PARFOR temporary variables warning

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Matt J
Matt J 2017-3-22
评论： Walter Roberson 2019-4-13
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.
  2 个评论
Adam
Adam 2017-3-22
Do you still get the warnings if you name the variables differently to the class properties?
Matt J
Matt J 2017-3-22
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.

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采纳的回答

Ken
Ken 2017-5-12
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
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Matt J
Matt J 2019-4-12
编辑：Matt J 2019-4-12
@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
Walter Roberson 2019-4-13
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?

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