[R] R matrix multiplication slowdown

[Jeff Newmiller]

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.
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>I have two relatively large matrices:
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>A is 100x3072, all integers 0-255, not sparse
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>B is 1016x3072, all integers 0-255, not sparse
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>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.
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>Thanks,
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>Ken
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>(Macbook Pro running 10.11.6, 16Gb, R v 3.3.1)
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>	[[alternative HTML version deleted]]
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