Is there a way to merge multiple list index by index?

Question

For example, I have three lists (of the same length)

A = [1,2,3]
B = [a,b,c]
C = [x,y,z]

and i want to merge it into something like: [[1,a,x],[2,b,y],[3,c,z]].

Here is what I have so far:

define merger(A,B,C):
  answer = 
  for y in range (len(A)):
    a = A[y]
    b = B[y]
    c = C[y]
    temp = [a,b,c]
    answer = answer.extend(temp)
  return answer

Received error:

'NoneType' object has no attribute 'extend'

Solution 1

It looks like your code is meant to say answer = [], and leaving that out will cause problems. But the major problem you have is this:

answer = answer.extend(temp)

extend modifies answer and returns None. Leave this as just answer.extend(temp) and it will work. You likely also want to use the append method rather than extend - append puts one object (the list temp) at the end of answer, while extend appends each item of temp individually, ultimately giving the flattened version of what you're after: [1, 'a', 'x', 2, 'b', 'y', 3, 'c', 'z'].

But, rather than reinventing the wheel, this is exactly what the builtin zip is for:

>>> A = [1,2,3]
>>> B = ['a', 'b', 'c']
>>> C = ['x', 'y', 'z']
>>> list(zip(A, B, C))
[(1, 'a', 'x'), (2, 'b', 'y'), (3, 'c', 'z')]

Note that in Python 2, zip returns a list of tuples; in Python 3, it returns a lazy iterator (ie, it builds the tuples as they're requested, rather than precomputing them). If you want the Python 2 behaviour in Python 3, you pass it through list as I've done above. If you want the Python 3 behaviour in Python 2, use the function izip from itertools.

Solution 2

To get a list of lists, you can use the built-in function zip() and list comprehension to convert each element of the result of zip() from a tupleto a list:

A = [1, 2, 3]
B = [4, 5, 6]
C = [7, 8, 9]

X = [list(e) for e in zip(A, B, C,)]

print X
>>> [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

Solution 3

Assuming you are doing this for class and not learning all of the tricks that make Python a great tool here is what you need. You had two problems, first if you want to extend then you do it in place but your desired result shows that you want to append, not extend

def merger(A,B,C):
    answer = []
    for y in range (len(A)):
        a=A[y]
        b=B[y]
        c=C[y]
        temp = [a,b,c]
        answer.append(temp)
   return answer


>>> answer
[[1, 'a', 'x'], [2, 'b', 'y'], [3, 'c', 'z']]

Comments

[PyNEwbie]

To directly answer your question answer = [] not answer = though I am surprised that you did not get a syntax error but maybe that is swallowed by your function? also extend works in place so you need to do answer.extend(something) not answer = answer.extend()

[PyNEwbie]

Sorry I was wrong what is needed is append not extend and your solution still is not there as you have a list of tuples

[user3225528]

Ah, I was not familiar with the extend function. I understand the error now thanks!