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

Gabor Grothendieck
2006-09-06T16:19:46Z

Yet another one using the idempotent apply in reshape package
that eliminates the transpose:

library(reshape)
iapply(a, 1, "/", a[1,])

On 9/6/06, Gabor Grothendieck <ggrothendieck at gmail.com>
wrote:
> This last one could also be written slightly shorter as:
>
> t(apply(a, 1, "/", a[1,]))
>
> On 9/6/06, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> > And here is one more:
> >
> > t(apply(a, 1, function(x) x/a[1,]))
> >
> > On 9/6/06, Gabor Grothendieck <ggrothendieck at gmail.com>
wrote:
> > > Here are a few possibilities:
> > >
> > > a <- matrix(1:24, 4) # test data
> > >
> > > a / rep(a[1,], each = 4)
> > >
> > > a / outer(rep(1, nrow(a)), a[1,])
> > >
> > > a %*% diag(1/a[1,])
> > >
> > > sweep(a, 2, a[1,], "/")
> > >
> > >
> > > On 9/6/06, toby_marks at americancentury.com
> > > <toby_marks at americancentury.com> wrote:
> > > > 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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