A few weeks ago I noticed
> z <- matrix(rnorm(20000),10000,2)
> system.time(for (i in 1:1000) apply(z,2,sum)) [1] 13.44 0.48 14.08 0.00 0.00
> system.time(for (i in 1:1000) rep(1,10000) %*% z) [1] 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] 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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