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

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

[the cyclist]

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

[Shantanu K]

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;

Answer 1

[Walter Roberson]

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.

[Shantanu K]

Hello Walter,

Variable 'w' is not empty. It is having some finite value. 'za' is linear equation which should be solved to produce 'w'.