# Zip with list output instead of tuple

Posted on

### Question :

Zip with list output instead of tuple

What is the fastest and most elegant way of doing list of lists from two lists?

I have

``````In : a=[1,2,3,4,5,6]

In : b=[7,8,9,10,11,12]

In : zip(a,b)
Out: [(1, 7), (2, 8), (3, 9), (4, 10), (5, 11), (6, 12)]
``````

And I’d like to have

``````In : some_method(a,b)
Out: [[1, 7], [2, 8], [3, 9], [4, 10], [5, 11], [6, 12]]
``````

I was thinking about using map instead of zip, but I don’t know if there is some standard library method to put as a first argument.

I can def my own function for this, and use map, my question is if there is already implemented something. No is also an answer.

If you are zipping more than 2 lists (or even only 2, for that matter), a readable way would be:

``````[list(a) for a in zip([1,2,3], [4,5,6], [7,8,9])]
``````

This uses list comprehensions and converts each element in the list (tuples) into lists.

You almost had the answer yourself. Don’t use `map` instead of `zip`. Use `map` AND `zip`.

You can use map along with zip for an elegant, functional approach:

``````list(map(list, zip(a, b)))
``````

`zip` returns a list of tuples. `map(list, [...])` calls `list` on each tuple in the list. `list(map([...])` turns the map object into a readable list.

I love the elegance of the zip function, but using the itemgetter() function in the operator module appears to be much faster. I wrote a simple script to test this:

``````import time
from operator import itemgetter

list1 = list()
list2 = list()
origlist = list()
for i in range (1,5000000):
t = (i, 2*i)
origlist.append(t)

print "Using zip"
starttime = time.time()
list1, list2 = map(list, zip(*origlist))
elapsed = time.time()-starttime
print elapsed

print "Using itemgetter"
starttime = time.time()
list1 = map(itemgetter(0),origlist)
list2 = map(itemgetter(1),origlist)
elapsed = time.time()-starttime
print elapsed
``````

I expected zip to be faster, but the itemgetter method wins by a long shot:

``````Using zip
6.1550450325
Using itemgetter
0.768098831177
``````

I generally don’t like using lambda, but…

``````>>> a = [1, 2, 3, 4, 5]
>>> b = [6, 7, 8, 9, 10]
>>> c = lambda a, b: [list(c) for c in zip(a, b)]
>>> c(a, b)
[[1, 6], [2, 7], [3, 8], [4, 9], [5, 10]]
``````

If you need the extra speed, map is slightly faster:

``````>>> d = lambda a, b: map(list, zip(a, b))
>>> d(a, b)
[[1, 6], [2, 7], [3, 8], [4, 9], [5, 10]]
``````

However, map is considered unpythonic and should only be used for performance tuning.

``````>>> def list_(*args): return list(args)

>>> map(list_, range(5), range(9,4,-1))
[[0, 9], [1, 8], [2, 7], [3, 6], [4, 5]]
``````

Or even better:

``````>>> def zip_(*args): return map(list_, *args)
>>> zip_(range(5), range(9,4,-1))
[[0, 9], [1, 8], [2, 7], [3, 6], [4, 5]]
``````

# Using numpy

The definition of elegance can be quite questionable but if you are working with `numpy` the creation of an array and its conversion to list (if needed…) could be very practical even though not so efficient compared using the `map` function or the list comprehension.

``````import numpy as np
a = b = range(10)
zipped = zip(a,b)
result = np.array(zipped).tolist()
Out: [[0, 0],
[1, 1],
[2, 2],
[3, 3],
[4, 4],
[5, 5],
[6, 6],
[7, 7],
[8, 8],
[9, 9]]
``````

Otherwise skipping the `zip` function you can use directly `np.dstack`:

``````np.dstack((a,b)).tolist()
``````

``````a=[1,2,3,4,5,6]