Cycle around a sequence

Tim Golden mail at timgolden.me.uk
Wed Feb 8 09:36:16 CET 2012


On 08/02/2012 08:26, Mark Lawrence wrote:
> On 08/02/2012 01:26, Dennis Lee Bieber wrote:
>> On Wed, 08 Feb 2012 01:10:28 +0000, Mark Lawrence
>> <breamoreboy at yahoo.co.uk> wrote:
>>
>>> I'm looking at a way of cycling around a sequence i.e. starting at some
>>> given location in the middle of a sequence and running to the end before
>>> coming back to the beginning and running to the start place. About the
>>> best I could come up with is the following, any better ideas for some
>>> definition of better?
>>>
>>> PythonWin 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit
>>> (Intel)] on win32.
>>> Portions Copyright 1994-2008 Mark Hammond - see 'Help/About PythonWin'
>>> for further copyright information.
>>>>>> from itertools import chain
>>>>>> a=range(10)
>>>>>> g = chain((a[i] for i in xrange(4, 10, 1)), (a[i] for i in
>>>>>> xrange(4)))
>>>>>> for x in g: print x,
>>> ...
>>> 4 5 6 7 8 9 0 1 2 3
>>>>>>
>>
>> How large a sequence and, more important, is it fully known at the
>> start...
>>
>>>>> a = range(20)
>>>>> a
>> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>>>> a_shift = a[5:] + a[:5]
>>>>> a_shift
>> [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 1, 2, 3, 4]
>>>>>
>>
>> IOWs, just slice and join: tail first, then front-end.
>>
>
> The sequences are small and the start is always known but the function
> that uses this is called thousands of times so I was trying to avoid
> building lists if at all possible.
>

I'm not an itertools expert, but does this do what you want?
(Untested - I might well be off by one)

<code>
import itertools

sequence = range (100)
split = 70
rslice = itertools.islice (sequence, split, len (sequence))
lslice = itertools.islice (sequence, split)

repeater = itertools.cycle (itertools.chain (rslice, lslice))

</code>

TJG



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