Once you know how to build the identity matrix (several ways on http://www.jsoftware.com/help/phrases/special_matrices.htm) you reverse it (http://www.jsoftware.com/help/dictionary/d231.htm) and that's it.
> -----Original Message----- > From: [email protected] [mailto:programming- > [email protected]] On Behalf Of Alex Gian > Sent: Friday, August 20, 2010 12:41 PM > To: Programming forum > Subject: [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 > > > > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
