Re: [q-lang-users] Re: Optimizations and tweaks

[Tim H. <q...@st...>]

Dr....@t-... (Albert Graef) writes:

> Tim Haynes wrote:
>> def HASH = ref emptyhdict;
>> public hashed F X;
>> hashed F X      = get HASH!(F,X) if member (get HASH) (F,X);
>>                 = put HASH (update (get HASH) (F,X) Y) || Y
>>                     where Y = F X otherwise;
>> hashed isprime = hisprime;
>> hisprime N = not (any ((=0).(N mod))) (primes (intsqrt N)) ;
>> hashed primes = hprimes;
>> hprimes N        = [2] if N=2;                  = [2,3] if N=3;
>> = [2,3|filter isprime [5,7..N]] otherwise;
>
> Hmm, when I run this code over here, the hashed function is apparently
> never invoked.

Very strange! THe performance was completely different from the unhashed
version - I observed it staying all-but linear when the all-the-odds
algorithm, or even the unhashed one, were showing increases, here.. And yes
it seemed to be producing the right answers, for a few random spot-checks,
too :)

> In fact, with filter isprime [5000000..5000100] I even get a failed
> condition:
>
> ! Error in conditional
>    0>  stdlib.q, line 134: filter isprime [5000011,5000012,5000013,...] ==>
>    [5000011|filter isprime [...]] if isprime 5000011

Very weird. I did actually see a glitch with one large number, so I'll have
to go back and investigate it more...

> I think the applications of hashed are the wrong way round (it's easy to
> fall into this trap, it has happened to me, too). I've attached my
> (hopefully corrected) version of the script to this post. This one *does*
> invoke hashed now (verified with tbreak).
>
> I'm ready to add this version to the examples now (if you don't complain
> -- better do that quickly, as I'm currently doing the final touches for Q
> 6.0 ;-).

If it's not too late, I'd like to hold back on that - I'm travelling atm
and will be back home tomorrowish. Can run yours and mine through the mill
again then. I'm particularly worried about those error-in-conditional
problems... :/

~Tim
-- 
<http://spodzone.org.uk/>