Calculate the sum of even Fibonacci numbers

Question

This is my solution for problem 2 of Project Euler using Python:

a, b = 1, 1
total = 0
while a <= 4000000:
 if a % 2 == 0:
 total += a
 a, b = b, a+b # the real formula for Fibonacci sequence
print (total)

I put the codes in the function IsEven:

def IsEven(n):
 if n % 2 == 0:
 return True
 else:
 return False

• Mast: Did you take a good look at the feedback on your previous question?
• Amir Motefaker: @Mast - Yes. I took a good look at the feedback on my previous question. Is there a problem I did not fix?
• Toby Speight: The question still is not self-contained; please edit to add a short summary of the problem statement. You can keep the link as a reference, but your question does need to be complete even if the linked material isn't accessible. Thanks.
• Toby Speight: Have you looked at other reviews of code solving this problem? Some of them even appear in the "Related" sidebar, normally on the right.

Answer 1

Modularity

We have the printing and the constraints (the constant 4000000) mixed in with the logic of the code. If we create a function, we can unit-test it, and we can time its execution (both of which will be useful to you if you attempt other Project Euler challenges). Perhaps something like

def sum_even_fibonacci(limit)
 '''Return the sum of all even Fibonacci numbers
 not greater than LIMIT
 '''
 a, b = 1, 1
 total = 0
 while a <= limit:
 if a % 2 == 0:
 total += a
 a, b = b, a+b
 return total

Simplify

The function that's unused is very long winded. Any time we write if condition: return True; else: return False, we could simply write return condition. I.e.

def IsEven(n):
 return n % 2 == 0

Algorithm

Instead of generating all the Fibonacci numbers, use your mathematical knowledge to generate just the even ones. Can you work out a formula for their sum?

• miracle173: I think "not greater than 'limit'" would be more appropriate in the comments "not greater than LIMIT", because name in python are case sensitive
• Toby Speight: I'm not really sure of conventions for Python doc-comments, but upcasing the argument names in Emacs Lisp documentation doesn't seem to cause any confusion there, and I find it does help users to quickly find where each argument is used.