[R] R matrix multiplication slowdown

[ken eagle]

I've done more searching and found a problem with the data in one of the
matrices that was corrupting the calculation.  Data now fixed and problem
is solved.  Apologies if anyone wasted time on this.

Ken

On Fri, Jan 27, 2017 at 8:29 AM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
wrote:

> You are asked by the Posting Guide to provide a reproducible example and
> to post in plain text (because HTML gets mangled).  I would guess your
> problem has nothing to do with multiplication, but without the code there
> is no way to say for sure.
> --
> Sent from my phone. Please excuse my brevity.
>
> On January 26, 2017 2:21:07 PM PST, ken eagle <eaglek2011 at gmail.com>
> wrote:
> >Hi all.  A question about performance of matrix multiplication.
> >
> >
> >
> >I have two relatively large matrices:
> >
> >
> >
> >A is 100x3072, all integers 0-255, not sparse
> >
> >B is 1016x3072, all integers 0-255, not sparse
> >
> >
> >
> >The command z<-B %*% t(A) works fine and takes roughly 0.2 seconds .
> >If I
> >add one row to B, the same command takes 2.4 seconds; at 1050 rows in
> >B,
> >the command is up to almost 4 seconds.  Just trying to understand why
> >the
> >big slowdown is occurring, especially since the matrices I actually
> >want to
> >multiply have 1000 and 5000 rows, not 100 and 1017.
> >
> >
> >
> >Thanks,
> >
> >Ken
> >
> >
> >
> >(Macbook Pro running 10.11.6, 16Gb, R v 3.3.1)
> >
> >       [[alternative HTML version deleted]]
> >
> >______________________________________________
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> >PLEASE do read the posting guide
> >http://www.R-project.org/posting-guide.html
> >and provide commented, minimal, self-contained, reproducible code.
>

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