Python - Creating a new list using existing lists of the same length?

Question

"Given the lists list1 and list2 that are of the same length, create a new list consisting of the first element of list1 followed by the first element of list2, followed by the second element of list1, followed by the second element of list2, and so on (in other words the new list should consist of alternating elements of list1 and list2). For example, if list1 contained [1, 2, 3] and list2 contained [4, 5, 6], then the new list should contain [1, 4, 2, 5, 3, 6]. Associate the new list with the variable list3."

    list1 = []
    list2 = []
    list3 = []
    for i in range(len(list3)):
        list3.append(list1)
        list3.append(list2)

I'm pretty sure this is dead wrong. What should I improve? By the way, I believe this must include both len and range.

well you wont go into your for loop since len is 0 when you get there... thats one thing — Joran Beasley, Mar 26 '13 at 05:20

score 5 · Answer 1 · answered Mar 26 '13 at 05:25

I would do it with a list comprehension, rather than anything with len or range. e.g.:

>>> list1 = [1, 2, 3]
>>> list2 = ['a', 'b', 'c']
>>> zip(list1, list2)
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> [x for pair in zip(list1, list2) for x in pair]
[1, 'a', 2, 'b', 3, 'c']

score 5 · Answer 2 · answered Mar 26 '13 at 05:28

5

>>> from itertools import chain
>>> list1 = [1, 2, 3]
>>> list2 = [4, 5, 6]
>>> list(chain.from_iterable(zip(list1, list2)))
[1, 4, 2, 5, 3, 6]

answered Mar 26 '13 at 05:28

jamylak

128,818
30
231
230

score 2 · Answer 3 · answered Mar 26 '13 at 06:25

2

list1 = [1, 2, 3]
list2 = [4, 5, 6]
list3 = []

for x1, x2 in zip(list1, list2):
    list3.extend([x1, x2])

answered Mar 26 '13 at 06:25

Yarkee

9,086
5
28
29

score 0 · Answer 4 · answered Mar 26 '13 at 05:23

0

Please look at the following snippet, maybe it will help

>>> list1 = [1,2,3]
>>> list2 = [4,5,6]
>>> list3 = []
>>> for i in range(len(list1)):
...     list3.append(list1[i])
...     list3.append(list2[i])
...
>>> list3
[1, 4, 2, 5, 3, 6]

answered Mar 26 '13 at 05:23

Sidharth Shah

1,569
1
11
14

1

Better would be `for x1, x2 in zip(list1, list2):` in my opinion. – wim Mar 26 '13 at 05:26
@wim yes that would work too. Thank you for pointing out. – Sidharth Shah Mar 26 '13 at 05:28

Python - Creating a new list using existing lists of the same length?

4 Answers4