parfor loop with continue gives incorrect results

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Steve Chang 2018-10-8

2
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/422906-parfor-loop-with-continue-gives-incorrect-results

评论： Steve Chang 2018-10-11

Consider the following code:

N = 1000;
failed = false( 1, N );
values = cell( 1, N );
n_failed = 0;
parfor idx = 1:N
  try
    if ( rand() > 0.7 )
      n_failed = n_failed + 1;
      error( '' );
    end
  catch err
    failed(idx) = true;
    continue;
  end
    values{idx} = rand( 1e3, 1 );
end
fprintf( 'N failed 1: %d\n\n', sum(failed) );
fprintf( 'N failed 2: %d\n', n_failed );

If I run this on my machine (macOS 10.12, r2017a), `sum(failed)` is 0, while `n_failed` is, as expected, ~300. What am I missing here? I don't see anything in the documentation about `continue` not being supported in a parfor loop?

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Steve Chang 2018-10-9

Using nnz instead of sum gives the same result. Also, regarding the randomization -- removing the if statement gives the same behavior (i.e., sum(failed) is 0, while n_failed is 1000).

I guess there's a bug in how continue is handled in a try / catch context, or else it is not meant to be used in this way in a parfor loop.

Matt J 2018-10-9

编辑：Matt J 2018-10-9

The problem doesn't seem to have anything to do with 'continue'. Even when the continue is commented out, I still obtain sum(failed)=0. It might have more to do with try...catch.

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回答（1 个）

Matt J 2018-10-9

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/422906-parfor-loop-with-continue-gives-incorrect-results#answer_340531

编辑：Matt J 2018-10-9

在 MATLAB Online 中打开

Don't pass an empty string '' to error() if you want the catch block to be triggered. An empty string apparently does not result in an error being thrown.

In other words, this works fine:

N = 1000;
failed = false( 1, N );
n_failed = 0;
parfor i = 1:N
  try
    if ( rand() > 0.7 )
      n_failed = n_failed + 1;
      error('an error');
    end
  catch 
    failed(i) = true;  
    continue
  end
end
fprintf( 'N failed 1: %d\n\n', sum(failed) );
fprintf( 'N failed 2: %d\n', n_failed );

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Steven Lord 2018-10-9

Don't pass an empty string '' to error() if you want the catch block to be triggered. An empty string apparently does not result in an error being thrown.

That is correct. The last item in the Tips section on the documentation page for the error function states that if all the inputs are empty, error does not throw an error.

Historically, error was often used with functions like nargchk that would return '' if the number of inputs was in the allowed range and an error message if not. Now, as the warning box at the top of the documentation page states, you should use narginchk (and/or nargoutchk) instead. Those will directly throw the error, so you don't need to wrap them in error calls.

The code above works, but I'm experiencing a different error with my original code.

That suggests that your original code is doing something different than the code you posted in Answers, and the differences between those two sections of code is relevant. Post your original code, or simplify the original code down to the minimal working example that reproduces the exact error you're seeing, and we may be able to offer guidance relevant to that original code.

Edric Ellis 2018-10-11

在 MATLAB Online 中打开

There's an existing problem in the parfor machinery that causes the values assignment to fail. The problem relates to the combination of try / catch and the assignment to values. You can trick the parfor machinery into operating correctly by changing how it analyses values. The following should work:

N = 1000;
failed = false( 1, N );
values = cell( 1, N );
n_failed = 0;
parfor idx = 1:N
    % Dummy reference to "values(idx)":
    if false
        values(idx);
    end
    try
        if ( rand() > 0.7 )
            n_failed = n_failed + 1;
            assert(false);
        end
    catch err
        failed(idx) = true;
        continue;
    end
    values{idx} = rand( 1e3, 1 );
end
fprintf( 'N failed 1: %d\n\n', sum(failed) );
fprintf( 'N failed 2: %d\n', n_failed );