Thanks for your replies.
as_coeff_exponent(...) does the right thing for the (similar) Integral
operation on f(x), so I'm not sure big changes are needed to fix this
little glitch.
I think this traces back to something slightly strange in sympy.collect
(...) which is used by as_coeff_exponent. The behaviour for the fourth
line of input below seems odd.
I'll dig a little more and see what I find.
Cheers,
andy
>>> Integral(f(x),x).as_coeff_exponent(f(x)) #This is OK
(Integral(f(x), x), 0)
>>> Derivative(f(x),x).as_coeff_exponent(f(x)) #This is what I first posted
>>> about, seems very odd.
(1, 1)
>>> collect(Integral(f(x),x),f(x),evaluate=False) #This seems OK.
{1: Integral(f(x), x)}
>>> collect(Derivative(f(x),x),f(x),evaluate=False) #This looks odd to me -
>>> collect has lost track of the derivative.
{f(x): 1}
also:
>>> collect(Derivative(2*f(x),x,x),f(x),evaluate=False) #Probably OK...
{1: D(2*f(x), x, x)}
>>> collect(2*Derivative(f(x),x,x),f(x),evaluate=False) #Weird result again -
>>> collect loses derivative operation again.
{f(x): 2}
On Oct 22, 10:47 pm, Ondrej Certik <ond...@certik.cz> wrote:
> On Thu, Oct 22, 2009 at 3:43 PM, Ronan Lamy <ronan.l...@gmail.com> wrote:
>
> > Le jeudi 22 octobre 2009 à 14:42 -0700, Ondrej Certik a écrit :
> >> On Thu, Oct 22, 2009 at 2:34 PM, andy2O <and...@hotmail.com> wrote:
>
> >> > Hi all,
>
> >> > as_coeff_exponent(...) seems to ignore derivatives, as shown below
>
> >> > Is this behaviour intended? If so I can report it as an issue - but
> >> > last time recorded an issue it turned out I just misunderstood atoms
> >> > (), so I thought I'd check first here! :)
>
> >> Sure. :)
>
> >> > Thanks,
> >> > andy
>
> >> >>>> ================================ RESTART
> >> >>>> ================================
> >> >>>> from sympy import *
> >> >>>> var('x')
> >> > x
> >> >>>> f=Function('f')
> >> >>>> d=diff(f(x),x)
> >> >>>> d
> >> > D(f(x), x)
> >> >>>> d.as_coeff_exponent(f(x))
> >> > (1, 1)
>
> >> What do you suggest to be a correct answer? I never used
> >> as_coeff_exponent inside a derivative.
>
> >> Ondrej
>
> > The correct answer is clearly (D(f(x), x), 0), by analogy with
> > cos(x).as_coeff_exponent(sin(x)) == (cos(x), 0). So, yes, Andy, you've
> > uncovered a real issue.
>
> > I think that the fundamental problem here is that there is no object
> > representing the derivative of a function (the current Derivative is an
> > operation applying to expressions, not functions).
>
> Yes, I think the Derivative should be improved to be able to represent
> a general derivative of a function f(x).
>
> Ondrej
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