How to Loop Through a List by Twos

How do I iterate through two lists in parallel?

Python 3

for f, b in zip(foo, bar):
    print(f, b)

zip stops when the shorter of foo or bar stops.

In Python 3, zip
returns an iterator of tuples, like itertools.izip in Python2. To get a list
of tuples, use list(zip(foo, bar)). And to zip until both iterators are
exhausted, you would use
itertools.zip_longest.

Python 2

In Python 2, zip
returns a list of tuples. This is fine when foo and bar are not massive. If they are both massive then forming zip(foo,bar) is an unnecessarily massive
temporary variable, and should be replaced by itertools.izip or
itertools.izip_longest, which returns an iterator instead of a list.

import itertools
for f,b in itertools.izip(foo,bar):
    print(f,b)
for f,b in itertools.izip_longest(foo,bar):
    print(f,b)

izip stops when either foo or bar is exhausted.
izip_longest stops when both foo and bar are exhausted.
When the shorter iterator(s) are exhausted, izip_longest yields a tuple with None in the position corresponding to that iterator. You can also set a different fillvalue besides None if you wish. See here for the full story.

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Note also that zip and its zip-like brethen can accept an arbitrary number of iterables as arguments. For example,

for num, cheese, color in zip([1,2,3], ['manchego', 'stilton', 'brie'], 
                              ['red', 'blue', 'green']):
    print('{} {} {}'.format(num, color, cheese))

prints

1 red manchego
2 blue stilton
3 green brie

How do I loop through a list by twos?

You can use a range with a step size of 2:

Python 2

for i in xrange(0,10,2):
  print(i)

Python 3

for i in range(0,10,2):
  print(i)

Note: Use xrange in Python 2 instead of range because it is more efficient as it generates an iterable object, and not the whole list.

Is there a better way to iterate over two lists, getting one element from each list for each iteration?

This is as pythonic as you can get:

for lat, long in zip(Latitudes, Longitudes):
    print(lat, long)

Iterating over every two elements in a list

You need a pairwise() (or grouped()) implementation.

def pairwise(iterable):
    "s -> (s0, s1), (s2, s3), (s4, s5), ..."
    a = iter(iterable)
    return zip(a, a)

for x, y in pairwise(l):
   print("%d + %d = %d" % (x, y, x + y))

Or, more generally:

def grouped(iterable, n):
    "s -> (s0,s1,s2,...sn-1), (sn,sn+1,sn+2,...s2n-1), (s2n,s2n+1,s2n+2,...s3n-1), ..."
    return zip(*[iter(iterable)]*n)

for x, y in grouped(l, 2):
   print("%d + %d = %d" % (x, y, x + y))

In Python 2, you should import izip as a replacement for Python 3's built-in zip() function.

All credit to martineau for his answer to my question, I have found this to be very efficient as it only iterates once over the list and does not create any unnecessary lists in the process.

N.B: This should not be confused with the pairwise recipe in Python's own itertools documentation, which yields s -> (s0, s1), (s1, s2), (s2, s3), ..., as pointed out by @lazyr in the comments.

Little addition for those who would like to do type checking with mypy on Python 3:

from typing import Iterable, Tuple, TypeVar

T = TypeVar("T")

def grouped(iterable: Iterable[T], n=2) -> Iterable[Tuple[T, ...]]:
    """s -> (s0,s1,s2,...sn-1), (sn,sn+1,sn+2,...s2n-1), ..."""
    return zip(*[iter(iterable)] * n)

Iterating over two lists one after another

This can be done with itertools.chain:

import itertools

l1 = [1, 2, 3, 4]
l2 = [5, 6, 7, 8]

for i in itertools.chain(l1, l2):
    print(i, end=" ")

Which will print:

1 2 3 4 5 6 7 8 

As per the documentation, chain does the following:

Make an iterator that returns elements from the first iterable until it is exhausted, then proceeds to the next iterable, until all of the iterables are exhausted.

If you have your lists in a list, itertools.chain.from_iterable is available:

l = [l1, l2]
for i in itertools.chain.from_iterable(l):
    print(i, end=" ")

Which yields the same result.

If you don't want to import a module for this, writing a function for it is pretty straight-forward:

def custom_chain(*it):
    for iterab in it:
        yield from iterab

This requires Python 3, for Python 2, just yield them back using a loop:

def custom_chain(*it):
    for iterab in it:
        for val in iterab:
            yield val

In addition to the previous, Python 3.5 with its extended unpacking generalizations, also allows unpacking in the list literal:

for i in [*l1, *l2]:
    print(i, end=" ")

though this is slightly faster than l1 + l2 it still constructs a list which is then tossed; only go for it as a final solution.

For loop iterating through two lists python

i assume that you have the same list length for list1 and list 2

in order to print it you just need to specify the position of the index on list like list[index], this will print the list value.

list1 = [0,1,2,3,4,5]
list2 = [6,7,8,9,10,11]

for i in range(len(list1)):
    print(f"{list1[i]}, {list2[i]}")
    

How to iterate over a list two at a time in Python 3?

You can use range like this by using step (indexing):

list_1 = [1,3,2,4,3,1,2,7]

for i in range(0,len(list_1),2):
    print(list_1[i])

or just using python slice notation:

list_1 = [1,3,2,4,3,1,2,7]

for v in list_1[::2]:
    print(v)

Iterate through pairs of items in a Python list

From the itertools recipes:

from itertools import tee

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return zip(a, b)

for v, w in pairwise(a):
    ...

Loop through Python list with 2 variables

Here's an example how you can do it. Take second index as function of length minus index minus one:

l = [1, 2, 3, 4]
for i, _ in enumerate(l):
    print(l[i], l[len(l)-i-1])

This will output

(1, 4)
(2, 3)
(3, 2)
(4, 1)

Not printing indexes themselves, but you can print them, if you chose to do so.

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