On Sat, Dec 13, 2008 at 08:59:08AM -0800, Aaron wrote:
>
> I'm a newbie to sympy. The problem is, I do not know whether it is
> possible to get the derivative with respect to a function, not a
> symbol. The code is like this:
>
> t = Symbol("t")
> f = Function("f")
>
> theta = f(t)
> theta_dot = diff(theta,t,1)
> x = theta*theta_dot
> y = diff(x,theta_dot,1)
>
> Then the output is
> D(f(t)*D(f(t), t), D(f(t), t))
>
> However, my expected output should be "theta" or "f(t)" instead.
Now, I have this idea:
% isympy
In [1]: var('t xDot')
Out[1]: (t, xDot)
In [2]: g = lambdify((x, xDot), x*xDot)
In [3]: g(x, xDot)
Out[3]: x*xDot
In [4]: g(f(t), diff(f(t), t))
Out[4]:
d
f(t)*──(f(t))
dt
In [5]: gDot = lambdify((x, xDot), diff(g(x, xDot), xDot))
In [6]: gDot(f(t), diff(f(t), t))
Out[6]: f(t)
By,
Friedrich
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