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"DOUBLE cannot convert the input expression into a double array. If the input expression contains a symbolic variable, use the VPA function instead."

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My command window code is:
syms L C gamma lambda k K X U FOG SOG u1 u2 u3 eps1 eps2;
C=u1^2+3*u2^2+2*u3^2;
L = u1*u2+2*u1*u3+3*u2*u3;
U=[u1 u2];X=[u3];
C=u1^2+3*u2^2+2*u3^2;
gamma=4;
Cx=jacobian(C,X);
Cu=jacobian(C,U);
Lu=jacobian(L,U);
Lx=jacobian(L,X);
lambda_T=-Lx*inv(Cx);
Lbar_u=Lu+lambda_T*Cu;
Lbar_x=Lx+lambda_T*Cx;
Lbar_ux=jacobian(Lbar_u,X);
Lbar_xx=jacobian(Lbar_x,X);
Lbar_xu=jacobian(Lbar_x,U);
Lbar_uu=jacobian(Lbar_u,U);
Lxu=jacobian(Lx,U);
Lux=jacobian(Lu,X);
Lbar_uu_star=Lbar_uu-Lbar_ux*inv(Cx)*Cu-(Cu)'*((Cx)')^(-1)*Lbar_xu+(Cu)'*((Cx)')^(-1)*Lbar_xx*inv(Cx)*Cu;
u3=-1;u1=1;u2=1;
k=1;K=.500330;
FOG=0;SOG=1;
Opt(0,K,1,Lbar_uu_star,L,X,U,C,k,gamma,lambda_T,Cx,Cu,Lx,Lu,Lxu,Lux,Lbar_u,Lbar_uu,Lbar_x,Lbar_xu, Lbar_xx, Lbar_ux,eps1,eps2);
My M-file Opt.m is:
function[] = Opt(FOG,K,SOG,Lbar_uu_star,L,X,U,C,k,gamma,lambda_T,Cx,Cu,Lx,Lu,Lxu,Lux,Lbar_u,Lbar_uu,Lbar_x,Lbar_xu, Lbar_xx, Lbar_ux,eps1,eps2)
while abs(double(subs(C))-double(subs(gamma)))>=(double(subs(eps1)))
X=(double(subs(X)))-inv(double(subs(Cx)))*(double(subs(C))-(double(subs(gamma))));
if abs(double(subs(C))-(double(subs(gamma))))<(double(subs(eps1)))
if abs(double(subs(Lbar_u)))<(double(subs(eps2)))
break;
else
k=k+1;
if FOG == 1
U=(double(subs(U)))-K*(double(subs((Lbar_u)')));
elseif SOG == 1
U=(double(subs(U)))-inv(double(subs(Lbar_uu_star)))*(double(subs(Lbar_u)));
end
end
end
end
L_star=((double(subs(X^2/2))))+(double(subs((U^2)/2)))-2*(double(subs(X)))-2*(double(subs(U)));
X_star=double(subs(X));
U_star=double(subs(U));
if FOG == 1
display('First order gradient method')
elseif SOG == 1
display('Second order gradient method')
end
display(X_star);display(U_star);display(L_star);display(k);
end
errors I get after running the code are:
Error using mupadmex Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use the VPA function instead.
Error in sym/double (line 702) Xstr = mupadmex('symobj::double', S.s, 0);
Error in Opt (line 3) while abs(double(subs(C))-double(subs(gamma)))>=(double(subs(eps1)))
I have no Idea what is going on. Any help would be highly appreciated

Answers (1)

Carlos
Carlos on 4 Apr 2013
The problem comes in
double(subs(C))
From your code
C=u1^2+3*u2^2+2*u3^2;
so when you do
subs(C)
ans =
u1^2 + 3*u2^2 + 2*u3^2
C remains unchanged, so you cannot convert a symbol to a number.
If you did
subs(C,{u1,u2,u3},{1,1,1})
ans =
6
So in conclusion you cannot convert a symbolic expression containing variables into a number.
  4 Comments
upinderjeet
upinderjeet on 4 Apr 2013
Walter and Carlos
Thanks first of all for the quick reply. Actually If you notice the while loop the value of u1,u2 and u3 is getting changed if the certain conditions are met. So I want "C" to acquire the latest value of itself according to the updated values of u1,u2 and u3. So by using "double(subs())" command I want to accomplish that and actually everything worked fine at the end for my previous issues on which walter was helping me but this is similar and related problem but the only differnce is that X and U now are vector parameters unlike the scalar case of the past Walter.
Walter Roberson
Walter Roberson on 5 Apr 2013
When you subs() and expect symbolic variables to be replaced by what is in the workspace, the substitution is limited to variables existing in the current workspace. In your code, u1 and so on are defined in the caller of the routine that does the subs()

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