[python] How do I loop through a list by twos?


Answers

You can also use this syntax (L[start:stop:step]):

mylist = [1,2,3,4,5,6,7,8,9,10]
for i in mylist[::2]:
    print i,
# prints 1 3 5 7 9

for i in mylist[1::2]:
    print i,
# prints 2 4 6 8 10

Where the first digit is the starting index (defaults to beginning of list or 0), 2nd is ending slice index (defaults to end of list), and the third digit is the offset or step.

Question

Possible Duplicate:
What is the most “pythonic” way to iterate over a list in chunks?

I want to loop through a Python list and process 2 list items at a time. Something like this in another language:

for(int i = 0; i < list.length(); i+=2)
{
   // do something with list[i] and list[i + 1]
}

What's the best way to accomplish this?




This might not be as fast as the izip_longest solution (I didn't actually test it), but it will work with python < 2.6 (izip_longest was added in 2.6):

from itertools import imap

def grouper(n, iterable):
    "grouper(3, 'ABCDEFG') --> ('A,'B','C'), ('D','E','F'), ('G',None,None)"
    args = [iter(iterable)] * n

    return imap(None, *args)

If you need to go earlier than 2.3, you can substitute the built-in map for imap. The disadvantage is that it provides no ability to customize the fill value.




The simplest in my opinion is just this:

it = iter([1,2,3,4,5,6])
for x, y in zip(it, it):
    print x, y

No extra imports or anything. And very elegant, in my opinion.




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