Best pattern/idiom
Peter Hansen
peter at engcorp.com
Mon Aug 9 15:28:13 EDT 2004
Chris Connett wrote:
> I was wondering if there were a well known pattern/idiom for breaking a
> sequence into a list of lists, each n elements long, e.g.
>
> [0,1,2,3,4,5,6,7,8,9,10,11] -> [[0,1,2,3],[4,5,6,7],[8,9,10,11]]
>
> This comes up reasonably often in my work, and every time I re-think
> about it, and come up with
> [ lis[n:n+4] for n in range( 0, len( lis ), 4 ) ]
> which seems very kludgy to me, since it uses a range and len, 2 mentions
> of the list identifier and 2 literal 4's (which is the size I want to
> break into this time).
>
> Is there a better way?
Found from a Peter Otten post via Google Groups (searching for
"islice list split"):
>>> def chunks(s, size):
... for i in range(0, len(s), size):
... yield s[i:i+size]
>>> s
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>>> list(chunks(s, 4))
[[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]]
>>> list(chunks([], 4))
[]
>>> list(chunks(s[:10], 4))
[[0, 1, 2, 3], [4, 5, 6, 7], [8, 9]]
Of course, this uses range and len as well, and two mentions
of the identifier. A function would have let you avoid the
duplicated literal 4, as this does also.
Now, define "better". ;-) Some tasks have an inherent
complexity that can't be shrunk below a certain size...
-Peter
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