m$(,v)(;(|.&.>@{.,:{:)&.|:_2]\</.v)},v=.i.m=.4 4
0 1 5 6
2 4 7 12
3 8 11 13
9 10 14 15
--- bill lam <[EMAIL PROTECTED]> wrote:
> I found a problem that is said to an assignment for introductory C course but
> it
> turned to be not so trivial. It can be rephrased in J as: for a positive
> integer
> m find the array v or equivalently (m,m)$v so that result is a matrix with
> atoms
> of i.*:m filling in zig-zag order. (restricted to square matrix for time
> being)
>
> example
> 4 4$0 1 5 6 2 4 7 12 3 8 11 13 9 10 14 15
> 0 1 5 6
> 2 4 7 12
> 3 8 11 13
> 9 10 14 15
>
> I tackled the problem by noticing the sum of cell index pair for each layer is
> constant but run into trouble when need to eliminate extra pairs after passing
> the middle layer.
>
> All suggestions welcome.
>
> PS. zig-zag order is used in jpeg encoding http://en.wikipedia.org/wiki/JPEG
> --
> regards,
> bill
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>
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