How to use Python's enumerate and zip to iterate over two lists and their indices.

Date: 2008-04-18 | Modified: 2008-08-22 | Tags: datastructures, python | 7 Comments

enumerate - Iterate over indices and items of a list¶

The Python Cookbook (Recipe 4.4) describes how to iterate over items and indices in a list using enumerate. For example:

alist = ['a1', 'a2', 'a3']

for i, a in enumerate(alist):
    print i, a

Results:

0 a1
1 a2
2 a3

zip - Iterate over two lists in parallel¶

I previously wrote about using zip to iterate over two lists in parallel. Example:

alist = ['a1', 'a2', 'a3']
blist = ['b1', 'b2', 'b3']

for a, b in zip(alist, blist):
    print a, b

Results:

a1 b1
a2 b2
a3 b3

enumerate with zip¶

Here is how to iterate over two lists and their indices using enumerate together with zip:

alist = ['a1', 'a2', 'a3']
blist = ['b1', 'b2', 'b3']

for i, (a, b) in enumerate(zip(alist, blist)):
    print i, a, b

Results:

0 a1 b1
1 a2 b2
2 a3 b3

How to sort a list of dicts in Python — posted 2010-04-02
Python setdefault example — posted 2010-02-09
How to conditionally replace items in a list — posted 2008-08-22
How to invert a dict in Python — posted 2008-01-14
How to find the intersection and union of two lists in Python — posted 2008-01-03

7 Comments — Comments feed for this post

#1 Jeremy Lewis commented on 2009-05-29:

If you're working with last lists and/or memory is a concern, using the itertools module is an even better option.

from itertools import izip, count
alist = ['a1', 'a2', 'a3']
blist = ['b1', 'b2', 'b3']

for i, a, b in izip(count(), alist, blist):
    print i, a, b

yields the exact same result as above, but is faster and uses less memory.

#2 Jeremy Lewis commented on 2009-05-29:

>>> def foo():
...  for i, x, y in izip(count(), a, b):
...   pass
...
>>> def bar():
...  for i, (x, y) in enumerate(zip(a, b)):
...   pass
...
>>> delta(foo)
0.0213768482208
>>> delta(bar)
0.180979013443

where a = b = xrange(100000) and delta(f(x)) denotes the runtime in seconds of f(x).

#3 Eliot commented on 2009-05-29:

Jeremy,
Thanks for the tip and the clear example and demonstration of the performance benefit. I had heard of itertools but have not really used it. It was great to talk to you today and I hope I can talk to you again soon.

#4 Joshua Gourneau commented on 2011-04-27:

Thanks for the zip example, I grok it now.

#5 Jesus Carrero commented on 2011-09-11:

Jeremy, Thanks for the example, It is very helpful.

#6 Nitin commented on 2011-11-01:

I have set of n set, each with different number of elements I wanted to find all possible combinations of n elements, one from each set. Consider two sets (e1,e2,e3) (e4,e5)

output required is as follows

(e1,e4)
(e1,e5)
(e2,e4)
(e2,e5)
(e3,e4)
(e3,e5)

I do not know the number of such sets in advance.

#7 Eliot commented on 2011-11-01:

Nitin: http://www.saltycrane.com/blog/2011/11/find-all-combinations-set-lists-itertoolsproduct/


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