Python weave lists

Question

I want to "weave" two lists of numbers together in Python.

Example:

x = [1,2,3]
y = [4,5,6]

result = [1,4,2,5,3,6]

This is my function, I can't find out why it doesn't work:

def weave(list1, list2):
    lijst = []
    i = 0
    for i <= len(list1):
        lijst += [list1[i]] 
        lijst += [list2[i]] 
        i + 1

Answer 1

Python's for-loops aren't like other languages where you can have a conditional. You need to use a while loop instead or alter your for-loop:

def weave(list1,list2):
    lijst = []
    i = 0
    while i < len(list1):
        lijst.append(list1[i])
        lijst.append(list2[i]) 
        i += 1
    return lijst

I've altered your code in a variety of ways:

• Use a while loop if you have a condition you want to loop through
• You want the conditional to be less than the length, not less than or equal to. This is because indexing starts at 0, and if you do <=, then it will try to do list1[3] which is out of bounds.
• Use list.append to add items to list
• You forgot the = in i += 1
• Finally, don't forget to return the list!

You could also use zip():

>>> [a for b in zip(x, y) for a in b]
[1, 4, 2, 5, 3, 6]