Cycle around a sequence
tjreedy at udel.edu
Wed Feb 8 21:15:59 CET 2012
On 2/8/2012 9:25 AM, Neil Cerutti wrote:
> On 2012-02-08, 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?
> 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
> 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)
i = k
while i < n:
i += 1
Terry Jan Reedy
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