MX function free variable

[Mosc1]

Hello,

I am getting this error in matlab,

my matlab code is:

q0 = MX.sym('q0');

qx = MX.sym('qx');

qy = MX.sym('qy');

qz = MX.sym('qz');

q = [q0, qx, qy, qz]';

R_I2B = MX.sym('R_I2B',3,3);

R_I2B(1,1) = 2*(q0^2+qx^2)-1;

R_I2B(1,2) = 2*(qx*qy+q0*qz);

R_I2B(1,3) = 2*(qx*qz-q0*qy);

R_I2B(2,1) = 2*(qx*qy-q0*qz);

R_I2B(2,2) = 2*(q0^2+qy^2)-1;

R_I2B(2,3) = 2*(qy*qz+q0*qx);

R_I2B(3,1) = 2*(qx*qz+q0*qy);

R_I2B(3,2) = 2*(qy*qz-q0*qx);

R_I2B(3,3) = 2*(q0^2+qz^2)-1;

rotational_matrix_I2B = Function('rotational_matrix_I2B',{q},{R_I2B});

aa = rotational_matrix_I2B([0, 0, 0, 1]); <-- the error appears here

and the error is: 

Error using casadi.Function/call (line 942)

.../casadi/core/mx_function.cpp:404: Cannot evaluate "rotational_matrix_I2B:(i0[4])->(o0[3x3]) MXFunction" since variables [R_I2B]

are free.

Error in casadi.Function/paren (line 1770)

        res = self.call(varargin);

Error in casadi.Function/subsref (line 1751)

        [varargout{1:nargout}]= paren(self, s.subs{:});

Error in uav (line 86)

aa = rotational_matrix_I2B(ctr.x0(7:10));

Anyone know how I can fix it?

Best Regards

[Joris Gillis]

The expression R_I2B you end up with after all the assignments is still dependant on the symbol R_I2B.

You cannot evaluate Functions that omit some symbols from the list of inputs.

Use MX.zeros to initialze R_I2B, not a symbol.

  Joris

[Mosc1]

You are right, my fault. Many thanks =D

[Mosc1]

I have tested the same code with the only difference that insted to use MX variable I have used SX, and It works without the initializzation of R_I2B to zero. Could you explain why ?

Many thanks for your support.

[Joris Gillis]

SX comes with more advancee simplication logic,

such that the dependence on the stray symbol is dropped.

This is bad style regardless, don't create symbols if you don't need them..

  Joris

[Mosc1]

Many thanks for your advice. I will apply them. 

[yingjie jiang]

Thank you for you explanation. I'm new to CasADi, could you please tell me why R_I2B is still dependant on R_I2B after the assignment? Isn't it already defined by q? Besides, I don't understand how R_I2B can depend on itself, what does this mean?

Thanks!