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

Question

Looking for some help with this project Euler question: By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

I am sure there are other, simpler ways to do this but I am just starting out! I have managed to get the code to output the sum of the even terms of the fibonacci sequence, but I have no idea how to set the output limit to four million (I have just set the range 1 - 10 for testing). Is there anyway to do this with the current code I have written, rather than starting again?

def fibonacci(n): 
    if n==0:
        return 0
    elif n==1 or n==2:
        return 1
    elif n>2:
        return (fibonacci(n-1)+fibonacci(n-2))


fib_list=[fibonacci(n) for n in range (1, 10) if fibonacci(n)%2==0]
fib_even=sum(fib_list)
print(fib_list)
print(fib_even)

Answer 1

Here is my solution:

def fibonacci():
   sequence = [1, 2]
   total = 0

   while sequence[-1] < 4000000:
       if sequence[-1] % 2 == 0:
           total += sequence[-1]
       sequence.append(sequence[-1] + sequence[-2])

   print(total)

The check on the last element in the list ensures that it does not run over 4 million. This is also what sgfw meant by their response. I am not sure how you would go about implementing that for a list comprehension - it wouldn't be my first choice for solving this problem.

Answer 2

you can use the built-in functions sum and filter :

def fib(limit):
    a, b = 0, 1
    while a < limit:
        yield a
        a, b = b, a + b

sum(filter(lambda x: x%2==0, fib(4_000_000)))

output:

4613732

the fib function will generate all the Fibonacci numbers while the filter function will filter out those numbers that are not even, and finally, the sum built-in function will add all the even numbers

Answer 3

A "while" loop may be more suitable for this problem than a list comprehension. It may even be easiest to use a "while True" loop with a conditional "break" statement.