Need to get rid of an eval and loop

Question

Christopher 2015-2-3

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https://ww2.mathworks.cn/matlabcentral/answers/174852-need-to-get-rid-of-an-eval-and-loop

编辑： Stephen23 2015-2-3

I have the code

 Fe = zeros(maxthick,numfiles);
 for i=1:numfiles
    eval(sprintf('tnum = numel(data%d.results(:,13));',i));
    for j=2:tnum
        eval(sprintf('Fe(j,i)=(data%d.results(j,13)-data%d.results(j-1,13))-(abspor(j,i)-abspor(j-1,i));',i,i));
    end
 end

where tnum=8000 and numfiles=441.

The line:

 for j=2:tnum
        eval(sprintf('Fe(j,i)=(data%d.results(j,13)-data%d.results(j-1,13))-(abspor(j,i)-abspor(j-1,i));',i,i));
 end

is extremely expensive. This should be made into a vectorized procedure, but the eval is making it complicated. How can this be improved?

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Stephen23 2015-2-3

编辑：Stephen23 2015-2-3

A classic example of why using eval to generate sequential variable names is really poor programming practice. Do not do this. Dynamically assigning variable names is a really bad idea in MATLAB:

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

http://www.mathworks.com/matlabcentral/answers/56124-eval-is-evil-using-variables-created-dynamically-info-retrieval

http://www.mathworks.com/matlabcentral/answers/51946-systematic-do-not-use-global-don-t-use-eval

http://www.mathworks.com/matlabcentral/answers/79281-create-a-structure-name-based-on-the-string-of-a-variable-create-fieldnames-in-this-structure

The best alternatives are to keep your data in an array (e.g. as they are returned from your file-reading function), or if you require key-value access to your data then use a structure . Structures do allow dynamic fieldnames , and this is much more robust than dynamic variables.

If you have a newer version of matlab you can also use a table , which stores the data together in one array but also allows key-name access to the columns.

Summary: use a cell, structure or table to store your data, and learn about vectorization .

Stephen23 2015-2-3

编辑：Stephen23 2015-2-3

And of course i and j are the names of the inbuilt imaginary unit , so they should not be used for loop variable names.

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Answer 1

Titus Edelhofer 2015-2-3

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Hi,

to start with, I would suggest to do a

data_i = eval(sprintf('data%d', i));

at the very beginning of the i-loop and then you work with data_i and no further eval/sprintf.

Titus

PS: I assume you "have" to work with the variables data1, data2, etc. If you have control over their creation, I would recommend to use a cell array data{1}, data{2}, ... to get fully rid of the eval constructs.