[Tutor] Re: rotating a list

[Bob Gailer]

At 03:09 AM 9/20/2003, Andrei wrote:

>Hi Kevin,
>
>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?
><snip>
>># 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]
>
>Works fine here:
> >>> rotate([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11])
>[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0]
> >>> rotate([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0])
>[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 1]
> >>> rotate([2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 1])
>[3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 1, 2]
>
>>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)):
>
>Here's your problem your rotate x and assign the result to y:
>
>>         y = rotate(x)
>
>Now x is unchanged, y is the result, so you go back and rotate the same 
>old x again. Obviously the result won't change.
>
>>         print y

No that is NOT the problem. That is an alternative algorithm. I explained 
the problem in my post.


>Here's the improved version:
>     for i in range(len(x)):
>         x = rotate(x)
>         print x
>
>Results:
> >>> test()
>[0, 1, 2, 3, 4]
>
>[0, 1, 4, 2, 3]
>
>[1, 4, 2, 3, 0]
>[4, 2, 3, 0, 1]
>[2, 3, 0, 1, 4]
>[3, 0, 1, 4, 2]
>[0, 1, 4, 2, 3]
>
>--
>Yours,
>
>Andrei
>
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Bob Gailer
bgailer at alum.rpi.edu
303 442 2625
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