Index with for loop variable name and definition

I have a matrix A and avector y. I want to create a loop which names and defines them the matrix A from A1,...,A10.

for i = 1:10
    A{i} = X(y==i)
end

Unfortunately this code does not work. It would be nice to end up with a double and not a cell

[Stephen23]

"I want to create a loop which names and defines them the matrix A from A1,...,A10."

Dynamically accessing variable names is one way that beginners force themselves into writing slow, complex buggy, code. Read this to know why:

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

Using a cell array with indexing is simple, neat, and very efficient. You should use a cell array with indexing.

Note that when you put a number into the variable name like that then you are using it as pseudo-indexing: note that processing pseudo-indexing is complex and slow. In contrast, real indexing of a cell array (as your question shows) is extremely fast, efficient, and easy to debug.

So far you have not provided any reason why you need to write much slower, more complex, buggier code. Cell arrays are designed to efficiently hold data arrays, exactly like you want to now.

Answer 1

[D. Plotnick]

If I understand your question, it is parallel to the one here, which as Stephen Cobeldick points out is a bad idea.

There is sort of a way around this using a structure.

 for i = 1:10
 fname = ['A',num2str(i)];
 B.(fname) = y(i);
 end

[Stephen23]

"which as Stephen Cobeldick points out is a bad idea."

I did not not write the MATLAB documentation which advises against this:

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

I did not write the thousands of threads where expert MATLAB users advise against doing this:

https://www.mathworks.com/matlabcentral/answers/57445-faq-how-can-i-create-variables-a1-a2-a10-in-a-loop

https://www.mathworks.com/matlabcentral/answers/143-how-do-i-make-a-series-of-variables-a1-a2-a3-a10

I did not write the MATLAB wiki page:

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

I am not a member of TMW staff who advises against doing this:

https://www.mathworks.com/matlabcentral/answers/302359-access-variables-in-several-mat-files#comment_389856

https://www.mathworks.com/matlabcentral/answers/235119-can-you-explain-this-weird-behavior

@Daniel Plotnick: please do not describe me as the source of this advice, as it distracts from the real source of this information: the writers of the MATLAB documentation, and MATLAB users with much more knowledge than I have. It would be appreciated if you referred to the actual sources in your future comments.

[Stef]

Ok, say I create the variables A1 to A10 manually. Is there a possibility to create a loop through the index of A, so that I can choose the A I want through its name in a loop?

[Stephen23]

"Is there a possibility to create a loop through the index of A, so that I can choose the A I want through its name in a loop?"

If you really want to write slow, complex, buggy code then the links in my comments ( here and here) reference how you can do this.

Note that both creating variable names dynamically and accessing variable names dynamically suffers exactly the same problems: slow, inefficient, more liable to bugs, much harder to debug. Also note that once you start defining/accessing variable names dynamically then you force yourself into writing the rest of your code inefficiently, because for the rest of the code you will have to use the same slow, complex, buggy ways to access your data.

What is the problem with using a cell array (which is simple, efficient, easy to debug, etc.)?

[Stef]

Thanks, it worked!

[Stephen23]

@Stef: note that your original code, using a cell array and indexing, is faster and simpler than this answer.

You might also find this to be of interest:

https://www.mathworks.com/matlabcentral/answers/225435-save-variable-as-string-from-user-input