Split list in tuples of three

Question

I have a list

L = [1, 2, 3, 4...]

which have n*3 elements. I want to be able to do something like

for a, b, c in three_tuple_split(L)

in a pythonic way but can't come up with one.

And it is! http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks-in-python — mgilson, Apr 03 '13 at 13:17
@mgilson Not this is not, here, the aim is to split the list into 3 (or n) equally sized parts, not of size n, but of size len(l)//n. — pradyunsg, Apr 04 '13 at 08:53
@Schoolboy -- I don't think so. If that was the case, OP would say that they wanted `a,b,c = three_tuple_split(L)` without the loop. — mgilson, Apr 04 '13 at 10:01

jamylak · Accepted Answer · 2013-04-04T11:40:29.293

3

Inefficient but pythonic solution:

for a, b, c in zip(*[iter(seq)]*3): pass

For a more efficient implementation, look at the itertools grouper recipe:

from itertools import izip_longest

def grouper(n, iterable, fillvalue=None):
    "Collect data into fixed-length chunks or blocks"
    # grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx
    args = [iter(iterable)] * n
    return izip_longest(fillvalue=fillvalue, *args)

for a, b, c in grouper(3, seq):
    pass

edited Apr 04 '13 at 11:40

answered Apr 03 '13 at 13:16

jamylak

128,818
30
231
230

I thought I was looking for something shorter, but I must have been mistaken. Thanks! – Jonas Byström Apr 03 '13 at 13:20
2

@JonasByström That's the efficient version. The short version is simply `for a, b, c in zip(*[iter(seq)]*3): pass` – jamylak Apr 03 '13 at 13:22
@JonasByström There is a shorter version. [Look](http://stackoverflow.com/a/15788596/1931274)... – pradyunsg Apr 03 '13 at 13:39
@Schoolboy that isn't shorter!! – jamylak Apr 03 '13 at 13:54
@jamylak: That's beautiful! – twig Oct 24 '13 at 01:15

score 0 · Answer 2 · answered Apr 03 '13 at 13:23

0

#!/usr/bin/env python
mylist = range(21)
def three_tuple_split(l):
    if len(l)%3 != 0:
        raise Exception('bad len')
    for i in xrange(0,len(l),3):
        yield l[i:i+3]
for a,b,c in three_tuple_split(mylist):
    print a,b,c

answered Apr 03 '13 at 13:23

Hal Canary

2,154
17
17

This returns a list containing lists of size 3. Not 3 lists. – pradyunsg Apr 03 '13 at 13:42

pradyunsg · Answer 3 · 2013-04-04T04:14:01.050

0

Just use slices and a for loop.

def break_into_chunks(l,n):
    x = len(l)
    step = x//n
    return [l[i:i+step] for i in range(0,x,step)]

Or a slower one:

def break_into_chunks(l,n):
    return [l[i:i+len(l)//n] for i in range(0,len(l),len(l)//n)]

To use:

for a, b, c in break_into_chunks(L,3):

edited Apr 04 '13 at 04:14

answered Apr 03 '13 at 13:24

pradyunsg

18,287
11
43
96

Split list in tuples of three

3 Answers3