Function composition for user-defined functions

[Amit Saha]

Hi all,

I understand that the compose() function defined in
sympy.polys.polytools can be used as follows:

>>> from sympy import compose, Symbol
>>> x = Symbol('x')
>>> compose(x+1, x+1)
x + 2


Now, let's say I have a user defined function defined as follows:

from sympy import Function, Symbol, solve, compose, sin, cos

class fx(Function):

@classmethod
def eval(cls, x):
return x**2

def _eval_evalf(self, prec):
return self.eval()

I tried to use the compose() function as follows:

compose(fx(x), x+1) and I get the answer as follows, x**2 + 2*x + 1
which is correct.

Is this the best way to demonstrate function composition for user
defined functions?

Thanks,
Amit.


--
http://echorand.me

[Aaron Meurer]

That only works because fx(x) always gives x**2. If it gave anything other than a polynomial (e.g., being unevaluated), it wouldn't. 

compose() is polynomial function. It's designed to efficiently compose two polynomials and expand the result. compose(a, b) is effectively the same as a.subs(x, b).expand(), but more efficient.

For general functions, I would just use subs, or Subs if you want something unevaluated.

Aaron Meurer

 

Thanks,
Amit.


--
http://echorand.me

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