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

Prof Brian Ripley
2006-09-06T16:10:21Z

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))


>
> -Christos
>
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of
> toby_marks at americancentury.com
> Sent: Wednesday, September 06, 2006 11:49 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Matrix multiplication using apply() or lappy() ?
>
> I am trying to divide the columns of a matrix by the first row in the
> matrix.
>
> I have tried to get this using apply and I seem to be missing a concept
> regarding the apply w/o calling a function but rather command args %*% /
> etc.  Would using apply be more efficient than this approach?
>
> I have observed examples in the archives using this type of approach. Does
> anybody have a snippet of a call to apply() that would accomplish this as
> well?
>
> Thanks!
>
>
> seed=50
> $a = array(rnorm(20),dim=c(4,5))
> $b = matrix(a[1,],dim(a)[1],dim(a)[2],byrow=T)
> $a
>            [,1]       [,2]       [,3]        [,4]       [,5]
> [1,] -1.3682810 -0.4314462 1.57572752  0.67928882 -0.3672346 [2,]
0.4328180
> 0.6556479 0.64289931  0.08983289  0.1852306 [3,] -0.8113932  0.3219253
> 0.08976065 -2.99309008  0.5818237 [4,]  1.4441013 -0.7838389 0.27655075
> 0.28488295  1.3997368
>
> $a/b
>            [,1]       [,2]       [,3]       [,4]      [,5]
> [1,]  1.0000000  1.0000000 1.00000000  1.0000000  1.000000 [2,] -0.3163225
> -1.5196515 0.40800157  0.1322455 -0.504393 [3,]  0.5930018 -0.7461539
> 0.05696457 -4.4062113 -1.584338 [4,] -1.0554128  1.8167710 0.17550671
> 0.4193841 -3.811560
>
>
>
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--
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595