Is it possible to assign variables in the workspace to other variables using for loop?

I have variables like a_1_1, a_1_2, a_1_3, a_2_1, a_2_2, etc...... in the workspace. I want to assign these variables to another variable inside a for loop. Such that for each loop each variable will be loaded. For instance,

for i=1:100

for j=1:3

e=a_i_j

end

end

The abovementioned is just a pseudo code only for understanding.

How this can be done inside a for loop?

[Stephen23]

"I have variables like a_1_1, a_1_2, a_1_3, a_2_1, a_2_2, etc...... in the workspace"

Why?

Forcing meta-data into variable names makes accessing your data slow, inefficient, buggy, and complex:

https://www.mathworks.com/matlabcentral/discussions/tips/849901-tutorial-why-variables-should-not-be-named-dynamically-eval/2593476

So far you have not told us the most important information: how did you get all of those variables into the workspace? The place where those variables are loaded or imported into the workspace is the correct place to fix this bad data design. For example, by LOADing into an output variable:

S = load(..);

Answer 1

[Ayush Modi]

Hi Bipin,

Yes, it is possible. To dynamically access variables such as a_1_1, a_1_2, a_1_3, a_2_1, a_2_2, etc from the workspace within a loop, you can use the eval function.

Here is an example to demonstrate how you can achieve this:

for i = 1:100
    for j = 1:3
        varName = sprintf('a_%d_%d', i, j);       
        e = eval(varName);
        ...
    end
end

Refer to the following MathWorks documentation for more information on "eval" function:

https://www.mathworks.com/help/matlab/ref/eval.html

[BIPIN SAMUEL]

Thank you @Ayush Modi, is there any alternative way to do this instead of using 'eval'?

[Stephen23]

"is there any alternative way to do this instead of using 'eval'?"

Yes: by not forcing meta-data into variable names. We could show you how, if you answered my question here:

https://www.mathworks.com/matlabcentral/answers/2120936-is-it-possible-to-assign-variables-in-the-workspace-to-other-variables-using-for-loop#comment_3167836

[Fake Name Johnson]

Sure, anything is possible, but even the docs will tell you that it's a bad idea. I don't know why this is a recommendation.

The link given leads us here:

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

Variables with Sequential Names

A frequent use of the eval function is to create sets of variables such as A1, A2, ..., An, but this approach does not use the array processing power of MATLAB and is not recommended. The preferred method is to store related data in a single array. If the data sets are of different types or sizes, use a structure or cell array.

...

When somebody says "I'm painting the floor but the walls keep getting closer", the answer isn't "don't worry, it's possible to keep painting until you reach the wall". The answer is "stop painting yourself into the corner".

[BIPIN SAMUEL]

@Stephen23 These variables are from the database which was already created before; and dynamic naming was required for that data to give the labeling accordingly.

[Stephen23]

"These variables are from the database which was already created before... "

You still have not answered the question: how did that data get into the workspace?

"... and dynamic naming was required for that data to give the labeling accordingly."

It is very unlikely that forcing meta-data into variable name was required. Much better data design would simply and efficiently use actual indices, rather than awkwardly and inefficiently forcing pseudo-indices into variable names.

[BIPIN SAMUEL]

@Stephen23 data was loaded using load function.

[Stephen23]

"data was loaded using load function."

Do not LOAD directly into the workspace. Always LOAD into an output variable:

S = load(..);

S will be a scalar structure. You can access its fields using e.g.:

https://www.mathworks.com/help/matlab/ref/fieldnames.html

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

C = fieldnames(S);
S.(C{1}) % the first field

or generate the fieldnames using e.g. SPRINTF.

Much better: use actual indices rather than forcing pseudo-indices into variable names.

Answer 2

[Steven Lord]

Can you dynamically create or work with variables with numbered names like a_1_1, a_1_2, a_1_3, a_2_1, a_2_2, etc.? Yes.

Should you do this? The general consensus is no. That Discussions post explains why this is generally discouraged and offers several alternative approaches.