Question

I want to create a function that takes a 2 dimensional list and outputs a 1 dimensional list with the same contents.
Here is what I have:

twoDlist= [[23, 34, 67],[44,5,3],[7,8,9]]
def twoone (list1):
    for x in range(len(list1)):
        for y in range(len(list1)):
            list2=[]
            list2.append(list1[x][y])
print(twoone(twoDlist))

Only problem it returns ‘None’. What is wrong, and how can I fix it?

Answer 1

Two issues here, the first is that you are not returning a value and the second is that you are resetting list2 to an empty list inside of your nested loop, so at the end you would only have a single element in it.

Here is how you could fix your code:

def twoone(list1):
    list2 = []
    for x in range(len(list1)):
        for y in range(len(list1)):
            list2.append(list1[x][y])
    return list2

>>> twoone(twoDlist)
[23, 34, 67, 44, 5, 3, 7, 8, 9]

However, there are much better ways to do this, see the link in jtbandes’ comment.

The best way is itertools.chain():

>>> from itertools import chain
>>> list(chain.from_iterable(twoDlist))
[23, 34, 67, 44, 5, 3, 7, 8, 9]