Speed of repeated matrix calculation
- To: mathgroup at smc.vnet.net
- Subject: [mg49717] Speed of repeated matrix calculation
- From: Gregory Lypny <gregory.lypny at videotron.ca>
- Date: Thu, 29 Jul 2004 07:43:39 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Hello everyone,
Just curious to know whether I'm doing this repeated matrix calculation
efficiently by using the Table command.
I've got matrices x and y, which are both 7000x3, and a matrix V which
is 3x3.
If I create a 7000x1 vector, q, whose elements are equal to each of the
row quadratic forms x.V.x, I can do it by creating a table.
q = Table[{x[[i, All]].V.x[[i, All]]}, {i, 1, 7000}];
The calculation is done in a split second on a G4 iBook.
However, if I use Table to create a 7000x1 vector, d, of the dot
products of the rows of x and y, the calculation takes more than two
minutes!
d = Table[{x[[i, All]].y[[i, All]]}, {i, 1, 7000}];
Why the big difference in time? Is there a better way?
Greg
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