MATLAB: How to identify variables in workspace from strings in cell arrays and then make changes to the variables themselves

cell arrayinterpolationregexpwhos

Hi all,

I have a cell array containing strings say for eg {'var1','var2','var3',….'varn'} . Now i have values of all these variables in my workspace.So what i want to do is , first to get values of these variables from workspace.Then i want to compare the lengths of these variables.If their lengths are not equal,i want to make them equal using interpolation.I am so very confused that i cant decide how to go ahead on this.Maybe i need to use regexp,whos,..but i dont know how to use them here.

Best Answer

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

The reason you are confused is because naming variables in this way is really not the way to solve anything. I don't know how you got the variables in the first place, maybe you individually named them all, but this is the type of problem you come to when you do that. Naming them this way in the first place is usually where people come unstuck, but even if they succeed in doing so accessing them afterwards is a real pain.

Use a struct with dynamic field names if you really want numbered items or even better just use an array - it is what they are for!

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MATLAB: Can I dynanically reference a variable name, similar to dynamic field references

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

You should be able to use the command form of load together with dynamic field names.

   thisdata = load('bigFile.mat', listOfVars{iVar});
  plot(thisdata.(listOfVars{iVar}).weight, thisdata.(listOfVars{iVar}).foodConsumed, 'x');

MATLAB: Does isfield return with false when the field does exist in a structure

Avoid creating dynamically named variables using eval 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 structures , where you can include fields for each kind of data (e.g. Process type, Flow data, Temperature data, Notes, Units, etc), or cell arrays . 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). Structures can be non-scalar , and you can even define structure fieldnames dynamically.

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://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

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

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