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Re: Cycle around a sequence

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On 2/8/2012 9:25 AM, Neil Cerutti wrote:
> On 2012-02-08, Mark Lawrence<breamoreboy@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?
>
> Python's indices were designed for these kinds of shenanigans.
>
> def rotated(seq, n):
>      """Iterate through all of seq, but starting from index n.
>
>      >>>  ", ".join(str(n) for n in rotated(range(5), 3))
>      '3, 4, 0, 1, 2'
>      """
>
>      i = n - len(seq)
>      while i<  n:
>          yield seq[i]
>          i += 1

This is really nice, in the category of "Why didn't I think of that?"
(Probably because I knew the % mod solution from C and never 'updated'!)

> if __name__ == "__main__":
>      import doctest
>      doctest.testmod()
>
> If you have merely an iterable instead of a sequence, then look
> to some of the other clever stuff already posted.

To make a repeating rotator is only a slight adjustment:

     k = n - len(seq)
     while True:
         i = k
         while i < n:
             yield seq[i]
             i += 1

-- 
Terry Jan Reedy

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Thread

Cycle around a sequence Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-02-08 01:10 +0000
  Re: Cycle around a sequence Christoph Hansen <ch@radamanthys.de> - 2012-02-08 03:01 +0100
  Re: Cycle around a sequence Neil Cerutti <neilc@norwich.edu> - 2012-02-08 14:25 +0000
    Re: Cycle around a sequence Terry Reedy <tjreedy@udel.edu> - 2012-02-08 15:15 -0500
    Re: Cycle around a sequence Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-02-08 22:47 +0000
    Re: Cycle around a sequence Serhiy Storchaka <storchaka@gmail.com> - 2012-02-09 12:10 +0200
  Re: Cycle around a sequence Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-09 03:55 +0000
    Re: Cycle around a sequence Chris Angelico <rosuav@gmail.com> - 2012-02-09 15:16 +1100
    Re: Cycle around a sequence Peter Otten <__peter__@web.de> - 2012-02-09 09:33 +0100
    Re: Cycle around a sequence Chris Angelico <rosuav@gmail.com> - 2012-02-09 21:34 +1100

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