# Cycle around a sequence

Mark Lawrence breamoreboy at yahoo.co.uk
Wed Feb 8 09:26:33 CET 2012

```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.
>>>>> 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.

--
Cheers.

Mark Lawrence.

```