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Re: Rotation

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On 05/01/2016 04:59, Kaz Kylheku wrote:
> On 2016-01-05, BartC <bc@freeuk.com> wrote:
>> On 04/01/2016 17:25, Paul wrote:

>>> I can verify that this code works for special cases but it's not
>>> intuitively clear that it should always work.

>> proc rotate(&a,middle)=
>>       first := a.lwb
>>       last := a.upb+1
>>       next := middle
>>
>>       while first <> next do
>>           swap(a[first++], a[next++])
>>           if next = last then
>>               next := middle
>>           elsif first = middle then
>>               middle := next
>>           fi
>>       od
>> end
>
> Lol, blub languages.
>
> $ ./txr
> This is the TXR Lisp interactive listener of TXR 129.
> Use the :quit command or type Ctrl-D on empty line to exit.
> 1> (defparm a (range 0 9))
> a
> 2> (defparm b (range 0 9))
> b
> 3> a
> (0 1 2 3 4 5 6 7 8 9)
> 4> b
> (0 1 2 3 4 5 6 7 8 9)
> 5> (rotate [a 0..8] [a 8..:])
> (0 1 2 3 4 5 6 7)
> 6> (rotate [b 0..2] [b 2..:])
> (0 1)
> 7> a
> (8 9 0 1 2 3 4 5 6 7)
> 8> b
> (2 3 4 5 6 7 8 9 0 1)

How does this relate to the problem of verifying the original C++ code?

Is it any easier to understand? It seems you are not actually 
implementing a 'rotate' function function at all (AFAICS), but simply 
using some built-on operator.

Then of course it becomes very easy! Any language can 'implement' rotate 
as something like:

   rotate_inplace(a,shiftcount)

because the work has already been done. But the same 'blub' language as 
above can also define the rotation like this:

proc rotate_left(&a,count)=
     a:=right(a,-count) concat left(a,count)
end

(But it requires extra workspace compared with the plain code above.)

-- 
Bartc

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Thread

Rotation Paul <pepstein5@gmail.com> - 2016-01-04 09:25 -0800
  Re: Rotation David Brown <david.brown@hesbynett.no> - 2016-01-04 23:51 +0100
  Re: Rotation BartC <bc@freeuk.com> - 2016-01-05 01:24 +0000
    Re: Rotation Kaz Kylheku <kaz@kylheku.com> - 2016-01-05 04:59 +0000
      Re: Rotation BartC <bc@freeuk.com> - 2016-01-05 12:22 +0000
      Re: Rotation BartC <bc@freeuk.com> - 2016-01-05 12:55 +0000
        Re: Rotation Kaz Kylheku <kaz@kylheku.com> - 2016-01-05 14:37 +0000
          Re: Rotation BartC <bc@freeuk.com> - 2016-01-05 18:48 +0000
  Re: Rotation Melzzzzz <mel@zzzzz.com> - 2016-01-05 09:13 +0100

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