PARFOR temporary variables warning

I am seeing a slew of warnings below which seem to be arising from the use of temporary variables in one of my parfor loops. The warnings would make sense to me if I were using variables of the same name prior to entering the loop - parfor would clear those variables - but I am not doing so. Also, the warnings seem to be triggered by some, but not all of my temporary variables. Does anyone have any idea what else could cause this?

One possible clue: the variables named in the warnings are also the names of class properties in a class I have defined. I do have an object of this class prior to the loop and access its properties as obj.A, obj.B, etc... but I don't see why that would be a cause for the warnings.

Warning: File: LineSegParsing.m Line: 120 Column: 33
The temporary variable coords will be cleared at the beginning
of each iteration of the parfor loop.
Any value assigned to it before the loop will be lost. 
If coords is used before it is assigned in the parfor
loop, a runtime error will occur.

See Parallel for Loops in MATLAB, "Temporary Variables". 
Warning: File: LineSegParsing.m Line: 116 Column: 38
The temporary variable A will be cleared at the beginning 
of each iteration of the parfor loop.
Any value assigned to it before the loop will be lost.  
If A is used before it is assigned in the parfor
loop, a runtime error will occur.

See Parallel for Loops in MATLAB, "Temporary Variables". 
Warning: File: LineSegParsing.m Line: 152 Column: 25
The temporary variable B will be cleared at the beginning of 
each iteration of the parfor loop.
Any value assigned to it before the loop will be lost.  
If B is used before it is assigned in the parfor
loop, a runtime error will occur.

[Adam]

Do you still get the warnings if you name the variables differently to the class properties?

[Matt J]

That would be a good test, but unfortunately, I'm finding that the warnings, even without renaming the variables, are not reproducible on subsequent runs. I'm not sure what it takes yet to reproduce them.

Answer 1

[Ken]

Uninitialized Temporaries Temporary variables in a parfor-loop are cleared at the beginning of every iteration. MATLAB can sometimes detect cases in which loop iterations use a temporary variable before it is set in that iteration.

Examples include conditionally setting a variable before any use of the variable.

function myFunction
parfor i = 1:10
    if rand < 0.5
        a = MyClass;          
    end
    disp(a.SomeValue);
end
end

Likewise, not setting a variable on all possible code paths.

function myFunction
parfor i = 1:10
    if rand < 0.5
        a = MyClass;
    else          
        % a is not set 
    end
    disp(a.SomeValue);
end
end

[Matt J]

Thanks, Ken. You might be on to something. I do set my variables inside a switch statement, similar to the example below, but I cannot get this example to throw the warning, so I'm not sure what's different about my actual code

 classdef myclass 

     properties 

        p=1; 
     end

     methods  

         function  obj=myclass(n,j)

             if ~nargin
                return; 
             end

             obj(n).p=[];

             parfor i=1:n

                 switch j

                  case 1
                    p=1;
                    obj(i).p=p;
                  case 2
                     p=2;
                    obj(i).p=p;
                  otherwise

                         error 'Unrecognized'

                 end
             end

         end

     end

 end

[Pierre Massé]

Yeah, I'm getting this warning in a parfor loop where the temporary variable is set in a while loop. I understand that this counts as conditional (in the case where the boolean activating the while loop is false from the get-go) but it would be nice if MATLAB recognized that said boolean is initialized to true at each parfor loop iteration, thus guaranteeing at least one while loop iteration and the proper setting of the aforementioned temporary variable. In fact, the looping condition literally can't be false without setting the temporary variable first in my case. Basically:

parfor ii=1:N

       cond = true;

       while cond

            tempVar = someThing;

            if someBool(tempVar)

                 cond = false;

            end

       end

       someThingElse = someFnctn(tempVar);

end

Surely this should be perfectly valid, no?

[David Lovell]

I'm willing to bet Matlab does NOT notice that you set cond = true before using it as the condition for the while loop. Instead, being more myopic, Matlab can easily notice that tempVar is only used inside a while loop, which in general are never guaranteed to execute, and therefore it throws this warning. Your code will work fine. If you want to get rid of the warning, just put a useless statement like tempVar = someThing; on the line before the while loop.

[David Lovell]

And to Matt J, in the example you posted, the only variables whose values can change inside the switch..case statement are p and obj(i), both of which existed in the workspace before the parfor command. Thus, they are not temporary variables. In your code, something else must be the case.

[Matt J]

@David,

No, p did not exist in the workspace before parfor. More importantly, though, it is the target of a non-indexed assignment inside the parfor loop, so it should be classified as temporary, as far as I can see.

[Walter Roberson]

Does it make a difference that p is an object property as well as the name of the temporary ? e.g., if you had written the code in terms of new_p for the temporary, would it have the same problem?