Hi all !
Roger Hui once wrote a small function, magic, as a solution to a problem
we discussed in this forum.
I work on a twisted version of J, JWithATwist, mostly to learn F#. This
is the first program larger than a line of code I write in this
language. It is an implementation of the magic algorithm within the
present limitations of JWithATwist.
It is incredibly slow, probably because of the strange way I have to do
Cut without any Cut adverb.
magic=: ; @ (<@(</\&.|.);.2)~
The start of the discussion:
http://www.jsoftware.com/pipermail/general/2005-September/024408.html
All opinions are welcome !
{
//Take 100 to show
100 <.-
// y, I switched x and y
( 40000 $ 0 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 1 0 0 0 0 0 0 1 0 0 0
1 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0 0 1 )
{!
//Catenate and unbox boxed results. Fold here has a left argument with
the initial accumulator value.
|>
( |< |i. 0 ) {! |< ( |> [ ) , |> ] } /
//Very complicated implementation of cut. Create boxed indices to take
from x.
(
( |i. 1 + 0 + / [ )
{!
|<
( [ = ] ) /:: |i. |# ]
} '/ 0 1 /
0 + \ [
)
{!
|<
//Reverse the group
{!
( |i. |- |# ] ) <- ]
}
//First one in the group
{!
( |_ 1 = 0 + \ ] ) * ]
}
//Reverse the group
{!
( |i. |- |# ] ) <- ]
}
//Pack up indexes. Take the group items from x.
( |> [ ) <- ]
} '/ 0 1 / ]
}
//x
( 40000 $ 0 0 1 0 0 1 0 0 1 0 1 0 0 1 0 1 0 1 0 1 0 1 0 0 1 0 0
0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0 0 )
}
Cheers !
Erling
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