Dot product confusion
- To: mathgroup at smc.vnet.net
- Subject: [mg109940] Dot product confusion
- From: "S. B. Gray" <stevebg at ROADRUNNER.COM>
- Date: Tue, 25 May 2010 06:32:38 -0400 (EDT)
- Reply-to: stevebg at ROADRUNNER.COM
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