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better way to write this function

P: n/a
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

Thanks!
Nov 26 '07 #1
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14 Replies


P: n/a
Kelie wrote:
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv
You can use slicing:
>>def chunks(items, n):
.... return [items[start:start+n] for n in range(0, len(items)-n+1, n)]
....
>>for i in range(1,10):
.... print chunks(range(5), i)
....
[[0], [1], [2], [3], [4]]
[[0, 1], [2, 3]]
[[0, 1, 2]]
[[0, 1, 2, 3]]
[[0, 1, 2, 3, 4]]
[]
[]
[]
[]

Or build a generator that works with arbitrary iterables:
>>from itertools import *
def chunks(items, n):
.... items = iter(items)
.... while 1:
.... chunk = list(islice(items, n-1))
.... chunk.append(items.next())
.... yield chunk
....
>>list(chunks(range(5), 2))
[[0, 1], [2, 3]]

Peter
Nov 26 '07 #2

P: n/a
On Nov 26, 9:42 am, Kelie <kf9...@gmail.comwrote:
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

Thanks!
x = ['1', '2', '3', '4', '5', '6', '7', '8']
def divide_list(lst, n):
rv = []
for i in range(int(round((len(lst)/n),0))):
rv.append(lst[i*n:(i+1)*n])
return rv

tmp = divide_list(x, 3)
tmp
[['1', '2', '3'], ['4', '5', '6']]

One way to do it.
Nov 26 '07 #3

P: n/a
Chris <cw****@gmail.comwrites:
for i in range(int(round((len(lst)/n),0))): ...
Ugh!!! Not even correct (under future division), besides being ugly.
I think you mean:

for i in xrange(len(lst) // n): ...

Really though, this grouping function gets reimplemented so often that
it should be built into the stdlib, maybe in itertools.
Nov 26 '07 #4

P: n/a
On Nov 26, 10:51 am, Paul Rubin <http://phr...@NOSPAM.invalidwrote:
Chris <cwi...@gmail.comwrites:
for i in range(int(round((len(lst)/n),0))): ...

Ugh!!! Not even correct (under future division), besides being ugly.
I think you mean:

for i in xrange(len(lst) // n): ...

Really though, this grouping function gets reimplemented so often that
it should be built into the stdlib, maybe in itertools.
Beauty is in the eye of the beholder, but true... Looks crap :p
Nov 26 '07 #5

P: n/a
Kelie <kf****@gmail.comwrites:
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

Thanks!
See the last recipe from:
http://docs.python.org/lib/itertools-recipes.html. It's not doing
quite the same thing, but gives an illustration of one way to approach
this sort of thing.
Nov 26 '07 #6

P: n/a
On Nov 25, 10:51 pm, Paul Rubin <http://phr...@NOSPAM.invalidwrote:
Really though, this grouping function gets reimplemented so often that
it should be built into the stdlib, maybe in itertools.
thanks Paul.
itertools? that was actually the first module i checked.
Nov 26 '07 #7

P: n/a
On Nov 25, 11:24 pm, Paul Rudin <paul.nos...@rudin.co.ukwrote:
See the last recipe from:http://docs.python.org/lib/itertools-recipes.html. It's not doing
quite the same thing, but gives an illustration of one way to approach
this sort of thing.
Thanks for the link!
Nov 26 '07 #8

P: n/a
On Nov 26, 2007 3:24 AM, Paul Rudin <pa*********@rudin.co.ukwrote:
Kelie <kf****@gmail.comwrites:
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

Thanks!

See the last recipe from:
http://docs.python.org/lib/itertools-recipes.html. It's not doing
quite the same thing, but gives an illustration of one way to approach
this sort of thing.

--
http://mail.python.org/mailman/listinfo/python-list
The one in the sample consumes the entire sequence up front, too. It's
trivial to write a fully generator based one (and only slightly more
work to implement an iterator that doesn't rely on generators, if you
want to avoid the slight performance hit), but there's a few subtle
issues and I too think that we really should have a good one ready for
use in itertools. Maybe I should write a patch.
Nov 26 '07 #9

P: n/a
On Nov 26, 1:42 am, Kelie <kf9...@gmail.comwrote:
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

Thanks!
>>lst = list("ABCDE")
for j in range(1,6):
.... print j,':',[lst[i:i+j] for i in xrange(0,len(lst),j)]
....
1 : [['A'], ['B'], ['C'], ['D'], ['E']]
2 : [['A', 'B'], ['C', 'D'], ['E']]
3 : [['A', 'B', 'C'], ['D', 'E']]
4 : [['A', 'B', 'C', 'D'], ['E']]
5 : [['A', 'B', 'C', 'D', 'E']]

Or if you want to discard the uneven leftovers:
>>for j in range(1,6):
.... print j,':',[lst[i:i+j] for i in xrange(0,len(lst),j) if i
+j<=len(lst)]
....
1 : [['A'], ['B'], ['C'], ['D'], ['E']]
2 : [['A', 'B'], ['C', 'D']]
3 : [['A', 'B', 'C']]
4 : [['A', 'B', 'C', 'D']]
5 : [['A', 'B', 'C', 'D', 'E']]

Or define a lambda:
>>chunksWithLeftovers = lambda lst,n: [lst[i:i+n] for i in xrange(0,len(lst),n)]
chunksWithoutLeftovers = lambda lst,n: [lst[i:i+n] for i in xrange(0,len(lst),n) if i+n<=len(lst)]
chunksWithLeftovers(lst,2)
[['A', 'B'], ['C', 'D'], ['E']]
>>chunksWithoutLeftovers(lst,2)
[['A', 'B'], ['C', 'D']]
-- Paul
Nov 26 '07 #10

P: n/a
On Nov 26, 7:42 am, Kelie <kf9...@gmail.comwrote:
Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

Thanks!
Here's a terrible way to do it:

def divide_list(lst, n):
return zip(*[lst[i::n] for i in range(n)])

[It produces a list of tuples rather than a list of lists, but it
usually won't matter].

--
Paul Hankin
Nov 26 '07 #11

P: n/a
Peter Otten wrote:
>>>def chunks(items, n):
... return [items[start:start+n] for n in range(0, len(items)-n+1, n)]
Ouch, this should be

def chunks(items, n):
return [items[start:start+n] for start in range(0, len(items)-n+1, n)]

Peter
Nov 26 '07 #12

P: n/a
On Nov 26, 8:19 am, Peter Otten <__pete...@web.dewrote:
[...]
>
Or build a generator that works with arbitrary iterables:
>from itertools import *
def chunks(items, n):

... items = iter(items)
... while 1:
... chunk = list(islice(items, n-1))
... chunk.append(items.next())
... yield chunk
...>>list(chunks(range(5), 2))

[[0, 1], [2, 3]]

Peter
I was about to send this before I saw your post :)
Well here it is anyway...
Using the fact that StopIteration exceptions fall through list
comprehensions (as opposed to islices):

def chunks(iterable, size):
next = iter(iterable).next
while True:
yield [next() for i in xrange(size)]

--
Arnaud

Nov 26 '07 #13

P: n/a
Peter Otten wrote:
Kelie wrote:
>Hello,

This function does I what I want. But I'm wondering if there is an
easier/better way. To be honest, I don't have a good understanding of
what "pythonic" means yet.

def divide_list(lst, n):
"""Divide a list into a number of lists, each with n items. Extra
items are
ignored, if any."""
cnt = len(lst) / n
rv = [[None for i in range(n)] for i in range(cnt)]
for i in range(cnt):
for j in range(n):
rv[i][j] = lst[i * n + j]
return rv

You can use slicing:
>>>def chunks(items, n):
... return [items[start:start+n] for n in range(0, len(items)-n+1, n)]
...
>>>for i in range(1,10):
... print chunks(range(5), i)
...
[[0], [1], [2], [3], [4]]
[[0, 1], [2, 3]]
[[0, 1, 2]]
[[0, 1, 2, 3]]
[[0, 1, 2, 3, 4]]
[]
[]
[]
[]

This won't work(e.g. you don't define "start", you change the value of n
through the loop). I guess you meant :

def chunks(items, n) :
return [items[i:i+n] for i in range(0, len(items)-n+1, n)]

Nov 27 '07 #14

P: n/a
Ricardo Aráoz wrote:
Peter Otten wrote:
>You can use slicing:
>>>>def chunks(items, n):
... return [items[start:start+n] for n in range(0, len(items)-n+1, n)]
...
>>>>for i in range(1,10):
... print chunks(range(5), i)
...
[[0], [1], [2], [3], [4]]
[[0, 1], [2, 3]]
[[0, 1, 2]]
[[0, 1, 2, 3]]
[[0, 1, 2, 3, 4]]
[]
[]
[]
[]


This won't work(e.g. you don't define "start", you change the value of n
through the loop). I guess you meant :

def chunks(items, n) :
return [items[i:i+n] for i in range(0, len(items)-n+1, n)]
Indeed; I'm still wondering how I managed to copy'n'paste the version with
the typo together with the demo of the correct one.

Peter
Nov 27 '07 #15

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