On Sat, Sep 15, 2012 at 12:12 AM, andrea crotti
<andrea.crott...@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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