Question

The solution is probably rather simple, but I just can’t figure it out. Here is the code, it’s a simple fibonacci number generator. The goal is to sum up all even fibonacci numbers below 4,000,000.

My approach is to first generate all fibonacci numbers below 4,000,000, and then either:
a) generate a new list (“even”) with the even ones (this works fine)
b) removing the odd ones from the list “all”

However, in the latter case, the output is, for reasons I don’t understand, this:
[2, 5, 8, 21, 34, 89, 144, 377, 610, 1597, 2584, 6765, 10946, 28657, 46368, 121393, 196418, 514229, 832040, 2178309, 3524578]

Any help is much appreciated. Thanks!

all = []
even = []

def fibonacci():
    a, b = 1, 2
    while a < 4000000:
        all.append(a)
        a, b = b, a + b
    print all

##Putting all the even fibonacci numbers in a different list and summing them up works fine
#    for i in all:
#        if i % 2 == 0:
#            even.append(i)
#    print even                   
#    print sum(even)


# But for some strange reason I can't figure out how to remove the odd numbers from the list
    for i in all:
        if i % 2 != 0:
            all.remove(i)

    print all
    print sum(all)

fibonacci()

Answer 1

This is a “gotcha” situation: you’re removing items from a list while iterating the list, thus changing the list, causing your iteration to behave unexpectedly. Try this:

...
# But for some strange reason I can't figure out how to remove the odd numbers from the list
    for i in all[:]:
        if i % 2 != 0:
            all.remove(i)
...

This is what’s called “slice” notation, and causes you to iterate a throwaway copy of the list so that your iteration is not affected by the all.remove() calls.