That’s a good approach.
The _ q: representation is really nice for this task.
Thanks,
—
Raul
On Monday, March 11, 2019, Eugene Nonko <eno...@gmail.com> wrote:
> Here's the solution I ended up using:
>
> _&q:^:_1 >./ _ q: >: i. 10000x
>
> Just factorize to prime exponents representation, find maximums and convert
> back from prime exponent representation.
>
> On Sun, Mar 10, 2019 at 2:35 PM Raul Miller <rauldmil...@gmail.com> wrote:
>
> > J's extended precision integer implementation is part of it. But
> > floating point numbers don't really work for this kind of problem for
> > anything longer than i.11
> >
> > But, also, I imagine a part of this also might be that Haskell has
> > optimizations which improve performance of this kind of problem, which
> > we don't have in J.
> >
> > Here's a J approach that gets the solution to this kind of problem a bit
> > faster:
> >
> > lcmseq=:3 :0
> > primes=. i.&.(p:inv) y
> > maxsq=. 1+primes I.%:y
> > */primes^x:1>.(#primes){.<.(maxsq{.primes)^.y
> > )
> >
> > lcmseq 100000x
> >
> > 695283836241707197000307586526418388339874291768035113536027
> 537561504144217502123750625798682860204776361287769787645489
> 273366008105870757535968316298519927347209547516689789186138
> 157883056062709938348338270956051626062862418050487468112737
> 2319705939469099...
> > 6!:2'lcmseq 100000x'
> > 0.398073
> >
> > I hope this helps,
> >
> > --
> > Raul
> >
> > --
> > Raul
> >
> > On Sun, Mar 10, 2019 at 5:10 PM james faure <james.fa...@epitech.eu>
> > wrote:
> > >
> > > That, I suspect, can be blamed mostly on the abysmally slow extended
> > precision integers in J, and the fact that *. must manipulate these
> > extended precision integers more often than other verbs.
> > >
> > > Indeed, If you remove the 'x', it runs extremely fast.
> > > ________________________________
> > > From: Programming <programming-boun...@forums.jsoftware.com> on behalf
> > of Eugene Nonko <eno...@gmail.com>
> > > Sent: Sunday, March 10, 2019 9:00 PM
> > > To: programm...@jsoftware.com
> > > Subject: [Jprogramming] LCM performance
> > >
> > > I need to find the smallest number that divides all numbers from 1 to
> n.
> > > The solution, of course is this:
> > >
> > > *./ >: i. n
> > >
> > > What I don't understand is why this solution seems to scale so poorly:
> > >
> > > 6!:2 '*./ >: i.10000x'
> > > 0.326128
> > > 6!:2 '*./ >: i.11000x'
> > > 1.00384
> > > 6!:2 '*./ >: i.12000x'
> > > 4.133
> > > 6!:2 '*./ >: i.13000x'
> > > 11.8082
> > >
> > > When I perform similar calculation in Haskell it produces result in
> > > negligible time, even when n = 100,000.
> > >
> > > λ: foldr1 lcm [1 .. 100000]
> > > 695283836241707197000307586...
> > >
> > > If I use a verb other than *. it runs very quickly, as expected.
> > >
> > > What's so special about LCM?
> > >
> > > Thanks,
> > > Eugene
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