Re: Dot product confusion
- To: mathgroup at smc.vnet.net
- Subject: [mg109952] Re: Dot product confusion
- From: Mark Adler <madler at alumni.caltech.edu>
- Date: Wed, 26 May 2010 07:08:56 -0400 (EDT)
- References: <htg90l$k7h$1@smc.vnet.net>
On 2010-05-25 03:32:53 -0700, S. B. Gray said:
> ptsa = {{x1, y1, z1}, {x2, y2, z2}, {x3, y3, z3}};
The dot product of two vectors commutes, but your ptsa is a matrix. In
general matrix multiplication does not commute.
> {aa, bb, cc}.ptsa (* expression 1 *)
> ptsa.{aa, bb, cc} (* expression 2 *)
Mathematica treats ptsa as a column vector (1x3) in expression 1, and
as a row vector (3x1) in expression 2.
Mark