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Speed of repeated matrix calculation

  • To: mathgroup at
  • Subject: [mg49717] Speed of repeated matrix calculation
  • From: Gregory Lypny <gregory.lypny at>
  • Date: Thu, 29 Jul 2004 07:43:39 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

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 

d = Table[{x[[i, All]].y[[i, All]]}, {i, 1, 7000}];

Why the big difference in time?  Is there a better way?


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