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

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ehsan on 18 Dec 2017
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,
##### 3 CommentsShow 1 older commentHide 1 older comment
ehsan on 18 Dec 2017
Thank you Stephen,
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 on 18 Dec 2017
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.

Kaushik Lakshminarasimhan on 18 Dec 2017
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 on 18 Dec 2017