MATLAB: Manipulating through sequential variables using for loop

for loopvariables

Hi,

(I tried to search on the forum and found many similar questions and answers, but none seems to be exactly the same as my question, so here we go…)

I have successfully loaded 32 sequential variables from a MAT file.

A_001, A_002, …, A_032

Each variable is an array of different length (1 x N). I want to manipulate these variables (e.g. plot all 32 of them), and I want to use a for loop to do this. How can I do this? I imagine things will be a lot easier to handle once I create a structure containing all of these variables, but I have no idea how to do that.

It looks like eval can help me do that, but many suggest not to use eval.

Many thanks in advance!

Hemmings

Best Answer

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

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MATLAB: How to split and save .txt columns using for loop

Ugh... this is a really bad idea!

Avoid creating dynamically named variables in MATLAB. This is poor practice as has been explained many times on this forum, and is not recommended by TMW/MATLAB themselves:

http://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

When you are a beginner it seems like a cunning and fast way to store information, but actually it is really bad practice to name your variables dynamically. MATLAB is also not intended for this kind of variable naming: if you continue to include data in the variable names then you will find yourself fighting many more of these battles against MATLAB.

However when you use more appropriate storage for your data (and meta-data) then you will suddenly find lots of MATLAB functions that do many useful operations for you, quickly and easily.

In your case a much more robust solution would be to use:

the dimensions of numeric arrays: fully vectorized code is often the fastest MATLAB data processing!
cell arrays
structures, where you can include fields for each kind of data (e.g. Process type, Flow data, Temperature data, Notes, Units, etc). Structures can even be non-scalar, which is extremely useful! And yes, you can even define structure fieldnames dynamically.

There are many functions that support working on structures and cell arrays and can access their data easily, and many functions operate on complete numeric arrays all at once without any loops (i.e. vectorized code, which is something you need to learn about).

Placing your data in a structure or cell array also makes it much easier to pass to functions: can you imagine the fight you would have trying to pass hundreds of dynamically named variables to a function?

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. This might be a good alternative for your data.

Not only that but using the function eval makes your code very slow, extremely difficult to debug, and it obscure the code's meaning. Beginners often use eval for everything, and then complain that they cannot debug it, or that their code is very slow:

http://www.mathworks.com/matlabcentral/answers/72799-any-ideas-how-to-make-the-code-more-efficient-now-it-takes-a-few-days

Solution: do not use eval! It really is that simple.

Here are discussions of why it is a bad idea to include meta-data (such as an index) in a variable name:

http://www.mathworks.com/matlabcentral/answers/225435-save-variable-as-string-from-user-input

http://www.mathworks.com/matlabcentral/answers/267329-list-values-of-variables-with-wildcard

Here is a discussion of why using eval is a very insecure way to dynamically select variables:

http://www.mathworks.com/matlabcentral/answers/42752-assigning-input-string-as-variable-name

And here is a discussion of why it is a really bad idea to make variables magically appear any workspace (even though beginners love doing this):

http://www.mathworks.com/matlabcentral/answers/249044-code-fails-to-run-in-function-works-in-command-window-works-in-debug-evaluate-selection

Finally, here are some pages explaining why dynamically assigning variable names is a really bad idea in MATLAB:

http://www.mathworks.com/matlabcentral/newsreader/view_thread/240867

http://blogs.mathworks.com/loren/2005/12/28/evading-eval/

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

http://ww.mathworks.com/matlabcentral/answers/143-how-do-i-make-a-series-of-variables-a1-a2-a3-a10

http://www.mathworks.com/matlabcentral/answers/257103-dynamic-variable-eval-evil-any-other-solution

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

http://www.mathworks.com/matlabcentral/answers/74778-loop-file-and-variablenames-when-importin

http://stackoverflow.com/questions/10272522/use-and-implications-of-evalexpression-in-matlab-code

http://blogs.mathworks.com/loren/2005/12/13/use-dynamic-field-references/

http://blogs.mathworks.com/pick/2008/08/20/advanced-matlab-dynamic-field-names/

And in case you thought this is a MATLAB restriction, here is the same discussion for some other languages, advising "DO NOT create dynamic variable names":

http://nedbatchelder.com/blog/201112/keep_data_out_of_your_variable_names.html

http://www.techrepublic.com/blog/it-security/never-use-dynamic-variable-names/

http://stackoverflow.com/questions/11118486/python-list-as-variable-name

http://www.reddit.com/r/learnpython/comments/2a9o4s/why_is_it_a_bad_idea_to_create_dynamic_variables/

http://stackoverflow.com/questions/25076883/creating-dynamically-named-variables-in-a-function-in-python-3-understanding-e

http://perl.plover.com/varvarname.html

https://24ways.org/2005/dont-be-eval

MATLAB: Plotting a struct in a loop

Try this:

% S.A601 = rand(10, 2); % Create sample data.
% S.A602 = rand(10, 2);
% S.A603 = rand(10, 2);
% S.A604 = rand(10, 2);
% S.A605 = rand(10, 2);
% S.A606 = rand(10, 2);
fn = fieldnames(S)
for k = 1:length(fn)
  thisS = S.(fn{k})
  x = thisS(:, 1);
  y = thisS(:, 2);
  fprintf('Printing field #%d.\n', k);
  semilogy(x, y);
  hold on
  drawnow;
end
grid on;

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