Assignin in parfor loop

Question

Sascha 2013-8-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/85005-assignin-in-parfor-loop

Hello everybody,

i have a problem with assignin in a parfor loop. For that I made a short example which explains the problem:

for i = 1:10
    beta = i;
    disp(beta);
end
parfor pi = 1:10
    assignin('base','beta',pi);
    disp(beta)
end

In the first loop beta gets values from 1 to 10, but in the parfor-loop beta always has value of 10. Why does this happen? I wanted to assignin for a variable as a input for simulink simulations.

Many thanks in advance,

Greetings

Sascha

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Jan 2013-8-18

I cannot image a situation, in which assignin is useful inside a parfor loop.

Sascha 2013-8-18

I tried to run multiple simulink simulations in parallel, as I have seen it in: http://blogs.mathworks.com/seth/2010/10/17/parallel-computing-with-simulink-running-thousands-of-simulations/#11

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Answer 1

Walter Roberson 2013-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/85005-assignin-in-parfor-loop#answer_94523

Doing an assignin('base') is going to change the value in the base workspace, but code executed in parallel is executed in a non-base workspace (because a new process if formed.)

I am surprised assignin() is allowed in a parfor. If the value being assigned varies from loop to loop, the result would be entirely dependent on the order the loops ended up being executed, which is not determinate in parfor()

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Answer 2

Edric Ellis 2013-8-19

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https://ww2.mathworks.cn/matlabcentral/answers/85005-assignin-in-parfor-loop#answer_94578

在 MATLAB Online 中打开

When you run your code at the command-line, all of the FOR loop body is run in the base workspace - whereas the PARFOR body is not. So, your ASSIGNIN statement is operating correctly, but the second line of your PARFOR loop body is not running in the base workspace - so it actually sees the value of beta set by the prior FOR loop. If you change your PARFOR loop to:

parfor pi = 1:10
  assignin('base','beta',pi);
  disp(evalin('base', 'beta'));
end

You should get the expected behaviour. Simulink models should also correctly see the value that you assign.

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Sascha 2013-8-20

在 MATLAB Online 中打开

Thanks for your answer. I already tried it with a simulink model. But it doesn't work as I expected, so the next step was to try it with disp, to see what values arre assigned to beta.

I tried it with simulink as in the following code-fragment:

parfor pi = 1:n
    load_system('invertedPendulum');
    assignin('base','beta',pi);
    sim('invertedPendulum');
    outputsParfor(pi) = simout;
end
figure;
for j = 1:n 
    plot(outputsParfor(j));hold all;
end
hold off;