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
