Enumerate two python lists simultaneously?

Question

How do I enumerate two lists of equal length simultaneously? I am sure there must be a more pythonic way to do the following:

for index, value1 in enumerate(data1):
    print index, value1 + data2[index]

I want to use the index and data1[index] and data2[index] inside the for loop.

score 219 · Accepted Answer · edited Sep 07 '20 at 18:31

219

Use zip for both Python2 and Python3:

for index, (value1, value2) in enumerate(zip(data1, data2)):
    print(index, value1 + value2)  # for Python 2 use: `print index, value1 + value2` (no braces)

Note that zip runs only up to the shorter of the two lists(not a problem for equal length lists), but, in case of unequal length lists if you want to traverse the whole list then use itertools.izip_longest.

edited Sep 07 '20 at 18:31

Sabito stands with Ukraine

4,271
8
34
56

answered May 01 '13 at 21:31

Ashwini Chaudhary

244,495
58
464
504

2

thanks this solved `zip(data1, data2).index(val1, val2)` – Chebhou Dec 06 '15 at 08:48

Fred Foo · Answer 2 · 2013-05-01T21:40:35.870

17

for i, (x, y) in enumerate(zip(data1, data2)):

In Python 2.x, you might want to use itertools.izip instead of zip, esp. for very long lists.

edited May 01 '13 at 21:40

answered May 01 '13 at 21:31

Fred Foo

355,277
75
744
836

17

Does anyone else hate that `enumerate` is nine characters long? – Steven Rumbalski May 01 '13 at 21:47
@StevenRumbalski: now that you mention it, it could have been named `enum` to be consistent with `len` and `str`. – Fred Foo May 01 '13 at 21:50
1

@jamylak: true, so maybe it should be named `enu` ;) – Fred Foo May 02 '13 at 08:54
1

@larsmans yeah that would fit in with `zip` and `map`! should make a request for Python 4 ;) ;) – jamylak May 02 '13 at 08:57
1

@jamylak: don't forget to request `range` → `rng` as well to make a stronger case. Oh, and `rpr`. – Fred Foo May 02 '13 at 09:04
1

@larsmans `chain.from_iterable` → `chain.fi` and then we should make `input()` into `<>` and change `print` to `say` (you know to make up for the extra 2 characters I have to type each time I use Py3) – jamylak May 02 '13 at 09:14
@jamylak: `re.match` → `~`. Feel like forking Python to start the Pthn prg lng? – Fred Foo May 02 '13 at 09:37
@larsmans of course, but I'm afraid that once I start it I won't be able to go back... – jamylak May 02 '13 at 09:48
4

@StevenRumbalski: Readability first! - enum() was considered but rejected: [PEP 279](http://www.python.org/dev/peps/pep-0279/) – Quant Metropolis May 02 '13 at 17:36

score 11 · Answer 3 · answered Jul 04 '16 at 09:55

11

from itertools import count

for index, value1, value2 in zip(count(), data1, data2):
    print(index, value1, value2)

Source: http://www.saltycrane.com/blog/2008/04/how-to-use-pythons-enumerate-and-zip-to/#c2603

answered Jul 04 '16 at 09:55

Marco Sulla

15,299
14
65
100

score 2 · Answer 4 · answered May 01 '13 at 21:37

2

Althought this is not very clear what you look for,

>>> data1 = [3,4,5,7]
>>> data2 = [4,6,8,9]
>>> for index, value in enumerate(zip(data1, data2)):
    print index, value[0]+value[1]


0 7
1 10
2 13
3 16

answered May 01 '13 at 21:37

kiriloff

25,609
37
148
229

score 1 · Answer 5 · answered Jul 04 '16 at 10:00

1

Since it has been mentioned that the length are equal,

for l in range(0, len(a)):
   print a[l], b[l]

answered Jul 04 '16 at 10:00

SuperNova

25,512
7
93
64

score 0 · Answer 6 · edited May 01 '13 at 21:33

0

Suppose you want to use zip:

   >>> for x in zip([1,2], [3,4]):
    ...     print x
    ... 
    (1, 3)
    (2, 4)

edited May 01 '13 at 21:33

Ashwini Chaudhary

244,495
58
464
504

answered May 01 '13 at 21:31

ddzialak

1,042
7
15

2

The index is need inside the loop too. – Quant Metropolis May 01 '13 at 21:42

Enumerate two python lists simultaneously?

6 Answers6

Linked