Saltycrane logo

SaltyCrane Blog

Notes on Python, Django, and web development on Ubuntu Linux

    

Iterating through two lists in parallel using zip()

Date: 2007-12-19  |   Modified: 2008-09-07  |   Tags: datastructures, python

From the Python docs, zip returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. This is useful for iterating over two lists in parallel. For example, if I have two lists, I can get the first element of both lists, then the second element of both lists, then the third, etc.

Python 2.5.1 (r251:54863, May 18 2007, 16:56:43)
[GCC 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> a = [1,2,3]
>>> b = ['a','b','c']
>>> for i,j in zip(a,b):
...     print i, j
...
1 a
2 b
3 c
>>>

If the lists are different lengths, zip truncates to the length of the shortest list. Using map with None is similar to zip except the results are padded with None.

>>> a = [1,2,3]
>>> b = ['a','b','c','d']
>>> zip(a,b)
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> map(None,a,b)
[(1, 'a'), (2, 'b'), (3, 'c'), (None, 'd')]
>>>

If I have a list of keys and a list of values, I can create a dictionary by passing the output of zip to dict.

>>> mykeys = ['a', 'b', 'c']
>>> myvalues = [1, 2, 3]
>>> dict(zip(mykeys, myvalues))
{'a': 1, 'c': 3, 'b': 2}
>>>

See also this thread on the Python mailing list: Iterating through two lists

Post a comment
Required
Required, but not displayed
Optional

Format using Markdown. (No HTML.)
  • Code blocks: prefix each line by at least 4 spaces or 1 tab (and a blank line before and after)
  • Code span: surround with backticks
  • Blockquotes: prefix lines to be quoted with >
  • Links: <URL>
  • Links w/ description: [description](URL)
Created with Django | Hosted by Slicehost