Subbing into a constant function in symbolic toolbox

All,

This doesn't work.

syms x y

f=3*y^2-2*y^3-3*x^2+6*x*y;

fx=diff(f,x);

fy=diff(f,y);

fxx=diff(f,x,2);

fyy=diff(f,y,2);

fxy=diff(diff(f,x),y);

S=solve(fx,fy);

FXX=subs(fxx,{x,y},{S.x,S.y});

H=fxx*fyy-fxy^2;

subs(H,{x,y},{S.x,S.y});

[S.x,S.y,FXX,H]

Wondering if it is a bug. The subs command in the symbolic toolbox does not produce a vector result when subbing a vector.

>> FXX=subs(fxx,{x,y},{S.x,S.y})

FXX =

      -6

This should be a two by one vector with two -6's in it.

Is this a bug?

Workaround?

David.

[bym]

why do you think it is a bug? fxx = -6; there is nothing to substitute into?!?

Answer 1

[Abhishek Gupta]

It does feel like a bug to me. I came up with a crude workaround/hack:

syms x y
f=3*y^2-2*y^3-3*x^2+6*x*y;
fx=diff(f,x);
fy=diff(f,y);
fxx=diff(f,x,2);
fyy=diff(f,y,2);
fxy=diff(diff(f,x),y);
S=solve(fx,fy);

% Add a variable which you do not expect to be in the calculation before substitution
syms z
FXX=subs(fxx+z,{x,y},{S.x,S.y})
FXX =
z - 6
z - 6
% Substitute zero for the variable to get your original calculation
FXX = subs(FXX,z,0)
FXX =
    -6
    -6

Hope this helps.