Cycle around a sequence

Neil Cerutti neilc at norwich.edu
Wed Feb 8 15:25:33 CET 2012


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

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.

-- 
Neil Cerutti



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