Zipping lists in python

Question

Let's say I have two lists:

["aa", "bb", "cc"], ["dd", "ee", "ffg"]

I have to get output like this:

["aadd", "bbee", "ccffg"]

Zipping came in my mind, but I think it's a little more advanced than that, so I have really no idea, what to do.

Answer 1

It is slightly more complex than just zip(), but only just. Just add the item pairs together after zipping:

>>> [a+b for a, b in zip(list1, list2)]
['aadd', 'bbee', 'ccffg']

Where list1 and list2 are your lists.

This works because zip() returns an object made up of a series of tuples, which contain corresponding elements from the two iterables. Therefore to concatenate these items we just add them together each time, creating a new list.

Answer 2

Just throwing this map version in. List comprehensions are more Pythonic, but here's an alternate.

>>> list1 = ["aa", "bb", "cc"]
>>> list2 = ["dd", "ee", "ffg"]
>>> map("".join, zip(list1, list2))
['aadd', 'bbee', 'ccffg']

You can even create a generator out of this one.

>>> from itertools import izip, imap
>>> final = imap("".join, izip(list1, list2))
>>> next(final)
'aadd'
>>> next(final)
'bbee'

Here are some timing results for the three different solutions. (The generator one beats everyone)

>>> timeit('[a+b for a, b in zip(list1, list2)]', 'list1 = ["aa", "bb", "cc"]*100; list2 = ["dd", "ee", "ffg"]*100', number=10000)
0.4470004917966719
>>> timeit('map("".join, zip(list1, list2))', 'list1 = ["aa", "bb", "cc"]*100; list2 = ["dd", "ee", "ffg"]*100', number=10000)
0.43502864982517053
>>> timeit('imap("".join, izip(list1, list2))', 'from itertools import imap, izip; list1 = ["aa", "bb", "cc"]*100; list2 = ["dd", "ee", "ffg"]*100', number=10000)
0.011020268755800089
>>> timeit('[a+b for a, b in izip(list1, list2)]', 'from itertools import izip; list1 = ["aa", "bb", "cc"]*100; list2 = ["dd", "ee", "ffg"]*100', number=10000)
0.32172862839627214
>>> timeit('map(lambda x,y: x + y, list1, list2)', 'list1 = ["aa", "bb", "cc"]*100; list2 = ["dd", "ee", "ffg"]*100', number=10000)
0.5423113458890612

Answer 3

So essentially you have two lists, and you need to perform the same operation ( concatenation ) between two elements at the same index position.

A map function can take elements from lists and perform the required function

list_a = ["aa", "bb", "cc"]
list_b = ["dd", "ee", "ffg"]

result = map(lambda x,y: x + y, list_a, list_b)

Answer 4

A list comprehension in combination with zip will work; first zip them to pairs, then append the pairs individually using the comprehension.

>>> list_a=["aa", "bb", "cc"]
>>> list_b=["dd", "ee", "ffg"]

# Using zip will pair the strings up in tuples
>>> zip(list_a, list_b)
[('aa', 'dd'), ('bb', 'ee'), ('cc', 'ffg')]

# ...so we just append them pair wise using a comprehension
>>> [x+y for x,y in zip(list_a, list_b)]
['aadd', 'bbee', 'ccffg']

Answer 5

l1=["aa", "bb", "cc"]
l2=["dd", "ee", "ffg"]
print [i+j for i,j in zip(l1,l2)]

Output:

['aadd', 'bbee', 'ccffg']

Access 2 list elements using zip and concatenate them in list comprehension