[R] Matrix multiplication using apply() or lappy() ?

Rolf Turner
2006-09-06T16:52:21Z

Prof. Brian Ripley wrote:

> On Wed, 6 Sep 2006, Christos Hatzis wrote:
>
> > See ?sweep
> >
> > sweep(a, 2, a[1,],"/")
>
> That is less efficient than
>
> a/rep(a[1,], each=nrow(a))

*My* first instinct was to use

	t(t(a)/a[1,])

(which has not heretofore been suggested).

This seems to be more efficient still (at least in respect of Prof.
Grothendieck's toy example) by between 20 and 25 percent:

	> a <- matrix(1:24,4)
	> system.time(for(i in 1:1000) junk <- a / rep(a[1,], each = 4))
	[1] 0.690 0.080 1.051 0.000 0.000
	> system.time(for(i in 1:1000) junk <- t(t(a)/a[1,]))
	[1] 0.520 0.120 0.647 0.000 0.000
	> system.time(for(i in 1:10000) junk <- a / rep(a[1,], each = 4))
	[1]  7.08  0.99 10.08  0.00  0.00
	> system.time(for(i in 1:10000) junk <- t(t(a)/a[1,]))
	[1] 5.530 0.940 7.856 0.000 0.000

			cheers,

				Rolf Turner