On Fri, 28 Sep 2012, Sven Schreiber wrote: > Good morning, > > I just noticed the new (?) function diagcat() with the following help text: > > "Returns the direct sum of A and B, that is a block-diagonal matrix > holding A in its north-west corner and B in its south-east corner. " > > Why would that be a _sum_?
Because that's its name :) http://en.wikipedia.org/wiki/Matrix_addition#Direct_sum Intuitively, you can think of this operator as a sum if you view matrices as the space spanned by their columns. The direct sum of two matrices results in a space which is equal to the two original spaces "put together", so to speak. > Also, what happens if A and B are not square? (I know, I could just try > that out, but too lazy, and that wouldn't necessarily answer what's > intended as a result.) Nothing spectacular. You just have non-square zero blocks in the north-east and south-west corners of the result. > And while I'm at it: I also noticed trimr() which trims rows. Don't know > whether that's new, but why introduce a special function for this > operation, couldn't it always be easily done by indexing, as in > M[ttop+1:rows(M)-tbot] ? It's been around for a while. Maybe you're right. We may want to suppress a few redundant functions for 2.0. -------------------------------------------------- Riccardo (Jack) Lucchetti Dipartimento di Economia Università Politecnica delle Marche (formerly known as Università di Ancona) r.lucchetti(a)univpm.it http://www2.econ.univpm.it/servizi/hpp/lucchetti --------------------------------------------------
