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

Question

ehsan 2017-12-18

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/373430-how-do-i-create-a-for-loop-for-the-this-example

评论： Kaushik Lakshminarasimhan 2017-12-18

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,

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ehsan 2017-12-18

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 2017-12-18

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.

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Answer 1

Kaushik Lakshminarasimhan 2017-12-18

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https://ww2.mathworks.cn/matlabcentral/answers/373430-how-do-i-create-a-for-loop-for-the-this-example#answer_296680

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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