Re: Dot product confusion
- To: mathgroup at smc.vnet.net
- Subject: [mg109950] Re: Dot product confusion
- From: "S. B. Gray" <stevebg at ROADRUNNER.COM>
- Date: Wed, 26 May 2010 07:08:34 -0400 (EDT)
- References: <htg90l$k7h$1@smc.vnet.net>
- Reply-to: stevebg at ROADRUNNER.COM
On 5/25/2010 3:32 AM, S. B. Gray 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
>
Thanks to all who answered my question. I temporarily forgot about
matrix - vector multiplication.