[Tutor] rotating a list

[Bob Gailer]

At 10:21 PM 9/19/2003, kevin parks wrote:

>Hi. My python chops are a bit rusty but i need another pair of eyes to 
>tell me what i am doing wrong. I am sure that it is something very stupid 
>but i can't see what it is.. I am trying to cycle through a list like so
>
>[a, b, c]
>[b, c, a]
>[c, a, b]
>[a, b, c]
>
>only, that aint be what is coming out the other side...
>
>anyone see where things are going wrong?
>
>
>
>cheers,
>-kp--
>
>--^----------------------------------------
>
>kevin parks
>university of virginia
>k p p 9 c @ virginia . edu
>
>PS. If my man Danny is out there... 'Hi'
>
>P.P.S if there are any pythonistas out here at UVa, i'd love to hear from
>you. i am new here.
>
>
>import sys
>import random
>
># cyclically rotate a sequence
>
># should work on any sequence type (list, tuple, string) and should work with
># any hop(n) interval and also work in both directions (left or right)
>
>def rotate(seq, n=1):
>     if len(seq) == 0:
>         return seq
>     n = n % len(seq) # Normalize n, using modulo - even works for negative n
>
>     return seq[n:] + seq[:n]
>
>
>def test():
>         random.seed(720)        # reproducable results
>         x = range(0, 12, 1)     # create a list for test purposes
>         print x; print          # look at it
>         random.shuffle(x)       # scramble it up
>         print x; print          # look at again...
>         # Now rotate the list one at a time till we've done come to
>         # the beginning once again
>         for i in range(len(x)):
>                 y = rotate(x)
>                 print y
>
>if __name__ == "__main__":
>         test()
>
>output is something like:
>
>[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
>
>[2, 5, 4, 10, 11, 6, 0, 3, 9, 7, 1, 8]
>
>[5, 4, 10, 11, 6, 0, 3, 9, 7, 1, 8, 2]
>[5, 4, 10, 11, 6, 0, 3, 9, 7, 1, 8, 2]
>[5, 4, 10, 11, 6, 0, 3, 9, 7, 1, 8, 2]

Exactly what I'd expect. Why did you think you'd get a different result? Do 
you suppose you were thinking that n would have a different value for each 
pass thru the loop? Yet you are not passing a 2nd parameter in the function 
call, so n always takes its default value of 1.

Bob Gailer
bgailer at alum.rpi.edu
303 442 2625
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