Adding a second for loop to create different matrices

Question

Inti Vanmechelen 2015-12-22

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/261340-adding-a-second-for-loop-to-create-different-matrices

编辑： Mohammad Abouali 2015-12-22

I have this for loop (see below), and I want to create 10 empty 6x3 matrices (S1 to S10). Is there a way to add a second loop that loops over 1 to 10 so I don't have to repeat everything 10 times?

names2 = {'soleus' 'tibant' 'gaslat' 'vaslat' 'rectfem' 'hamlat'};

Vars2 = {soleus, tibant, gaslat, vaslat, rectfem, hamlat};

nvars = length(Vars2);

    for s = 1:1:nvars
        S1.(names2{s}) = zeros(100,3);
        S2.(names2{s}) = zeros(100,3);
        S3.(names2{s}) = zeros(100,3);
        S4.(names2{s}) = zeros(100,3);
        S5.(names2{s}) = zeros(100,3);
        S6.(names2{s}) = zeros(100,3);
        S7.(names2{s}) = zeros(100,3);
        S8.(names2{s}) = zeros(100,3);
        S9.(names2{s}) = zeros(100,3);
        S10.(names2{s}) = zeros(100,3);
    end

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Stephen23 2015-12-22

Although beginners always ignore this advice, here it is anyway: Do NOT define variable names dynamically. Read my answer below to know why this is a bad programming practice.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Mohammad Abouali 2015-12-22

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/261340-adding-a-second-for-loop-to-create-different-matrices#answer_204014

编辑：Mohammad Abouali 2015-12-22

在 MATLAB Online 中打开

Yes, there is a way; but (really really) not recommended: (why not, refer to Stephens answer) But if you insist, here is how you can do it (but again you shouldn't do it)

    for s = 1:1:nvars
        for idx=1:10
          eval(sprintf('S%d.(name2{s}) = zeros(100,3);',idx))
        end
    end

You better off using array of structure

    for s = 1:1:nvars
        for idx=1:10
          S(idx).(names2{s}) = zeros(100,3);
        end
    end

2 个评论
显示无隐藏无

Inti Vanmechelen 2015-12-22

Thank you, I'm using the structure input now.

I wanted to do something similar as you posted above, but it's not working.. I'm trying to create 10 empty 6x3 matrices and I tried this imput:

for idx = 1:10

subject(idx) = zeros(6,3);

end

If I try to run it, matlab says "Subscripted assignment dimension mismatch"

Mohammad Abouali 2015-12-22

编辑：Mohammad Abouali 2015-12-22

在 MATLAB Online 中打开

You are correct. That wouldn't work because:

subject(idx) = zeros(6,3);

will treat subject as a regular matrix therefore, subject(id) should have only one element while zeros(6,3) is a matrix.

The alternative is to use cell arrays as follow:

subject=cell(10,1);
for idx = 1:10
  subject{idx} = zeros(6,3);
end

Depending on what you want to do, you might not need cell or structure at all. Depending what your calculations are may be this would work too:

subject = zeros(6,3,10);

This is a three-dimensional array with third dimension being your 10 matrices. The data structure that you use is related to what type of calculation you want to do.

请先登录，再进行评论。

Answer 2

Stephen23 2015-12-22

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/261340-adding-a-second-for-loop-to-create-different-matrices#answer_204013

编辑：Stephen23 2015-12-22

在 MATLAB Online 中打开

Solution

Use a non-scalar array:

   S(1).(names2{s}) = zeros(100,3);
   S(2).(names2{s}) = zeros(100,3);
   S(3).(names2{s}) = zeros(100,3);
   ... etc

Obviously you can loop over the indices of the structure. Or you can learn how to write efficient MATLAB code without loops: this allocates all those zeros-arrays to all fields of a 1x10 structure, in just three lines:

>> names2 = {'soleus','tibant','gaslat','vaslat','rectfem','hamlat'};
>> X(1,:) = names2;
>> X(2,:) = {zeros(100,3)};
>> S = repmat(struct(X{:}),1,10);

Why waste your time writing loops when you can do that instead?

And whatever you do, do not define those variable names S1, S2, etc, dynamically.

Why Dynamic Variable Names are a 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 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.

In case you are interested, 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/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/196952-how-to-create-new-variables-in-batches-with-strcat

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

Here is a discussion 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

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

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