Maybe it would be possible to do this using either the optimizations or postprocess options to cse().
Aaron Meurer On Sat, Sep 1, 2012 at 12:05 PM, Dimas Abreu Dutra <[email protected]> wrote: > Hello all, > > I'm using sympy for automatic code generation and differentiation. Before > code generation I perform common subexpression simplification. I'm getting > weird results in one of my expressions, though. I get a substitution for > (_cse1, x0**3) and then in my expression I get _cse1**(2/3) instead of > x0**2. Although both expressions are mathematically equivalent, numerically > x0**2 is much more efficient and robust. Bellow is a code snippet of this > particular expression. > >>>> import sympy >>>> variables = sympy.var('x:10 hyperpar:10 u:10 u_dot:10 dt theta:10') >>>> e = 0.5*dt**2*hyperpar9**2*x1 + 0.5*dt**2*hyperpar9*theta0*x0**3 + >>>> 0.5*dt**2*hyperpar9*theta1*x0 - 0.5*dt**2*hyperpar9*u0 - >>>> 1.5*dt**2*theta0*x0**2*x1 - 0.5*dt**2*theta1*x1 + 0.5*dt**2*u_dot0 - >>>> dt*hyperpar9*x1 - dt*theta0*x0**3 - dt*theta1*x0 + dt*u0 + x1 >>>> sympy.cse(e, sympy.numbered_symbols('_cse')) > ([(_cse0, dt**2), (_cse1, x0**3), (_cse2, theta1*x0), (_cse3, 0.5*_cse0), > (_cse4, _cse1*theta0)], [-1.5*_cse0*_cse1**(2/3)*theta0*x1 + > _cse2*_cse3*hyperpar9 - _cse2*dt + _cse3*_cse4*hyperpar9 + > _cse3*hyperpar9**2*x1 - _cse3*hyperpar9*u0 - _cse3*theta1*x1 + _cse3*u_dot0 > - _cse4*dt - dt*hyperpar9*x1 + dt*u0 + x1]) > > > Does anyone have any idea how to prevent such substitutions in cse? > > Thanks in advance, > ~Dimas > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To view this discussion on the web visit > https://groups.google.com/d/msg/sympy/-/JfSiGL3LTMUJ. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/sympy?hl=en. -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
