Can you just get what you want by overriding _eval_derivative()? Aaron Meurer
On Fri, Jun 3, 2011 at 7:48 PM, Gilbert gede <gilbertg...@gmail.com> wrote: > I guess I should have asked this as well; is it considered bad to > write diff() and replace the current Sympy diff() within my code? Or > is that OK? > > -Gilbert > > > > On Jun 3, 6:38 pm, Gilbert gede <gilbertg...@gmail.com> wrote: >> Yeah, I had read some of them. I had already thought of writing my >> own Diff method or something and do substitution with it, but was >> hoping to have the functionality I want work like standard SymPy >> operations. That's what I've been trying to do with my PyDy classes; >> make them work more like you would expect other SymPy objects to. >> I have read through Derivative() and diff(), and couldn't really find >> a way to make them do what I want (like I said about my symbol >> extension no longer having its methods called once it is inside a >> SymPy add or mul). I guess what I was hoping for was input on whether >> I could make Derivative do what I want with my extended Symbol, as I >> couldn't really see how. But if writing my own Diff method is the >> only option, there's not much I can do then. >> >> Thanks, >> -Gilbert >> >> On Jun 3, 5:53 pm, "Aaron S. Meurer" <asmeu...@gmail.com> wrote: >> >> >> >> >> >> >> >> > This has actually been discussed quite a bit before (a lot of people want >> > to use Lagrangians). You can search the mailing list. From what I've >> > seen, you will either have to write your own custom diff routine or do >> > clever substitution of functions and derivatives with symbols. I don't >> > think I've ever seen anyone suggest extending Symbol to hold a time >> > derivative, which is essentially just a more formal way of doing the >> > substation method. It might work. >> >> > Aaron Meurer >> >> > On Jun 3, 2011, at 6:05 PM, Gilbert Gede wrote: >> >> > > Hi, >> > > I was trying to implement some functionality for PyDy for this year's >> > > GSoC, and was looking for some advice. >> > > In dynamics problems, you usually have time-varying quantities, like >> > > generalized coordinates, speeds, and accelerations. Often, you want to >> > > take the partial derivative of an expression with respect to the time >> > > derivative of one of these quantities. This come up when using >> > > Lagrange's Method (or Kane's Method). It's described to some degree >> > > here: >> > >http://en.wikipedia.org/wiki/Lagrangian_mechanics >> > >https://gist.github.com/1005937 >> > > In Lagrange's Method, you end up taking the partial derivative of the >> > > energy with respect to the time derivative of a generalized coordinate. >> > > I'm trying to figure out a way to make this work in PyDy/SymPy. >> > > Derivative won't take in anything but a Symbol. >> > > The only idea I have come up with is to extend Symbol and write my own >> > > .diff() method for it which returns a new symbol representing the time >> > > differentiation of the original extended Symbol. Once my new object is >> > > inside a Mul or Add sympy object, then my .diff() method is no longer >> > > called. >> > > Can anyone give some insight into how I could get this desired behavior, >> > > taking the derivative of an expression wrt a time-differentiated symbol, >> > > to work in a way consistent with existing SymPy behavior? Thanks. >> >> > > -Gilbert >> >> > > -- >> > > You received this message because you are subscribed to the Google >> > > Groups "sympy" group. >> > > To post to this group, send email to sympy@googlegroups.com. >> > > To unsubscribe from this group, send email to >> > > sympy+unsubscr...@googlegroups.com. >> > > For more options, visit this group >> > > athttp://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 sympy@googlegroups.com. > To unsubscribe from this group, send email to > sympy+unsubscr...@googlegroups.com. > 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 sympy@googlegroups.com. To unsubscribe from this group, send email to sympy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
