Project Euler: Even Fibonacci numbers

Question

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.

My program prints 4613731, but it is supposed to be 4613732. Where is the problem?

def fib(n):
    if n == 0:
        return 1
    elif n == 1:
        return 2
    else:
        return fib(n-1)+fib(n-2)

tot = 0
n = 0
while fib(n) <= 4000000:
    if fib(n) % 2 != 0:
        tot += fib(n)
    n += 1

print(tot, n)

You calculate `fib(n)` three times for every `n` , keep it optimize. — ᴀʀᴍᴀɴ, Dec 26 '17 at 20:13
@Arman Uh... three times would actually be very good compared to how often they *really* do it. — Stefan Pochmann, Dec 26 '17 at 20:14
Another reason to calculate `fib(n)` once and save it to a variable is that you could easily log it, if you did that you would've seen the problem instantly. — Akavall, Dec 26 '17 at 20:20
@StefanPochmann Why would that have been a clue by itself? The sum of an even number of odd numbers is even — ubadub, Dec 26 '17 at 20:20
@Arman I made it count the `fib` calls... 35563500 calls. For n from 0 to 32. That's on average 1077682 calls for each n, not just three. — Stefan Pochmann, Dec 26 '17 at 20:21
@ubadub Because they thought they're summing even numbers, and the sum of even numbers is even. So when they saw an odd sum, they should've realized they're not summing even values. — Stefan Pochmann, Dec 26 '17 at 20:23
@StefanPochmann , OMG , I just tried to make him optimize his code , it is a disaster! — ᴀʀᴍᴀɴ, Dec 26 '17 at 23:57
because `fib(n)` is a recursive function , and for big numbers in it will be called thousands of times. this is for your old code. — ᴀʀᴍᴀɴ, Dec 27 '17 at 20:56

score 1 · Answer 1 · answered Dec 26 '17 at 20:13

1

if fib(n) % 2 != 0:
    tot += fib(n)

This checks for odd values, since an even mod (%) 2 is 0

Also, you're calculating fib(n) three times. Might wanna do something about that.

answered Dec 26 '17 at 20:13

ubadub

3,571
21
32

score 1 · Answer 2 · answered Dec 26 '17 at 20:26

Thank you for the reply! I forgot what 'even' meant, sorry for wasting your time! I also improved my code,

tot = 0
a = 1
b = 1
h = 0

while h <= 4000000:
    if h % 2 == 0:
        tot += h
    a = b
    b = h
    h = a + b

print(tot)

Project Euler: Even Fibonacci numbers

2 Answers2