how to evaluate a function in string with parfor?

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Julie G 2017-11-14

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评论： OCDER 2017-11-17

Hi everybody, I have a problem evaluating the value of a function in string with parfor. Before I started using parfor, I used "eval", which worked quite well. I know "eval" does not work well with parfor and the one recommended the most online is "feval". However, I do not know how to use it. More specifically, i am doing genetic programming with the following code randomly generating a function for each individual.

parfor v = 1:8;
  popu{v} = gpols_init(popusize,maxtreedepth,symbols);
  tree{v} = popu{v}.chrom{jf}.tree;
  aa{1,jf,v} = tree_stringrc(tree{v},1,symbols);
end

The output in "aa" looks like the following.

val(:,:,1) = 
    'XX(iv,1)'    '(XX(iv,9))/(XX(iv…'    '(((XX(iv,7))*exp(…'    '(XX(iv,1))*sqrt(X…'    '((XX(iv,2))*log((…'

what I need to do is to evaluate the value of each expression, which I do not know how to do. I will appreciate if someone could help me with it. Thanks. Best, J.

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Julie G 2017-11-14

编辑：Walter Roberson 2017-11-17

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The reason is I have to run quite a lot simulations and parfor would save a lot of time. I tried "function handles" with the following code

str = 'a{1,jf,v}';
fh = str2func(str)

but still it had problems like the following.

Warning: The input to STR2FUNC "a{1,jf,v}" is not a valid function name. This will generate an error in a future release.
  In parallel_function>make_general_channel/channel_general (line 914)
  In remoteParallelFunction (line 38)
Warning: The input to STR2FUNC "a{1,jf,v}" is not a valid function name. This will generate an error in a future release.
  In parallel_function>make_general_channel/channel_general (line 914)
  In remoteParallelFunction (line 38)
Warning: The input to STR2FUNC "a{1,jf,v}" is not a valid function name. This will generate an error in a future release.
  In parallel_function>make_general_channel/channel_general (line 914)
  In remoteParallelFunction (line 38)

Stephen23 2017-11-14

编辑：Stephen23 2017-11-14

@Julie G: Why not generate those functions entirely using function handles, and avoid using strings entirely? Have you considered exclusively staying in the realm of function handles?

Also: what is XX ?

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Answer 1

OCDER 2017-11-14

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For str2func, you also need to include the "@(var1, var2, var3, ...)" string. This is used to tell Matlab what part of the string are variables, while the rest of the string are treated as function names and math operators.

XX = rand(10);  %What is XX? I'm assuming it's a matrix
iv = 10;        %Assuming this is just an index of XX
aa{1,1,1} = 'XX(iv,1) * sin(XX(iv,1))';        %Example string of a function. XX and iv are variables.
FuncHandle = str2func(['@(XX, iv)' aa{1,1,1}]);%Define "XX" and "iv" as variable names in the string using @(XX, iv).
Output = FuncHandle(XX, iv);                   %Evaluate the string as function.

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Julie G 2017-11-15

编辑：Stephen23 2017-11-15

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Hi Lee, sorry for not making my question clear. please forget about the "XX" thing. The main point is that i have a lot of functions stored in a cell array. As these functions are randomly generated, they are not in a common form (i have a set of variables X=[x(:,1),x(:,2),x(:,3),x(:,4),x(:,5),x(:,6),x(:,7),x(:,8),x(:,9),x(:,10)]. each function randomly chooses some of these variables to construct a function). The following functions give some examples of the functions generated.

val(:,:,5) = 
  'x(iv,9)'    'x(iv,3)'    '((((x(iv,9))*sqrt…'    'x(iv,3)'    'x(iv,2)'
val(:,:,6) = 
  '(((x(iv,10))*sqrt…'    'x(iv,9)'    '(x(iv,1))*exp(x(i…'    'x(iv,3)'    '((((x(iv,9))*log(…'

I know to use "str2func" and "feval" I need to include "@(var1, var2, var3, ...)". However, as i have quite a lot of functions which are not in common format, it seems unlikely to do it for every function. Do you have any idea about how to evaluate these functions? Thanks Best, J.

Stephen23 2017-11-15

编辑：Stephen23 2017-11-15

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"I need to include "@(var1, var2, var3, ...)". However, as i have quite a lot of functions which are not in common format, it seems unlikely to do it for every function."

You could easily add the complete list of variables to every function. It does not matter if they are not used inside that function. Simply providing all variables would be a trivially easy solution. Basically this is perfectly okay:

@(x,y,z) x+y; % z is not used: this is okay.

"Do you have any idea about how to evaluate these functions?"

Function handles are the by far the best way to handle functions. You should convert to function handle, or even better work exclusively with function handles. Note also that feval is not required to evaluate function handles: the whole point of function handles is that they can be called, just like any other function:

>> fun = @(x,y,z) x+y;
>> fun(1,2,4)
ans = 3

You could possibly simplify your code by putting all of the values into one variable, and then also providing a logical mask to select which elements of that variable get used. In this way you can easily adjust for different numbers of values without changing your functions. But it really depends on your requirements, which so far you have told us nothing about.

Stephen23 2017-11-15

编辑：Stephen23 2017-11-15

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@Julie G: Note that x(:,1), x(:,2), etc are not variables (they are indexing into variables). So the way you have defined your function makes no sense because a function must be defined with variables as the input arguments, not some operation on a variable (like indexing).

You appear to have only one variable x, so you could try this:

str2func(['@(x)','ab{1,jf,v}']);

But I notice that x is not used inside that function definition. This means that whatever value you supply to that function will be unused. If you want to use the input variable's values then it must be the name of a variable used within that function's workspace. See my last comment for complete working examples: do you see how I defined x, y, and z as input variables of that function, and used x and y within that function?

I would highly recommend that you read the MATLAB documentation for function handles and anonymous functions, because your current approach of guessing how to write MATLAB code is very inefficient and will just waste your time. You should also read the str2func help.

Julie G 2017-11-17

Hi Donald. i added "parpool(12)" to my matlab file. but error message appeared in unix as below.

{Undefined variable "com" or class "com.mathworks.toolbox.distcomp.pmode.SessionInfo.NULL_SESSION_INFO".

Error in parpool (line 82) sessionInfo = com.mathworks.toolbox.distcomp.pmode.SessionInfo.NULL_SESSION_INFO;

Error in gp1 (line 156) parpool(12);

}

can you help with this? Thanks.

J.

OCDER 2017-11-17

This could be caused if starting matlab with the -nojvm option. Here's a similar Q&A thread for this error. https://www.mathworks.com/matlabcentral/answers/230285-parpool-r2014a-fails-on-linux

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