2

I'd like to iterate over two lists simultaneously.

The first list is just a regular list of elements.

The second list is a list containing multiple lists, each containing elements which are in fact 3-tuples.

I want to perform some function on every element of every 3-tuple in every list inside the second list (if that's understandable). This function will be dependant on an element in the first list, and I'd like it all to run in order.

ie: If I have some list such as [0,1,2,3,1,5,6,7,1,2,3,5,1,1,2,3,5,6] as my first list, and [[(13,12,32),(11,444,25)],[(312,443,12),(123,4,123)],[(545,541,1),(561,112,560)]] as my second list, I'd like to perform such an operation such that it involves the first element of the first list and the first element of the first tuple of the first list of the second list. I'd like this operation to be iterated using the nth element of the first list, and the nth element of the the tuple bla bla

So if we're looking at what elements will be used together for the operation:

  • 0 goes with 13
  • 1 goes with 12
  • 2 goes with 32
  • 3 goes with 11
  • 1 goes with 444
  • 5 goes with 25
  • 6 goes with 312

and so on.

What's happening is that my function will take the element from the second list, its paired element from list 1, use these bits somehow, and output a new value to replace its original second list counterpart. I then want to a construct a replacement/new 'second list' which is the second list after running this function (which uses the information as described above)

Hope this was semi-cohesive, I tried to be as informative as possible. Please inquire if anything is unclear.

Cheers, and thank you for any help you can provide!

Edit:

I want to take list 1 and list 2, then flatten list 2. I then want to iterate over both lists, using some function. I then want to construct a new list, which uses the outputs from my function, which will just be some 'adjusted' values for my flattened second list, but in the format of how it was originally, tuples in a list of list.

ie: my output could be something like:

[[(13,13,32),(11,444,24)],[(313,443,12),(123,4,123)],[(546,542,1),(561,112,561)]]

with some of the values either changing a little bit or remaining the same, as a result of being 'passed through' my function.

Edit#2:

The ticked solution shows exactly what was needed for this problem!

Edit#3:

A few fixes that are needed to the top solution:
How would I fix the code such that if the first list is empty, the output will just be the original second list? And if the second list is empty, the output should just be an empty list. Also if len(list 1) > len(list 2), it will just iterate until the end of list 2, then ignore the excess in list 1, outputting a new list of same format as the original list 2. And if len(list 2) > len (list 1), it will just iterate for where list 1 can be matched up, but leave the remainder of list 2 intact, rather than chopping it off.

Can I perform all these fixes with some if/elseif/else loops?

EDIT: I'm still looking to fix these issues, which could be separated/postulated to be a separate query. I've posed a new question here https://stackoverflow.com/questions/19118641/making-a-function-only-run-for-certain-conditions-in-python if anyone is interested in helping out.

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3
  • It sounds like what you really want to do is flatten the second list, and then you can just e.g. zip together with the first. Commented Oct 1, 2013 at 12:54
  • So once I've flattened the second list using one of the methods suggested below, performed my operation using an element from each list, how would I create an unflattened replacement/new version of my second list from my outputted data? Commented Oct 1, 2013 at 12:57
  • It would help to, for your given example, do the work yourself and show us exactly what you want to end up with. Commented Oct 1, 2013 at 12:58

5 Answers 5

Reset to default
4

You can use zip and itertools.chain like this:

from itertools import chain

first = [0, 1, 2, 3, 1, 5, 6, 7, 1, 2, 3, 5, 1, 1, 2, 3, 5, 6]
second = [
    [(13, 12, 32), (11, 444, 25)],
    [(312, 443, 12), (123, 4, 123)],
    [(545, 541, 1), (561, 112, 560)]
]    
zip(first, chain(*(chain(*second))))

UPDATE

def add(x, y):
    return x + y

# Flatten the second list
second_flattened = list(chain(*(chain(*second))))

# There is probably a better way to achieve this
foo = [add(x, y) for x, y in zip(first, second_flattened)]

# If second is longer we should append unprocessed values
if len(second_flattened) > len(first):
    foo += second_flattened[len(foo): ]

bar = [foo[i:i+3] for i in range(0, len(foo), 3)]
second = [bar[i:i+2]  for i in range(0, len(foo) / 3, 2)]
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9 Comments

Okay, so if I wanted to say, take the elements out of each list, pair them together, use them in some function which will give an element to use for my new version of list no.2, then put those outputs into a new version of list no.2, how could I go about such a task?
Did you actually try out the code? Hint: the zip result is iterable; try iterating over it and see what you get. I don't understand what you mean by "put into a new version of list no. 2".
I did, and I'm fine with iterating over the zip function (might have to double check the syntax). I want to use the output of my function (which uses the data from list 1 and the flattened version of list 2) and construct a new list in the same format as list 2, but the values are the outputs of my function.
Success! Your edit was absolutely perfect. Thank you so much!
A few bonus queries: How would I fix the code such that if the first list is empty, the output will just be the original second list. And if the second list is empty, the output will just be an empty list. Also if len(list 1) > len(list 2), it will just iterate until the end of list 2, then ignore the excess in list 1. And if len(list 2) > len (list 1), it will just iterate for where list 1 can be matched up, but leave the remainder of list 2 intact, rather than chopping it off.
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1

Your second list needs to be flattened by two levels:

import itertools
flatten = itertools.chain.from_iterable
for i, j in zip(list1, flatten(flatten(list2))):
    # whatever
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0

you can use something like this to get each element from second list :

from itertools import chain
l=list(chain.from_iterable(list2))
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0

python's itertools contains izip which allows you to zip an arbitrary number of lists using the same syntax as zip. i.e. izip(l1, l2, l3)

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1 Comment

In Python 3 zip_longest()
0

Do this.

dict(zip(ListA, eval('[' + repr(ListB).replace('(', '').replace(')', \
'').replace('[', '').replace('[', '') + ']'))

I know it's pretty long, but it will return a dictionary containing the output you need.

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