MATLAB: Defining a variable based on input

define variableMATLAB

I am trying to create a set of variables based on input.

For example, I have the matrix

angle=[30,45,60]

I do some calculations using each angle and I want to put the results into individual matrices for each angle. I want each matrix to be named partially using the respective input angle. The number of inputs in angle(:) and their values are arbitrary.

For example, the first matrix would be named Qbar30, the second Qbar45, and the third Qbar60. Is there a way to declare variables with an input and a preceding string like 'Qbar'?

Best Answer

If that is what you want, try eval

doc eval

However, you can skip making each variable with a new name and try cell or structure.

Related Solutions

MATLAB: How to give different inputs for a loop in each and every iteration

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

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

MATLAB: How can use a for loop to name a matrix

Use a cell array like this:

out = cell(1,max_trials);
for trial = 1:max_trials
    out{trial} = ... your calculation here
end

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 cell arrays or structures, where you can include fields for each kind of data (e.g. Process type, Flow data, Temperature data, Notes, Units, etc). 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). And yes, 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://de.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

Best Answer

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MATLAB: How to give different inputs for a loop in each and every iteration

MATLAB: How can use a for loop to name a matrix

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