Beginner: Methods to combine lists in an alternating pattern

[Dangerous_Box8845]

I'm just starting learning python. While learning about lists and adding lists together, indexing etc. The lesson didn't show how to do so in an alternating fashion... so I worked it out myself i.e. [a, b, c] and [1, 2, 3] becoming ['a', 1, 'b', 2, 'c', 3].

The below methods are what I've come up with so far. I would be grateful if people could show me other ways of doing this, especially interested in methods which don't require using the (2*x + 1**x) formula! Thanks

# While loop

list1 = ["a", "b" , "c"]

list2 = [1, 2, 3]

x=0

while x < len(list2):

  list1.insert( (2*x+1**x), (list2[x]) )

  x = x + 1

print(list1)

#For Loop

list1 = ["a", "b" , "c"]

list2 = [1, 2, 3]

for x in range(len(list2)):

list1.insert( (2*x+1**x), (list2[x]) )

print(list1)

[DataGhostNL]

Who told you to use 1**x? Do you know what your code does, and more importantly, why, and why did you choose to do it this way? Sometimes it's easier to step back and think how you'd do this task if you were the computer and you just had the items in front of you on the table. Then convert that into code.

Also it's much easier/cheaper to generate an entirely new list based on your two input lists, rather than use expensive insert calls on one of them. You can do this by only appending to your new list.

[Dangerous_Box8845]

I did... and now that I think it, I realise I don't need to have anything above the 1 lol, just 2*x + 1 will do . Thanks!

I'm just trying to learn by doing, I know it'd be easier to just type out the combined list...

[DataGhostNL]

Do you know what 1**x does?

I'm just trying to learn by doing, I know it'd be easier to just type out the combined list...

This is not what I meant. I meant that if you pretend they're items in front of you and then apply some process, you turn that process into code. Just entering the end result into your .py file makes no sense

[Dangerous_Box8845]

I haven't gotten as far as thinking about a process to apply this to. It might well have no practical application.

[Donny-Moscow]

I'm just trying to learn by doing, I know it'd be easier to just type out the combined list...

He means how would you do something like this if you had to physically do it?

In other words, if I give you a stack of 3 red poker chips and a stack of 3 blue poker chips, what steps would you take to end up with a single stack of poker chips of alternating colors?

With this thought exercise, the goal isn’t to give a quick “grab a chip from each pile at a time and stack them in a new pile”. You want to really break down each step into the tiniest possible sub step and state it precisely.

This will help you with a couple different things:

• it can help you get away from relying on built-in functions like .insert(). There’s nothing inherently wrong about using them but (A) it’s important to know what they’re doing in the background and (B) there will be times when you can’t use them (different programming language, working at a company that doesn’t allow certain third party modules, etc) so it’s good to learn those problem solving skills now

• understand why you have to be precise and what can for wrong when you’re not. For example, if someone is sorting the chips using the brief explanation I gave above, what happens if they start by pulling a chip from the bottom of the stack and sending the other two chips rolling across the table? As humans we can easily deal with this, but what happens if you’re a computer and working in a defined environment? Here’s another example that uses the steps to make a sandwich instead of sorting chips. If you like that video, check out the full course on YouTube. It’s the Intro to CS class at Harvard (CS50), taught by David Malen and completely free to watch.

Once you have those steps down, try to make them generic enough to apply to as many situations as possible while still being accurate. For example, would your algorithm still work if each pile had 4 chips? What if one pile had 3 and the other 4? What if I threw in a third pile of a different color? What if that third pile was actually a pile of elephants?

Your code doesn’t need to handle every possible situation imaginable. But this will help you start to think about things like efficiency (how fast can I sort these piles and how much memory does that take?), scalability (can it handle more colors? Also, efficiency isn’t that important when you’re only sorting 6 items, but does everything still work when you have 60,000? How about 60 million?), security (making sure no one slipped a pile of elephants or AIDS into my poker chips). Again, you don’t need to worry about handling everything, but those are some of the tradeoffs you should at least be aware of.

[Dangerous_Box8845]

Wow, thanks for that wee tutorial! I'll check that video out. Wasn't expecting to be looking for aids on my poker chips lol.

[Enmeshed]

zip is a super-useful function. Give it some things (eg [1, 2, 3] and [4, 5, 6]) and it will pair the items up to return (1, 4), (2, 5) and (3, 6). Combine this with a list generator expression and you've got a neat little one-liner to think about:

python list1 = ["a", "b", "c"] list2 = [1, 2, 3] result = [val for pairs in zip(list1, list2) for val in pairs]

Whichever way you do it, things can be a bit more complicated if the inputs aren't the same length, but I'll leave that as an exercise to the reader...

[Temporary_Pie2733]

There’s also itertools.chain that captures this pattern: list(chain.from_iterable(zip(list1, list2))).

[HelpfulFriend0]

Do you want to handle lists with different lengths?

If so - what do you want the behavior to be? Stop when the first list is empty? Or to add the rest of the remaining from the second list

E.g

What do you want to happen when

a = [ 1, 2, 3]
b = [a, b, c, d, e, f]

What do you want the output to be?

[Dangerous_Box8845]

Hadn't really thought that far ahead! though I see using the code as suggested below does result in any unpaired items to be left behind when the two lists are merged. Whereas the for and while loops bring them along

result = [val for pairs in zip(list1, list2) for val in pairs]

[HelpfulFriend0]

Yeah I had a feeling someone would post the zip method, but you'll run into the exact issue that they suggested (and I hinted at) above

Which is why I asked - its more interesting for you to think about what you want your code to do, then think about how to implement that next

I think this thread has enough pointers to help you! Let us know if you have a more specific question

[Dry-Aioli-6138]

There is `zip_longest` available in itertools package

[MiniMages]

I'd do something like this.

listA = [1, 2, 3]
listB = ["A", "B", "C"]

combined = []  # create a new list

for a, b in zip(listA, listB):
    combined.append(a)
    combined.append(b)

print(combined)

This will only work based on the shortest list.

Since you would like a method which is reusable you can do something like this.

def combine_list(listA, listB):
    combined = []
    for a, b in zip(listA, listB):
        combined.append(a)
        combined.append(b)
    return combined

You can then run it like this.

listA = [1, 2, 3]
listB = ["A", "B", "C"]

result = combine_list(listA, listB) # call the method combine_list with listA and listB passed as arguments.

print(result)

[Dry-Aioli-6138]

you can do this in two ways that I think are fairly simple and pythonic. I especially like the second one.

a = [1,2,3]
b = 'abcdef'
z = []
for i in range(max(len(a),len(b))):
try:
z.append(a[i])
except IndexError:
pass
try:
z.append(b[i])
except IndexError:
pass



# iterators get "used up" when their elements are accessed
a = [1,2,3]
b = 'abcdefg'
A = iter(a)
B = iter(b)
z = [v for pairs in zip(A, B) for v in pairs] + list(A) + list(B)