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

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Wed 06 Sep 2006 - 16:19:46 GMT

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@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@gmail.com> wrote:
> > And here is one more:
> >
> > t(apply(a, 1, function(x) x/a[1,]))
> >
> > On 9/6/06, Gabor Grothendieck <ggrothendieck@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@americancentury.com
> > > <toby_marks@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
> > > >
> > > >
> > > >
> > > > ------------------------------------------------------------
> > > > CONFIDENTIALITY NOTICE: This electronic mail transmission (i...{{dropped}}
> > > >
> > > > ______________________________________________
> > > > R-help@stat.math.ethz.ch mailing list
> > > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > > and provide commented, minimal, self-contained, reproducible code.
> > > >
> > >
> >
>

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu Sep 07 02:38:10 2006

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