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Python Python Collections (Retired) Tuples Combo

Posted by Beth Singley
Beth Singley
3,491 Points

how to iterate through two lists at the same time

Hi - I completed this challenge "combo" on tuples, but I don't understand what I did. I got stuck at trying to iterate through two lists at the same time. Below is the solution I found on the internet, but the "zip" function was definitely not in the lesson. When I didn't use "zip", I got an error about expecting two variables. Is there anyone out there with a different solution/explanation for how to step through two lists at the same time? I'd love to understand this more.

Beth Singley
Beth Singley
3,491 Points

don't know why my code didn't come through... here it is...

def combo (iter1, iter2):
  new_list = []

  for a, b  in zip(iter1, iter2):
    new_tuple = (a, b)
    new_list.append(new_tuple)

  return (new_list)


print(combo ([1, 2, 3], 'abc'))

[MOD: Added ```python markdown formatting -cf]

Nathan Tallack
Nathan Tallack
22,159 Points
Nathan Tallack
Nathan Tallack
22,159 Points

Beth, your code looks like it should work. Take care to verify that the indentation is as the mod has formatted it here. It may be a simple indentation error. ;)

Keep up the great work!

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

To fully use the power of zip you can take advantage that it returns an interable that generates tuples. Wrapping that tuple generator in a list() function get you back a list:

def combo (iter1, iter2):
  return list(zip(iter1, iter2))

But that is way beyond where students are at this point. To solve it without using zip you can walk down one or both of the iterables using an index:

# using general index
def combo(iter1, iter2):
    new_list = []
    for idx in range(len(iter1)):
        new_list.append((iter1[idx], iter2[idx]))
    return new_list

# using index on one iterable
def combo(iter1, iter2):
    new_list = []
    for idx, item in enumerate(iter1):  # <-- returns index and one item from iter1
        new_list.append((item, iter2[idx]))
    return new_list
Beth Singley
Beth Singley
3,491 Points

Chris -

Thanks for your answer. I was looking for something that obviously used the lessons taught in the class. I see how the enumerate would provide an index and one item from iter1. Can you help me understand why you need the idx in the for loop? Does it have to do with the enumerate piece?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

enumerate is a useful function to loop through an iterable. For each item in the iterable, enumerate returns a tuple of a the current loop count (starting with 0) and the next item from the iterable.

Since enumerate returns a tuple of two items one needs to provide two variables to receive the count and the item from the iterable. I chose to use idx to receive the index or count value.

Without using enumerate you would need an explicit count to index into the second iterable. 

# using a counter and looping through one iterable
def combo(iter1, iter2):
    count = 0
    new_list = []
    for item in iter1:
        new_list.append((item, iter2[count]))
        count += 1
    return new_list

More on enumerate in the docs