code (except a "var squareArray : List." before the Awake function); I know it is just an idiot loop doing nothing but I was just "testing the water" before adding any real code in.

Answer 2

Answer by Lovrenc · May 30, 2014 at 11:30 PM

There are couple of possibilities.

1) Your value is changed by a different thread. From the snippet I suspect you are fairly new coder so it might be the code you copied/included from third party.

2) Square Null prints so fast, "What?" string goes out of bounds (if I remember correctly console holds 500 outputs) and your conclusions are wrong.

I can't think of anything else from what little you showed here.

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tw1st3d · May 30, 2014 at 11:32 PM 0

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I agree with 2. @rkGio, try changing your while statement to

 if(mySquare != myGoal){ 
  if(mySquare == null) {
   print("Square Null");
  }
 }

Jeff-Kesselman · May 30, 2014 at 11:38 PM 0

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Thats a good idea. Also turn on collapse on your output. That will prevent repeated identical error messages from filling up your console.

alt text

rkGio · May 30, 2014 at 11:45 PM 0

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@tw1st3d That did work, mySquare is no longer null. But, I'll need to use While when I implement the real code.

@Lovrenc I'm not using any third party code

tw1st3d · May 30, 2014 at 11:46 PM 0

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If it worked my way, then it's working your way, but it's logging too fast for you to see it. There's no technical problem with the code, just the delivery of logging.

Lovrenc · May 31, 2014 at 12:05 AM 0

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So it is point 2. Value of the mySquare variable never changes. It was null from the beginning. Everything works as intended.

Also, what @tw1st3d said in comment above is true and since you failed to realize this on your own I suggest you read about basic looping before you go on. (I am not saying that with bad intentions).

How can this possibly be null?

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