MATLAB: Dynamically allocated variables with indices..

bad horrible ideadynamically allocatederrorevalsprintf

Hello folks,

Quick question. I need to dynamically allocate variables from a cell array using a string value within the cell.. I know it is not good practice to use eval but my application requires me to do this. The expression I require to be evaluated is:

eval(sprintf('%s = ''%s''',cell2mat(stocks(i,1)),cell2mat(C(1,k))))  
- which has the output: AAVL = 300

However I need the output: AAVL(j,k) = 300

I am having trouble adding indices..

When I try this:

eval(sprintf('%s(%d,%d) = ''%s''',cell2mat(stocks(i,1)),j,k,cell2mat(C(1,k))))

which gives an error: Assignment has more non-singleton rhs dimensions than non-singleton subscripts

note:

    cell2mat(stocks(i,1)) % yields the AAVL string
    cell2mat(C(1,k))) % yields the value 300 which was initially a string

Any help would be much appreciated!

Best Answer

allvars.(stocks{i,1})(j,k) = str2double(C{1,k}));

That is, do not use eval, do not generate variable names on the fly. Use dynamic field names on a structure. Eventually you may need to save the structure as individual variables; you can do that with

save('AppropriateFilename.mat', 'allvars', '-struct')

which will create one variable in the .mat file for each field of structure allvars.

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MATLAB: Not sure why “eval(name of a variable)” cannot be assigned a value

This is why beginners love eval: because they think it solves problems. In reality it just makes problems that are hard to debug because using eval deactivates all of the useful debugging tools that MATLAB has. Once you learn to write faster, neater code without using eval, and start paying attention to those wavy orange underlines, tab completion, f1 help, mlint messages, etc, etc, you will wonder why beginners bother to use eval, when it removes all of the tools that help them to write code, and find and fix mistakes.

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

http://www.mathworks.com/matlabcentral/answers/196952-how-to-create-new-variables-in-batches-with-strcat

If you must use names then a much better solution would be to keep all of those variables as fields of a structure. Then looping over them would be neater and faster:

S.name1 = [...];
S.name2 = [...];
C = fieldnames(S);
for k = 1:numel(C)
    S.(C{K}) = sort(S.(C{K}));
end

Or indeed any kind of array would be better than using eval, with simple indexing: a non-scalar structure, a cell array, or a numeric array.

MATLAB: Saving a new table for every loop iteration

Don't do that. Dynamic variable creation is a bad idea and totally unnecessary.

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Use a cell array.

 data = cell(variable_max,1);
 column_name  = cell(variable_max,1);
 %and preallocate other variables, this is important if you want your code to be faster
 for k=1:variable_max
   ...
   ...
   column_name {k,1} = ...
   data{k,1} = dataextract(...)
 end

Now you have all your tables under one variable. You can access them by indexing.

Best Answer

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MATLAB: Not sure why “eval(name of a variable)” cannot be assigned a value

MATLAB: Saving a new table for every loop iteration

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