Generating a list of EVEN numbers in Python

Question

Basically I need help in generating even numbers from a list that I have created in Python:

[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, ...]

I have tried a couple different methods, but every time I print, there are odd numbers mixed in with the evens!

I know how to generate even/odd numbers if I were to do a range of 0-100, however, getting only the even numbers from the previous mentioned list has me stumped!

P.S. I've only been using python for a couple days, if this turns out to be extremely simple, thanks in advance!

EDIT: Thanks for all the replies, with your help I've gotten through this little problem. Here is what I ended up with to complete a little excercise asking to sum the even numbers of fibonacci sequence:

F = [1, 2]
while F[-1] < 4000000
    F.append(F[-1] + F[-2])

sum(F[1::3])
4613732

Solution 1

Use a list comprehension (see: Searching a list of objects in Python)

myList = [<your list>]
evensList = [x for x in myList if x % 2 == 0]

This is good because it leaves list intact, and you can work with evensList as a normal list object.

Hope this helps!

Solution 2

The following sample should solve your problem.

Newlist = []
for x in numList:
   if x % 2 == 0:
      print x          
      Newlist.append(x)

Solution 3

You can do this with a list comprehension:

evens = [n for n in numbers if n % 2 == 0]

You can also use the filter function.

evens = filter(lambda x: x % 2 == 0,numbers)

If the list is very long it may be desirable to create something to iterate over the list rather than create a copy of half of it using ifilter from itertools:

from itertools import ifilter
evens = ifilter(lambda x: x % 2 == 0,numbers)

Or by using a generator expression:

evens = (n for n in numbers if n % 2 == 0)

Solution 4

In your specific case my_list[1::3] will work. There are always two odd integers between even integers in fibonacci: even, odd, odd, even, odd, odd.....

>>> my_list = [1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
>>>         
... 
>>> my_list[1::3]
[2, 8, 34, 144, 610, 2584, 10946, 46368]

Solution 5

Just check this

A = [i for i in range(101)]
B = [x for x in A if x%2 == 0]
print B

View more solutions

Comments

[jathanism]

This case works, but because you're relying on slicing and stepping by index it's not a portable solution.

[Trufa]

I think list comprehension might be a little to advanced for the first days with python... but that's just my opinion!

[Erty Seidohl]

I see you're slicing the list, but there's two colons? What is this method called?

[Colin Dunklau]

@Trufa list comprehensions are one of the best parts about python... why not introduce them early?

[Erty Seidohl]

Oh I agree, but this is the best practice - and it leaves him with a list object.

[mgilson]

@Erty -- The third number is the "stride". You start at the first element and then take every 3rd element after that.

[Erty Seidohl]

This is if you want to print all the even numbers - if you want to get a list that you can work with, see the answers below. Edit: I see the new version appends it to a new list :)

[Trufa]

OP, this is a great answer but consider that lambda is a little too advanced for the first days of python :)

[Katriel]

Ugh, ifilter? Just (x for x in numbers if not x % 2) works.

[Trufa]

I just think it would be healthier to learn the for loops ins and outs before getting into list comprehension. But again, IMHO.

[David Webb]

Have added a generator expression, although I went for n % 2 == 0

[Ryan Haining]

nah, list comprehensions are way better for this kind of thing

[barbiepylon]

agreed...I've never seen them before. They look fast and efficient!

[jonrsharpe]

It would be nice if you included some information regarding what this does. Also, you could save a lot of code using the third step argument to range.

[bohnpessatti]

For a cleaner code, you could simply use if not x%2 instead of if x % 2 == 0