parfor warning and analysis of switch statement

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David Young
David Young 2015-5-28
评论： David Young 2015-5-28
I have two functions which are, I think, functionally identical. Both use a parfor loop. If the logic within the loop is expressed using switch, I get a warning at run-time. If the same logic is expressed using if, there is no warning. The functions return the same result.
This seems like a bug, but I haven't used the Parallel Computing Toolbox much so I thought I'd ask whether anyone could shed any light on this behaviour before I send in a service request.
Here is the version using switch:
function op = parfortest1
t = 'xyz';
op = zeros(1, 10);
parfor kk = 1:10
switch t
case 'abc'
q = rand;
otherwise
q = -1;
end
op(kk) = q;
end
end
which generates this message:
Warning: File: parfortest1.m Line: 11 Column: 14
The temporary variable q 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 q
is used before it is assigned in the parfor loop, a runtime
error will occur.
And here is the if version, which does not produce the warning:
function op = parfortest2
t = 'xyz';
op = zeros(1, 10);
parfor kk = 1:10
if strcmp(t, 'abc')
q = rand;
else
q = -1;
end
op(kk) = q;
end
end
I can thus work round the problem - my question is whether I have missed some obvious, or unobvious but documented reason for the difference in behaviour, or whether this is a failure of the parser to correctly analyse the switch case and recognise that q is always given a value before it is used.

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回答（1 个）

Sean de Wolski
Sean de Wolski 2015-5-28
It doesn't error does it? It looks like the warning is a feature of the switch statement letting you know what you can expect as behavior. You should be able to turn the warning off.
  1 个评论
David Young
David Young 2015-5-28
Thanks - yes, I can switch off the warning - at the expense of cluttering my code. But I suspect the warning should not be there, because q always receives a value before it is used, and the warning is being issued in error.

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