parfor warning and analysis of switch statement

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.

Answer 1

[Sean de Wolski]

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.

[David Young]

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.