I have 3 lists of numbers, how do I make a list which will contain all the numbers by their indexes?

Question

I have 3 lists:

list1 = [1, 2, 3]
list2 = [4, 5, 6]
list3 = [7, 8, 9]

And I need to write a function that will return this list: [1, 4, 7, 2, 5, 8, 3 ,6 ,9] How can I do that?

Answer 1

# Your lists
list1 = [1, 2, 3] 
list2 = [4, 5, 6]
list3 = [7, 8, 9]

new_list = [list1, list2, list3]
new_list = list(map(list, zip(*new_list)))

print(sum(new_list,[])) # Output: [1, 4, 7, 2, 5, 8, 3 ,6 ,9]

Expalnation:

Map:

The map() function is used to apply a function on all elements of a specified iterable and return a map object (We can iterate over its elements). map() will take 2 required positional arguments... A function to run against the iterables. and an iterable. ()

Zip:

The zip() function combines elements of 2 lists with matching indexes into tuples. zip() will take an undefined number of arguments where each argument is an iterable to zip.

The * in the zip() function call "unpacks" a list (or other iterable), making each of its elements a separate argument.

So basically this is what happens:

You have a list of lists [[1,2,3], [4,5,6], [7,8,9]]

The zip(*...) function will make it be ([1,2,3], [4,5,6], [7,8,9]) Then the map() function will get inside of it and will get every first element in each list. so it will go over the first list pick up the number 1 and put it in a list, then it will go to the next list and pick up the number 4 and put it in the list, then it will go to the next list and pick up the number 7 and put it in the list. then it will do it again but this time pick the second element from each list.. [2,5,8] and so on...

then it puts each of those lists it created inside a list [[1, 4, 7, 2, 5, 8, 3 ,6 ,9]]

Finally, to unnest the nested list we use the sum() function

Hope this helps :)

Answer 2

you can do something like this:

list1 = [1, 2, 3]
list2 = [4, 5, 6]
list3 = [7, 8, 9]

final_list = []

for i in range(len(list1)):

    final_list.append(list1[i])
    final_list.append(list2[i])
    final_list.append(list3[i])

print(final_list)

this solution works if the lists have the same size. You make a for loop which allows you to browse every elements in every list. After that, you just append the elements of each list to the end of you final list