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


Question

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?

1
129
7/10/2017 11:05:31 PM

Accepted Answer

This is as pythonic as you can get:

for lat, long in zip(Latitudes, Longitudes):
    print lat, long
246
12/17/2009 2:03:39 AM

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

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