How do I create a for loop for the this example?

Dear all, I have a code as follow:

 for i = 1:5
    syms (['X_' , num2str(i)])
 end 

 J = (X_2 - X_1)^2 + (X_3 - X_2)^2 + (X_4 - X_3)^2 + (X_5 - X_4)^2 ;

 B_1 = diff(J,X_1)/2  ; B_2 = diff(J,X_2)/2 ; B_3 = diff(J,X_3)/2 ; ...
 B_4 = diff(J,X_4)/2 ; B_5 = diff(J,X_5)/2 ;

 unknown = [X_1 X_3 X_4 X_5];

Now I would like to do the below operation for all from B_1 until B_5 in a loop

B_1sorted = collect(B_1,unknown); %Sort symbols from Psi_1 to Psi_100
[c,t] = coeffs(B_1,unknown);
index = 0;
newc = zeros(1,length(unknown));
for k = 1:length(unknown)
    if any(t==unknown(k)) 
        index = index + 1;
        newc(k) = c(index);
    end
end

any help would be greatly appreciated,

[Stephen23]

You should read this:

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

If you only have five variables then using a loop and eval is a total waste of time and only serves to make your code slow, complex, obfuscated, and pointlessly hard to read. Why bother?

Rather than writing this awkward code, simply use an array. Indexing is neat, fast, easy to debug, easy to understand, and highly efficient. Or perhaps define a simple function and call that. But using eval on this problem is like cracking a walnut with steamroller.

[ehsan]

Thank you Stephen,

Thanks for your useful link and prompt response.

Actually, you are right, but in my case, I have a lot of variables (it can be 200 or much more).

The way Kaushik did according to my codes, it makes code slow but it works.

Regards,

[Kaushik Lakshminarasimhan]

I agree with Stephen. I suggested eval only because your set up used symbolic variables and I assumed you were simply looking for a quick hack. This solution is not sustainable if you have many more variables.

Answer 1

[Kaushik Lakshminarasimhan]

You can use eval. Something like this should work, although you need to take care of saving your variables at the end of each iteration.

for i=1:5
    eval(['B_' num2str(i) 'sorted = collect(B_1,unknown)']);
    [c,t] = coeffs(eval(['B_' num2str(i)]),unknown);
    index = 0;
    newc = zeros(1,length(unknown));
    for k = 1:length(unknown)
        if any(t==unknown(k))
            index = index + 1;
            newc(k) = c(index);
        end
    end
    %%save your variables from the i^th iteration
end

[ehsan]

Thank you Kaushik, for your prompt reply. Your solution works nicely.