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