全局变量和 parfor [英] globals and parfor

问题描述

在 parfor 循环中，我试图调用一个访问 global 的函数但无济于事.

Inside a parfor loop, I am trying to call a function that accesses a global to no avail.

函数

function a = getA()
   global OPTIONS;
   a=OPTIONS.PROBLEM.A;
end

循环:

parfor i=1:3
    b=getA();
end

错误:

Error using parallel_function (line 589)

Attempt to reference field of non-structure array.

我做错了什么?

推荐答案

来自 parfor 上的 rel="nofollow noreferrer">文档:

From the documentation on parfor:

parfor 循环的主体不能包含全局或持久变量声明.

The body of a parfor-loop cannot contain global or persistent variable declarations.

在您的问题的上下文中，即在 parfor 中调用一个函数，该函数又引用了一个 global，这转化为:parfor 可能不会给出预期或有意义的结果".

In the context of your problem, i.e., calling a function within the parfor that in turn references a global, this translates into: "parfor will probably not give expected or meaningful results".

这是完全有道理的.考虑以下

This makes perfect sense. Consider the following

Lab 1:         Lab 2: 

GetB();        GetB();

如果GetB()的内容是这样的:

function GetB()
    global B;

    %# do something useful

    B = rand;

 end

当Lab 1 引用B 时，它的值是什么?在 Lab 2 上?rand 的不同结果如何传达?这将是一团糟！

what will the value of B be when it is referenced on Lab 1? and on Lab 2? How are the different outcomes of rand communicated? It's going to be a mess!

编写适用于 parfor 循环的代码可能会是一个真正的痛苦，因为该代码来自只考虑正常 for 循环的东西.通常，当您事先知道要编写一段计算密集型的 Matlab 代码时，请从一开始就将所有函数和循环编写为 parfor 循环.只有这样，才能避免此类 bug 花费您一天的时间来对您的函数进行转码.

Writing code suited for parfor loops can be a real pain when that code comes from something that only had normal for-loops in mind. Generally, when you know beforehand you're going to write a computationally intensive piece of Matlab code, write all functions and loops as parfor loops right from the beginning. That is the only way that bugs like these will not cost you a day on transcoding your functions.

从 for 转换为 parfor 绝非易事.

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