In general, lazy lists and lazy-style programming are not well-supported in
J. Even if you have a good representation of a lazy list, it won't work with
J's array-processing style. The main reason J is so fast is that it performs
each operation all at once, allowing for highly optimized operations that
can then be composed. Therefore it runs counter to J's design to try using
lazy lists or lazy evaluation.
Marshall
On Wed, Oct 12, 2011 at 11:24 AM, Raul Miller <rauldmil...@gmail.com> wrote:
> On Wed, Oct 12, 2011 at 10:34 AM, Zheng, Xin (NIH) [C]
> <zheng...@mail.nih.gov> wrote:
> > Hello all,
> >
> > 'label' I.@E. 'label1label2label3' could give where the label starts in
> the string. If the string is very long or infinite, only limited sites, say
> the first 100 starting sites, are needed. How to do it?
>
> If the statistical character of the right argument is well understood,
> you can sample it, choosing a likely length and just testing that
> part.
>
> For example, given:
>
> L=: 'label'
> D=: ,'p<label>0'8!:2 i.1e6
>
> ({.~ 100 <. #) L I.@E. D
> 0 6 12 18 24 30 36 42 48 54 60 67 74 81 88 95 102 109 116 123 130 137
> 144 151 15...
> ({.~ 100 <. #) L I.@E. 1e4{.D
> 0 6 12 18 24 30 36 42 48 54 60 67 74 81 88 95 102 109 116 123 130 137
> 144 151 15...
> ({.~ 100 <. #) L I.@E. 100{.D
> 0 6 12 18 24 30 36 42 48 54 60 67 74 81 88 95
>
> Or, if you want a more general approach:
>
> F=:1 :0 NB. x: label, m: max labels to find, y: target
> :
> b=. 2*m*#x NB. block size to search (smaller for testing, larger
> for typical use)
> r=. i.0 NB. result
> for_block. |: b -~/\@(] <. [ * 0 1 +/i.@>.@%~) #y do.
> if. m-:#r do. return. end.
> 'z b'=. block
> r=. r, ({.~ (m-#r) <. #)z+x I.@E. y{~z+i.b
> end.
> )
>
> L 100 F D
>
> That said, unless D is extremely large (perhaps gigabytes), the extra
> complexity of this approach is usually not worth the overhead of
> breaking the problem up into blocks.
>
> Also, it's quite possible with this kind of problem statement that you
> need to take a step back and redefine the problem. It's quite
> possible that restructuring the data or expressing the result
> differently would be advantageous.
>
> --
> Raul
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>
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