[R] Number of replications of a term

[Gabor Grothendieck]

Note that that assumes that all occurrences of a value
are contiguous.

On 1/24/06, Ray Brownrigg <ray at mcs.vuw.ac.nz> wrote:
> There's an even faster one, which nobody seems to have mentioned yet:
>
> rep(l <- rle(ids)$lengths, l)
>
> Timing on my 2.8GHz NetBSD system shows:
>
> > length(ids)
> [1] 45150
> > # Gabor:
> > system.time(for (i in 1:100) ave(as.numeric(factor(ids)), ids, FUN =
> length))
> [1] 3.45 0.06 3.54 0.00 0.00
> > # Barry (and others I think):
> > system.time(for (i in 1:100) table(ids)[ids])
> [1] 2.13 0.05 2.20 0.00 0.00
> > Me:
> > system.time(for (i in 1:100) rep(l <- rle(ids)$lengths, l))
> [1] 1.60 0.00 1.62 0.00 0.00
>
> Of course the difference between 21 milliseconds and 16 milliseconds is
> not great, unless you are doing this a lot.
>
> Ray Brownrigg
>
> > From: Gabor Grothendieck <ggrothendieck at gmail.com>
> >
> > Nice.  I timed it and its much faster than mine too.
> >
> > On 1/24/06, Barry Rowlingson <B.Rowlingson at lancaster.ac.uk> wrote:
> > > Laetitia Marisa wrote:
> > > > Hello,
> > > >
> > > > Is there a simple and fast function that returns a vector of the number
> > > > of replications for each object of a vector ?
> > > > For example :
> > > > I have a vector of IDs :
> > > > ids <- c( "ID1", "ID2", "ID2", "ID3", "ID3","ID3", "ID5")
> > > >
> > > >  I want the function returns the following vector where each term is the
> > > > number of replicates for the given id :
> > > > c( 1, 2, 2, 3,3,3,1 )
> > >
> > > One-liner:
> > >
> > >  > table(ids)[ids]
> > > ids
> > > ID1 ID2 ID2 ID3 ID3 ID3 ID5
> > >   1   2   2   3   3   3   1
> > >
> > >  'table(ids)' computes the counts, then the subscripting [ids] looks it
> > > all up.
> > >
> > >  Now try it on your 40,000-long vector!
> > >
> > > Barry
>