Thanks Aaron. Really appreciate. will start to migrate to python3. Thanks
again
On Thursday, 13 June 2019 19:48:03 UTC+1, Aaron Meurer wrote:
>
> Derivatives don't evaluate automatically after subs. You have to call
> doit().
>
> However, for your example, you are also replacing the derivatives. You
> will need to replace the derivatives first. Otherwise you will get 0
> because it will have Derivative(1, t) == 0. You can do the
> substitution in a single call with
>
> L.subs([(q1d, 3), (q2d, 7), (q1, 2), (q2, 1)])
>
> Or if you are using Python 3.6 or newer:
>
> L.subs({q1d: 3, q2d: 7, q1: 2, q2: 1})
>
> Prior to Python 3.6 dicts are not ordered, so that won't always work
> in older versions. But I do recommend using Python 3 for SymPy
> regardless as we will be dropping Python 2 support later this year.
>
> Aaron Meurer
>
> On Thu, Jun 13, 2019 at 12:43 PM Ash <[email protected] <javascript:>>
> wrote:
> >
> > Hello,
> >
> > I am having some doubts on how to use subs() in sympy. My aim is to do
> lagrangian mechanics which involves differentiation.The code below is a
> cooked up one but I guess it would serve the purpose.
> >
> > Please look at the code below
> >
> > from sympy.physics.mechanics import *
> > import cvxpy
> >
> > q1, q2 = dynamicsymbols('q1 q2')
> > q1d, q2d = dynamicsymbols('q1 q2', 1)
> > L = q1*q1d + q2*q2d
> > print L.subs(q1, 2).subs(q2, 1).subs(q1d, 3).subs(q2d, 7)
> >
> > # EXPECTED ANSWER: L = 2*3 + 1*7 = 13
> > # ACTUAL ANSWER: L = Derivative(1, t) + 2*Derivative(2, t)
> >
> > Please let me know how to modify the code to get the expected answer
> >
> > Thanks
> > Ash
> >
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