This solution is quite elegant--for those who didn't read the link, the
verb is
f3 =. 2 > (i.~ (] - {) /:@/:)
However, it's about as fast as f2, that is, a bit slow. I'll have to
stick with f1.
It seems I had forgotten the blazing speed of ~: . It's clear that f1
could be made faster in C by modifying ~:'s code, but that algorithm is
a bit too involved to write in J. I don't think any other approach will
dethrone f1, although that's an easy thing to be wrong about...
Marshall
On Thu, Jun 12, 2014 at 10:26:12AM -0700, Roger Hui wrote:
> See http://www.jsoftware.com/jwiki/Essays/Progressive%20Index-Of. Choose
> items with occurrence counts <: 2 .
>
>
> On Thu, Jun 12, 2014 at 10:17 AM, Marshall Lochbaum <[email protected]>
> wrote:
>
> > I came across this problem and was wondering if anyone could come up
> > with a solution that is both elegant and fast:
> >
> > Given a list of positive integers, return a mask (or index list) which
> > selects the first two of each unique value in the list.
> >
> > Of course ~: is the solution if you only want the first value. A fast
> > but ugly solution using ~: is
> >
> > f1 =. (] +. ~:@:(*-.)) ~:
> >
> > which relies on the fact that the list is always positive. A more
> > general solution using key, which returns an unordered list of indices,
> > is
> >
> > f2 =. (2&{.)^:(2<#)(<@)/.(;@:) i.@#
> >
> > which isn't exactly pretty, but at least allows you to change the number
> > of values you want. It's also much slower than f1:
> >
> > d =. 99 , 100 + 1e7?.@$10000
> > (I.@:f1 -: /:~@:f2) d
> > 1
> > 6!:2 'f1 d'
> > 0.273576
> > 6!:2 'f2 d'
> > 1.08829
> >
> > Any other ideas?
> >
> > Marshall
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> >
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