Whats mistake in this code....can anybody guide me?

Question

Shantanu K 2013-3-24

0
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关闭： MATLAB Answer Bot 2021-8-20

z = sym('2*x1 * x1 + x2*x2 + 3*x3*x3');
syms x1 x2 x3 a;
gradz = [diff(z,x1); diff(z,x2); diff(z,x3)]; 
x1(1)=2;
x2(1)=-2;
x3(1)=1;
for k=2:5
p=subs(gradz, [x1 x2 x3], [x1(k-1) x2(k-1) x3(k-1)]);
x1a=x1(k-1)+p(1)*a;
x2a=x2(k-1)+p(1)*a;
x3a=x3(k-1)+p(1)*a;
za=subs(z, [x1 x2 x3], [x1a x2a x3a]);
w=solve(za);
x1(k)=x1(k-1)+p(1)*w;
x2(k)=x2(k-1)+p(2)*w;
x3(k)=x3(k-1)+p(3)*w;
end

Its giving error

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the cyclist 2013-3-24

Which lines gives the error, and what is the full text of the error message?

Shantanu K 2013-3-24

在 MATLAB Online 中打开

Warning: Explicit solution could not be found. 
> In solve at 81
  In cauchyloop at 18
??? Error using ==> mupadmex
Error in MuPAD command: Subscripted assignment dimension mismatch
Error in ==> sym.sym>sym.subsasgn at 1420
            C = mupadmex('mllib::subsasgn',A.s,B.s,inds{:});
Error in ==> cauchyloop at 20
x1(k)=x1(k-1)+p(1)*w;

This is error i am geting at line x1(k)=x1(k-1)+p(1)*w;

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Answer 1

Walter Roberson 2013-3-24

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https://ww2.mathworks.cn/matlabcentral/answers/68410-whats-mistake-in-this-code-can-anybody-guide-me#answer_79710

When the explicit solution could not be found, solve() is returning the empty matrix, which you assign into "w". The empty matrix multiplied by p(1) gives the empty matrix. Add x1(k-1) to the empty matrix and you get the empty matrix. You then try to assign that empty matrix, which is of size 0, to x1(k) which is of size 1.