Re: Dot product confusion
- To: mathgroup at smc.vnet.net
- Subject: [mg109961] Re: Dot product confusion
- From: Bob Hanlon <hanlonr at cox.net>
- Date: Wed, 26 May 2010 07:10:34 -0400 (EDT)
The dot product is not commutative in general and the examples in the Documentation Center for Dot explicitly show this (Examples/Basic Examples/Products of matrices and vectors).
Bob Hanlon
---- "S. B. Gray" <stevebg at ROADRUNNER.COM> wrote:
=============
Given
ptsa = {{x1, y1, z1}, {x2, y2, z2}, {x3, y3, z3}};
I thought the following expressions would be identical:
{aa, bb, cc}.ptsa (* expression 1 *)
ptsa.{aa, bb, cc} (* expression 2 *)
but they are not. They evaluate respectively as:
{aa x1 + bb x2 + cc x3, aa y1 + bb y2 + cc y3,
aa z1 + bb z2 + cc z3}
{aa x1 + bb y1 + cc z1, aa x2 + bb y2 + cc z2,
aa x3 + bb y3 + cc z3}
Since ptsa is itself three xyz coordinates, the expressions might be
ambiguous, but I assumed the dot product would always commute. Should
there be a warning?
The first result is the one I want.
Steve Gray