That bug has been fixed in master. subs works for me with your expression beq.subs(reversed(sd)) gives the same result as what you get with replace (the reversed is important so that it replaces higher derivatives first).
Aaron Meurer On Tue, Dec 20, 2016 at 6:30 AM, 程迪 <[email protected]> wrote: > Hi, everyone > > I am going to solve Blasius equation in a tranformed variable. > > The equation looks like: > > U'''+2U*U''=0 > U' == diff(U(y),y) > > Change of variable: > > x = (y-L)/(y+L) > U = y+V > > I cannot implement it correctly using **subs** or **xreplace** due to some > limitations(Incorrect behavior with subs and second derivatives) > > Currently, I have to implement it this way: > > from sympy import * > U,V,x,y = symbols('U,V,x,y') > L = symbols('L') > init_printing() > get_ipython().magic(u'matplotlib inline') > > # Blasius equation > beq = 2*Derivative(U(y),y,3)+Derivative(U(y),y,2)*U(y) > Eq(beq,0) > > # ## substitution of variables > > # from y to x > > #new variable: x > #old variable: y > y2x = 2*y/(L+y)-1 > > # from x to y > x2y = solve(y2x-x,y)[0] > > # ## Dictionary for change of variable > > sd = zip([diff(U(y),y,i) for i in > range(4)],[diff(y+V(y2x),y,i).subs({y:x2y}).simplify().doit() for i in > range(4)]) > sd > > > # Change of variables from high order derivative to low order ones. > # > # *sympy* function **replace** is used rather than **subs** which will give > incorrect result. > for sdi in sd[::-1]: > beq = beq.replace(*sdi) > beq=beq.simplify() > > # ## some hacks to generate code for chebfun.chebop in matlab > DIFF=symbols('diff') > sd2 = zip([Derivative(V(x),x,i)for i in range(4)],[DIFF(V,i) for i in > range(4)]) > > beq2 = beq.args[-1] > for sdi in sd2[::-1]: > beq2 = beq2.replace(*sdi) > beq2 > > print octave_code(beq2.subs({L:1})) > > > My question is: is there any way to do this more tight and clean? > > > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at https://groups.google.com/group/sympy. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/7706f861-d4b7-47ea-a705-466e0f29be14%40googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAKgW%3D6KmfTGhGYPoKH0%3DbQj3AaK4VPXCizgh7BJqHjMRQ43PWw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
