How to get values assigned to a string?

Hi all,

My data are stored as below in the workspace:

white_left  400x129
white_right 400x129
black_left  400x90
black_right 400x90

Now I have to call each and do some operations in each column:

type = {'white','black'};
orientation = {'left','right'};
addOne = 0;
for a = 1:length (type)
  for b = 1:length (orientation)   
    for j = 1:size([neuron_type{a} '_' orientation{b}]), 2) % Suppose I want to repeat some operation for the number of columns in each dataset
      addOne = addOne + 1; % DO SOME OPERATION
    end
  end
end

But already in

size([neuron_type{a} '_' orientation{b}]), 2)

it does not find the size of data which should be 129 but it finds the size of string for example 10 for "white_left".

How can I make Matlab understand that I want the data.

Thanks a lot :)

[Stephen23]

@Max: read the comments that you are getting here: "do not use eval", and then have a read of the links that I gave you. You will learn something today if you try and understand why using eval is not a good way to solve this problem. Use cell arrays instead.

Answer 1

[Image Analyst]

See the FAQ to understand how cell arrays work: http://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F

orientation(b) is a cell but orientation{b} is the contents of the cell, which is a string.

neuron_type is not defined so we don't know what is in the cells of that cell array.

Anyway, you would not use a nested for loop for this kind of thing. You'd write a function (for example AnalyzeData) and just pass in the arrays

results = AnalyzeData(white_left)
results = AnalyzeData(white_right)
results = AnalyzeData(black_left)
results = AnalyzeData(black_right)

In AnalyzeData(), you'd extract and process the desired columns of whatever matrix got passed in.

[Max]

Image Analyst;

Thank you for your suggestion. That surely will work. What I needed now was actually "eval" command.

size(eval([type{nt} '_' orientation{ori}]), 2)

This gives 129 now.

[Cedric]

Avoid EVAL, and use fields and dynamic field names instead if you can.

[Image Analyst]

No, that's exactly the way NOT to do it.

Answer 2

[Stephen23]

Ugh... 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 these data easily, and they can also be used in 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/74778-loop-file-and-variablenames-when-importin

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 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

[Cedric]

Quite the reference on the topic => printed as PDF ;-)