from the desk of stinkpot

extract odd or even elements from a python list

October 12th, 2007 by Lawrence David

here’s how to extract every other element from a python list using one line of code. to get even elements, use “i%2 == 0″; for odd elements, use “i%2 == 1″.

>>> my_list = ['banana','apple','frog','octopus','pen','spiderman','flower','battery']
>>> map(lambda i: my_list[i],filter(lambda i: i%2 == 0,range(len(my_list))))
['banana', 'frog', 'pen', 'flower']
>>> map(lambda i: my_list[i],filter(lambda i: i%2 == 1,range(len(my_list))))
['apple', 'octopus', 'spiderman', 'battery']

Posted in Python | 4 Comments »

if that was helpful ...

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4 Responses to “extract odd or even elements from a python list”

1. on 02 Nov 2007 at 10:47 am   David

   An equivalent (but maybe easier to understand) approach would be to use list comprehensions:

   >>> [my_list[i] for i in range(len(my_list)) if i % 2 == 0]
   ['banana', 'frog', 'pen', 'flower']
   >>> [my_list[i] for i in range(len(my_list)) if i % 2 == 1]
   ['apple', 'octopus', 'spiderman', 'battery']

   (Found your site from the \argmax trick btw – thanks!)

2. on 02 Nov 2007 at 11:21 am   Lawrence David

   wow, that’s awesome david! thanks for teaching me about list comprehensions … those look very handy!

3. on 04 Dec 2007 at 5:01 pm   Eric Brunson

   This can be done more simply with subscripting:

   >>> x = range(1,10)
   >>> print x
   [1, 2, 3, 4, 5, 6, 7, 8, 9]
   >>> x[::2]
   [1, 3, 5, 7, 9]
   >>> x[1::2]
   [2, 4, 6, 8]

   The third arg to the subscript is a step value.

4. on 23 Apr 2011 at 9:23 pm   josh

   l[::2] and l[1::2] works too!