Re: Why Does Repeated Dot Product Take So Long?
- To: mathgroup at smc.vnet.net
- Subject: [mg67949] Re: [mg67938] Why Does Repeated Dot Product Take So Long?
- From: "Carl K. Woll" <carlw at wolfram.com>
- Date: Tue, 18 Jul 2006 05:50:34 -0400 (EDT)
- References: <200607171051.GAA02347@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Gregory Lypny wrote:
> Hello Everyone,
>
> x and y are both 2000 x 3 matrices. I wanted to created a 2000 x 1
> vector with each element equal to the dot product of the
> corresponding rows of x and y. So I tried this:
>
> Table[x[[i]].y[[i]], {i, 1, 2000}]
>
> It took more than two and a half minutes on my iBook G4. Is that
> normal? I've done seemingly more demanding computations in Do loops
> and Tables that are completed in a split second. Am I doing
> something wrong with this one?
>
> Regards,
>
> Gregory
When I tried your code with matrices containing machine reals, the
timing was practically instantaneous. Perhaps you could tell us what the
matrix elements are.
For this particular computation with very large numerical matrices, a
faster approach is to rely on the listable attribute of matrices:
Total[Transpose[x y]]
For large matrices such as 200000 x 3 the above method is significantly
faster.
Carl Woll
Wolfram Research
- References:
- Why Does Repeated Dot Product Take So Long?
- From: Gregory Lypny <gregory.lypny@videotron.ca>
- Why Does Repeated Dot Product Take So Long?