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Re: Decorators not worth the effort

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On Sat, Sep 15, 2012 at 12:12 AM, andrea crotti
<andrea.crotti.0@gmail.com> wrote:
> def fib(n):
>     if n <= 1:
>         return 1
>     return fib(n-1) + fib(n-2)
>
> @memoize
> def fib_memoized(n):
>     if n <= 1:
>         return 1
>     return fib_memoized(n-1) + fib_memoized(n-2)
>
>
> The second fibonacci looks exactly the same but while the first is
> very slow and would generate a stack overflow the second doesn't..

Trouble is, you're starting with a pretty poor algorithm. It's easy to
improve on what's poor. Memoization can still help, but I would start
with a better algorithm, such as:

def fib(n):
	if n<=1: return 1
	a,b=1,1
	for i in range(1,n,2):
		a+=b
		b+=a
	return b if n%2 else a

def fib(n,cache=[1,1]):
	if n<=1: return 1
	while len(cache)<=n:
		cache.append(cache[-1] + cache[-2])
	return cache[n]

Personally, I don't mind (ab)using default arguments for caching, but
you could do the same sort of thing with a decorator if you prefer. I
think the non-decorated non-recursive version is clear and efficient
though.

ChrisA

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Re: Decorators not worth the effort Jean-Michel Pichavant <jeanmichel@sequans.com> - 2012-09-14 11:28 +0200
  Re: Decorators not worth the effort Duncan Booth <duncan.booth@invalid.invalid> - 2012-09-14 11:26 +0000
    Re: Decorators not worth the effort andrea crotti <andrea.crotti.0@gmail.com> - 2012-09-14 15:12 +0100
    Re: Decorators not worth the effort Chris Angelico <rosuav@gmail.com> - 2012-09-15 00:41 +1000
      Re: Decorators not worth the effort 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-14 09:37 -0700
      Re: Decorators not worth the effort 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-14 09:37 -0700
    Re: Decorators not worth the effort andrea crotti <andrea.crotti.0@gmail.com> - 2012-09-14 17:15 +0100
    Re: Decorators not worth the effort Chris Angelico <rosuav@gmail.com> - 2012-09-15 03:30 +1000
      Re: Decorators not worth the effort Neil Cerutti <neilc@norwich.edu> - 2012-09-18 13:19 +0000
        Re: Decorators not worth the effort Chris Angelico <rosuav@gmail.com> - 2012-09-18 23:25 +1000
          Re: Decorators not worth the effort 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-18 08:14 -0700
          Re: Decorators not worth the effort 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-18 08:14 -0700
    RE: Decorators not worth the effort "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-09-14 23:14 +0000
  Re: Decorators not worth the effort Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-14 12:01 +0000
    Re: Decorators not worth the effort Tim Chase <python.list@tim.thechases.com> - 2012-09-14 08:06 -0500
      Re: Decorators not worth the effort Steve Howell <showell30@yahoo.com> - 2012-09-14 18:13 -0700
  Re: Decorators not worth the effort Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2012-09-14 13:49 +0200

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