Project Euler Problem 2: Fibonacci Numbers

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

Version 1: The obvious but inefficient way

In [1]:
from itertools import takewhile

def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b
In [2]:
sum(filter(lambda x: x%2==0, takewhile(lambda x: x <= 4*10**6, fibonacci())))
Out[2]:
4613732

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