How to iterate through two lists with one of them shifted?

Question

My question is how do you start one list at the next index when iterating?

for elem1, elem2 in zip(unigram_mixture_list, bigram_mixture_list):
    print elem1, elem2

I want to start looping through elem1 one index ahead.

How would I achieve this in python?

Answer 1

slice the first list with [1:]:

for elem1, elem2 in zip(unigram_mixture_list[1:], bigram_mixture_list):

You got everything else exactly right

---

Note that if the lists were the same length, but now truncated because you've shortened one, you have a couple choices:

• slice the second list to remove the tail: [:-1]
• replace zip with itertools.izip_longest (after import itertools)

Example with izip_longest:

import itertools

# ~~~ other code ~~~ #

for elem1, elem2 in itertools.izip_longest(unigram_mixture_list[1:], bigram_mixture_list):
    print elem1, elem2

---

Edit: In python 3, izip_longest was renamed zip_longest, so use that instead.

Answer 2

There is the zip approach, but if the lists are the same length the longer will be truncated:

>>> li1='abcdefg'
>>> li2='1234567'
>>> zip(li1[1:], li2)
[('b', '1'), ('c', '2'), ('d', '3'), ('e', '4'), ('f', '5'), ('g', '6')]
# NOTE -- '7' is dropped from li2...

With Python 2, you can use map:

>>> map(None, li1[1:], li2)
[('b', '1'), ('c', '2'), ('d', '3'), ('e', '4'), ('f', '5'), ('g', '6'), (None, '7')]
# NOTE -- We ran out of li1, so start using 'None'

For both Python 2 and 3, you can use izip_longest from itertools:

>>> from itertools import izip_longest
>>> list(izip_longest(li1[1:], li2))
[('b', '1'), ('c', '2'), ('d', '3'), ('e', '4'), ('f', '5'), ('g', '6'), (None, '7')]