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

Date:  |  Modified:  |  Tags: datastructures, python  |  12 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

Comments


#1 Jeremy Lewis commented on :

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 :
>>> 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 :

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 :

Thanks for the zip example, I grok it now.


#5 Jesus Carrero commented on :

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


#6 Nitin commented on :

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.


#8 S commented on :

In order to use zip to iterate over two lists - Do the two lists have to be the same size? What happens if the sizes are unequal? Thanks.


#9 Tim commented on :

Thx man helped me alot nice example btw


#10 matt commented on :

re:#8, unequal list length: the result is truncated to the shorter list. See below for a discussion of how to use the longest list instead: http://stackoverflow.com/questions/1277278/python-zip-like-function-that-pads-to-longest-length

short answer for py2.6+: use "map(None, alist, blist)"

dunno what the equivalent is in py3+


#11 ashutosh pandey commented on :

when iterating through unequal length lists using itertools

import itertools

a1=[1,2,3,6,7,9]

c1=['a','a','c','d']

b1=[10,20,30,40,50,60]

d1=[11,12,13,14,15,16,17,18]

mylist = list(itertools.izip_longest(a1,b1,c1,d1))

a1=[1,2,3,6,7,9]

for items in mylist:

litems=list(items)

if items[0] is not None and items[1] is not None:

a_old = items[0]

b_old = items[1]

if items[0] is None and items[1] is None:

litems[0]= a_old

litems[1]= b_old

a,b,c,d=litems

print a,b,c,d

is ther any other better way to give previous value if None occurs for any field.

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