I'm probably missing something basic, but why not use functions x(t),
v(t) etc. in the first place?
Renato
On Fri, Jun 3, 2011 at 11:56 PM, Aaron Meurer <asmeu...@gmail.com> wrote:
> I see. Yeah, that's because of the optimization built in to
> Derivative.__new__ that does diff(expr, x) => 0 if x is not in
> expr.free_symbols. This should probably be factored out to just be a
> default _eval_derivative action in Expr, so that subclasses can
> override it. On the other hand, if your object is going to be
> something akin to x(t) or x'(t), then t should be in the free_symbols.
>
> Aaron Meurer
>
> On Fri, Jun 3, 2011 at 8:53 PM, Gilbert gede <gilbertg...@gmail.com> wrote:
>> Yeah, if you try diff(x,y), the method is not called.
>> Ronan, I'm not sure how to do what you're describing? How would I
>> call that?
>>
>> -Gilbert
>>
>>
>> On Jun 3, 7:32 pm, Ronan Lamy <ronan.l...@gmail.com> wrote:
>>> Le vendredi 03 juin 2011 à 19:16 -0700, Gilbert gede a écrit :
>>>
>>> > You're talking about Symbol._eval_derivative? I tried that within my
>>> > extended Symbol class. It returns 0 or 1 testing self == symbol. I
>>> > tried making some changes to it, but I don't think I can use it. I
>>> > think it doesn't even get called unless you do something like:
>>> > t = timevaryingsymbols('t')
>>> > Derivative(2+3*t,t)
>>> > I think only when t is both part of (expr, symbols, ...) within
>>> > Derivative's __new__ definition does t's _eval_derivative() method get
>>> > called.
>>>
>>> That seems to be a recent "optimisation" from commit 2361dd86. You
>>> should revert this to the old behaviour: call
>>> expr._eval_derivative(symbol) in all cases.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> > On Jun 3, 6:53 pm, Aaron Meurer <asmeu...@gmail.com> wrote:
>>> > > 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
>>>
>>> > > >> > > --
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