I'm having some trouble with the itertools.count function, and I don't quite understand what it does. I expect the code below to accomplish Project Euler problem 2.
I know that I could write this with a simple while loop, but is there a way to do it with a list comprehension? This code just freezes as I guess it's going to go infinity with count(). I would have hoped it would stop after x > MAX, but I know that won't happen. Is there a way to stop count in a generator expression like below?
def fib(n):
if (n <= 1): return 1
else: return fib(n-1) + fib(n-2)
MAX = 4000000
infiniteFib = (fib(x) for x in count())
s = (x for x in infiniteFib if x < MAX and x % 2 == 0)
print sum(s)
We just need to tell the infiniteFib
generator when to stop yielding elements. itertools
provides a number of useful methods to help with this:
less_than_max = itertools.takewhile(lambda x: x<MAX, infiniteFib))
even = itertools.ifilter(lambda x: x%2==0, less_than_max)
print sum(even)
We get a generator for all the numbers yielded by infiniteFib
, until one returned is greater than MAX
. Then we filter that generator, choosing only the even numbers. And finally we can sum the result.