I like
antid=: 1 {."0~ -@(-i.)
antid 4
0 0 0 1
0 0 1 0
0 1 0 0
1 0 0 0
R.E. Boss
> -----Oorspronkelijk bericht-----
> Van: [email protected] [mailto:programming-
> [email protected]] Namens Alex Gian
> Verzonden: vrijdag 20 augustus 2010 12:41
> Aan: Programming forum
> Onderwerp: [Jprogramming] Creating a matrix (some polish required)
>
> The "flipped" identity matrix is quite interesting. (Basically it's the
> identity matrix flipped or mirrored about either the vertical or
> horizontal axis. IOW, for a 3x3:
> 0 0 1
> 0 1 0
> 1 0 0
> I don't know if it has any special name, I stumbled on it by chance
> while messing around with a simple FSA for a puzzle game.
> Pre-multiplying by this matrix flips any same size matrix vertically,
> post-multiplying flips it horizontally. Its square is obviously the
> identity matrix.
>
> Now, I have created this matrix on various systems, (e.g. HP/RPL,
> Scheme, TI-Basic), usually - if an id matrix is provided - just by
> reversing its rows, otherwise just by starting off with the vector, say,
> 0 0 0 1, and then rotating the "1" each time and appending that vector
> to a string, which is eventually converted to a matrix.
>
> I can do this using loops and explicit coding, but am looking for any
> suggestions to make it into a J one-liner, such that:
> flipidm 4
> 0 0 0 1
> 0 0 1 0
> 0 1 0 0
> 1 0 0 0
>
> Hope my explanation was clear enough,
> TIA
>
>
>
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