Clean way to iterate through a list in pairs in Python?

Question

I am using a string.split(':') function so that my list consists of firstname:lastname pairs (e.g. ["John", "Doe", "Alex", "Jacobson" ...]

I know how to use a basic for loop where I would increment the index by two each time (and compare to the length of the list), but I want to take care of it in a more Python specific way.

Are there any cleaner looping constructs I can take which would allow me to consider the first two indices, and then the next two, etc?

Possible duplicate of [What is the most "pythonic" way to iterate over a list in chunks?](http://stackoverflow.com/questions/434287/what-is-the-most-pythonic-way-to-iterate-over-a-list-in-chunks) and [alternative way to split a list into groups of n](http://stackoverflow.com/questions/1624883/alternative-way-to-split-a-list-into-groups-of-n/1625023) and [how do you split a list into evenly sized chunks in Python](http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks-in-python) — TessellatingHeckler, Oct 26 '15 at 18:41

Padraic Cunningham · Answer 1 · 2015-10-26T18:37:59.863

3

Call iter on the list and zip:

l = ["John", "Doe", "Alex", "Jacobson"]

it = iter(l)

for f,l  in zip(it, it):
    print(f,l)

John Doe
Alex Jacobson

zip(it,it) just consumes two elements from our iterator each time until the iterator is exhausted so we end up getting the first and last names paired. It is somewhat equivalent to:

In [86]: l = ["John", "Doe", "Alex", "Jacobson"]

In [87]: it = iter(l)

In [88]: f,l = next(it),next(it)

In [89]: f,l
Out[89]: ('John', 'Doe')

In [91]: f,l
Out[91]: ('Alex', 'Jacobson')

edited Oct 26 '15 at 18:37

answered Oct 26 '15 at 18:26

Padraic Cunningham

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Which can be generalised to chunks of `n` as [`zip(*[iter(xs)]*n)`](http://stackoverflow.com/a/16758257/478656) – TessellatingHeckler Oct 26 '15 at 18:51

R Nar · Answer 2 · 2015-10-26T18:30:21.107

1

for first,last in zip(string[::2],string[1::2]):

should work.

EDIT: sorry, I wasn't really being concise.

What zip does is create an object that allows two variables in a for loop. In this case, we are creating a zip object with a sliced list of every other object starting at index 0 (string[::2]) and a sliced list of every other object starting at index 1 (string[1::2'). We can then use two iterators (first,last) to iterate through that zip list.

ie.

>>> l1 = [1,2,3,4]
>>> l2= [5,6,7,8]
>>> zip(l1,l2)
<zip object at 0x02241030>
>>> for l,k in zip(l1,l2): print(l+k)
...
6
8
10
12
>>> for l,k in zip(l1,l2): print (l,k)
...
1 5
2 6
3 7
4 8

edited Oct 26 '15 at 18:30

answered Oct 26 '15 at 18:21

R Nar

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What is happening here exactly? – AlwaysQuestioning Oct 26 '15 at 18:24
Why should this work? If you have to say (and only say) "this should work" it's probably a bad answer. Explain yourself – Sterling Archer Oct 26 '15 at 18:26

score 1 · Answer 3 · answered Oct 26 '15 at 18:22

1

>>> import itertools
>>> L = ["John", "Doe", "Alex", "Jacobson"]
>>> for fname, lname in zip(*itertools.repeat(iter(L), 2)): print(fname, lname)
...
John Doe
Alex Jacobson

answered Oct 26 '15 at 18:22

inspectorG4dget

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score 0 · Answer 4 · edited Oct 26 '15 at 18:44

0

You can use splicing, ie, Assuming all the names are in x

fname = x[::2]
lname = x[1::2]
y = []
for a,b in zip(fname,lname):
    y.append({'fname':a,
              'lname':b,})

Now you can easily iterate by,

for i in y:
    print i['fname'], i['lname']

edited Oct 26 '15 at 18:44

sam

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answered Oct 26 '15 at 18:26

Yash Mehrotra

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Clean way to iterate through a list in pairs in Python?

4 Answers4