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

user_233382 0
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I have a list of Latitudes and one of Longitudes and need to iterate over the latitude and longitude pairs.

Is it better to:

  • A. Assume that the lists are of equal lengths:

    for i in range(len(Latitudes)):
    Lat,Long=(Latitudes[i],Longitudes[i])

  • B. Or:

    for Lat,Long in [(x,y) for x in Latitudes for y in Longitudes]:

(Note that B is incorrect. This gives me all the pairs, equivalent to itertools.product())

Any thoughts on the relative merits of each, or which is more pythonic?

iteration list python
7 Answers

This is as pythonic as you can get:

for lat, long in zip(Latitudes, Longitudes):
    print(lat, long)
Roberto_Bonvallet_131690

Another way to do this would be to by using map.

>>> a
[1, 2, 3]
>>> b
[4, 5, 6]
>>> for i,j in map(None,a,b):
    ...   print i,j
    ...
1 4
2 5
3 6

One difference in using map compared to zip is, with zip the length of new list is
same as the length of shortest list. For example:

>>> a
[1, 2, 3, 9]
>>> b
[4, 5, 6]
>>> for i,j in zip(a,b):
    ...   print i,j
    ...
1 4
2 5
3 6

Using map on same data:

>>> for i,j in map(None,a,b):
    ...   print i,j
    ...

    1 4
    2 5
    3 6
    9 None
user_582200

Good to see lots of love for zip in the answers here.

However it should be noted that if you are using a python version before 3.0, the itertools module in the standard library contains an izip function which returns an iterable, which is more appropriate in this case (especially if your list of latt/longs is quite long).

In python 3 and later zip behaves like izip.

user_1379020

in case your Latitude and Longitude lists are large and lazily loaded:

from itertools import izip
for lat, lon in izip(latitudes, longitudes):
    process(lat, lon)

or if you want to avoid the for-loop

from itertools import izip, imap
out = imap(process, izip(latitudes, longitudes))
user_2335070

Iterating through elements of two lists simultaneously is known as zipping, and python provides a built in function for it, which is documented here.

>>> x = [1, 2, 3]
>>> y = [4, 5, 6]
>>> zipped = zip(x, y)
>>> zipped
[(1, 4), (2, 5), (3, 6)]
>>> x2, y2 = zip(*zipped)
>>> x == list(x2) and y == list(y2)
True

[Example is taken from pydocs]

In your case, it will be simply:

for (lat, lon) in zip(latitudes, longitudes):
    ... process lat and lon
user_1258440 
for Lat,Long in zip(Latitudes, Longitudes):
user_1747280
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