[Rd] the incredible lightness of crossprod
bates at stat.wisc.edu
Thu Jan 27 21:33:20 CET 2005
Paul Gilbert wrote:
> A few weeks ago I noticed
> > z <- matrix(rnorm(20000),10000,2)
> > system.time(for (i in 1:1000) apply(z,2,sum))
>  13.44 0.48 14.08 0.00 0.00
> > system.time(for (i in 1:1000) rep(1,10000) %*% z)
>  6.46 0.11 6.84 0.00 0.00
> which seemed completely contrary to all my childhood teachings. Now
Must have had an interesting childhood if you spent it learning about
the speeds of various matrix multiplication techniques.
> > system.time(for (i in 1:1000) crossprod(rep(1,10000), z))
>  1.90 0.12 2.24 0.00 0.00
and there is a good chance that a significant portion of that time is
taken up by repeating the rep(1, 10000) function call 1000 times.
> makes sense because it is suppose to be faster than %*% , but why is
> apply so slow?
I believe that this is Bert Gunter's cue to comment on the internal (or
infernal) nature of the apply functions.
> (And should I go back and change apply in my code everywhere or is this
> likely to reverse again?)
> Paul Gilbert
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