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

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Wed 06 Sep 2006 - 16:08:35 GMT

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
> >
> >
> >
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