On 13-3-2009 12:24, Invisible wrote:
>>> How about these?
>>>
>>> ABC+*DE+*F*
>>> ABC+DE+**F*
>>
>> such rewriting indeed works for any operator $ that satisfies
>> (A$B)$C=A$(B$C) i.e. AB$C$=ABC$$ in RPN.
>
> So... any associative operation then?
yes. because associativity is the property that you are using in the
reordering.
> A quick check confirms that
>
> ABC*-DE*-F- ==> (A-(B*C))-(D*E)-F ==> A - BC - DE - F
> ABC*DE*--F- ==> (A-((B*C)-(D*E)))-F ==> A - BC + DE - F
>
> which aren't the same.
>
>> BTW Why did you choose this example with such a lot or irrelevant detail?
>
> My brain is like mush this morning. This was the first correct example I
> could find. It's about 10 years ago that I discovered this effect, so I
> wasn't even sure if I was remembering right!
I think that as soon as you find an example you try to reduce it to the
bare essentials and post that. The same actually as when posting a bug.
> (Currently attempting to build a Haskell parser, printer and type
> interence engine.
One word: why?
> This stuff is surprisingly non-trivial. Having an
> argument about RPN in parallel with these activities is exceeding my
> mental capacity!)
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