[R] How to repeat vectors ?

[Gabor Grothendieck]

Here are some timings.  From fastest to slowest
we have: #3, #4, #1, #2 so, yes, the apply
approach, even with the improvement (#2), is the
slowest and, in fact, on this test is an order
of magnitude slower than #3 which is the fastest one.

> m <- matrix(1:40000, 200) # test matrix
>
> # 1 - m must be numeric for this one to work
> system.time(for(i in 1:100)kronecker(m, rep(1,2)))
[1] 2.93 0.29 3.34   NA   NA
>
> # 2
> system.time(for(i in 1:100)apply(m, 2, rep, each = 2))
[1] 5.22 0.09 5.73   NA   NA
>
> # 3 - from Alex's post
> system.time(for(i in 1:100)m[rep(1:nrow(m), each = 2),])
[1] 0.50 0.07 0.60   NA   NA
>
> # 4
> system.time(for(i in 1:100)matrix(rbind(c(m), c(m)), nc = ncol(m)))
[1] 1.54 0.20 1.77   NA   NA

On 10/1/06, Tong Wang <wangtong at usc.edu> wrote:
> Hi,
>    Thanks you guys for all the help. I learned a lot from it.
>    It looks using apply() is not an efficient way, since all it does is looping through
> each row(or col) , which would be slow for large matrix, right ?
>
> cheers
>
> ----- Original Message -----
> From: Gabor Grothendieck <ggrothendieck at gmail.com>
> Date: Saturday, September 30, 2006 4:54 am
> Subject: Re: [R] How to repeat vectors ?
> To: Tong Wang <wangtong at usc.edu>
> Cc: r-help at stat.math.ethz.ch
>
> > Here are 4 approaches in order from most compact
> > to least.  #1 only works for numeric matrices, # 2 is
> > a shorter versio of your solution using rep.vec and # 3
> > is from Alex's post and is likely what I would
> > use in practice.
> >
> > m <- matrix(1:4, 2) # test matrix
> >
> > # 1 - m must be numeric for this one to work
> > kronecker(m, rep(1,2))
> >
> > # 2
> > apply(m, 2, rep, each = 2) # 2
> >
> > # 3 - from Alex's post
> > m[rep(1:nrow(m), each = 2),]
> >
> > # 4
> > matrix(rbind(c(m), c(m)), nc = ncol(m))
> >
> > On 9/30/06, Tong Wang <wangtong at usc.edu> wrote:
> > > I just figured out a way to do this:
> > >          rep.vec <- function(X,n)
> > return(t(array(rep(X,n),c(length(X),n))))>
> > >   Then,    apply(MyMatrix, 2, rep.vec,2)
> > >
> > > Is there a better way ?  Is there an internal function to repeat
> > a vector or matrix ?
> > >
> > > Thanks a lot.
> > >
> > >
> > > ----- Original Message -----
> > > From: Tong Wang <wangtong at usc.edu>
> > > Date: Friday, September 29, 2006 11:23 pm
> > > Subject: How to repeat vectors ?
> > > To: r-help at stat.math.ethz.ch
> > >
> > > > Hi,
> > > >    If I have a matrix  , say       a11   a12
> > > >                                                   a21  a22
> > > >    Is there a routine to get:      a11  a12
> > > >                                                     a11  a12
> > > >                                                     a21   a22
> > > >                                                     a21   a22
> > > >
> > > >     Thanks a lot for any help.
> > > >
> > > > best
> > > >
> > >
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> > >
> >
>