# [Rd] HOW TO AVOID LOOPS

Dan Davison davison at stats.ox.ac.uk
Sat Apr 12 22:32:49 CEST 2008

```On Sat, Apr 12, 2008 at 06:45:00PM +0100, Dan Davison wrote:
> On Sat, Apr 12, 2008 at 01:30:13PM -0400, Vincent Goulet wrote:
> > Le sam. 12 avr. à 12:47, carlos martinez a écrit :
> > >> Looking for a simple, effective a minimum execution time solution.
> > >>
> > >> For a vector as:
> > >>
> > >> c(0,0,1,0,1,1,1,0,0,1,1,0,1,0,1,1,1,1,1,1)
> > >>
> > > To transform it to the following vector without using any loops:
> > >
> > >> (0,0,1,0,1,2,3,0,0,1,2,0,1,0,1,2,3,4,5,6)
> > >>
> > > Appreciate any suggetions.
> >
> > This does it -- but it is admittedly ugly:
> >
> >  > x <- c(0,0,1,0,1,1,1,0,0,1,1,0,1,0,1,1,1,1,1,1)
> >  > ind <- which(x == 0)
> >  > unlist(lapply(mapply(seq, ind, c(tail(ind, -1) - 1, length(x))),
> > function(y) cumsum(x[y])))
> >   [1] 0 0 1 0 1 2 3 0 0 1 2 0 1 0 1 2 3 4 5 6
> >
> > (The mapply() part is used to create the indexes of each sequence in x
> > starting with a 0. The rest is then straightforward.)
>
>
> Here's my effort. Maybe a bit easier to digest? Only one *apply so probably more efficient.
>
> function(x=c(0,0,1,0,1,1,1,0,0,1,1,0,1,0,1,1,1,1,1,1)) {
>     d <- diff(c(0,x,0))
>     starts <- which(d == 1)
>     ends <- which(d == -1)
>     x[x == 1] <- unlist(lapply(ends - starts, function(n) 1:n))
>     x
> }
>

Come to think of it, I suggest using the existing R function rle(), rather than my dodgy substitute.

e.g.

g <- function(x=c(0,0,1,0,1,1,1,0,0,1,1,0,1,0,1,1,1,1,1,1)) {

runs <- rle(x)
runlengths <- runs\$lengths[runs\$values == 1]
x[x == 1] <- unlist(lapply(runlengths, function(n) 1:n))
x
}

Dan

p.s. R-help would perhaps have been more appropriate than R-devel

> Dan
>
>
> >
> > HTH
> >
> > ---
> >    Vincent Goulet, Associate Professor
> >    École d'actuariat
> >    Université Laval, Québec
> >    Vincent.Goulet at act.ulaval.ca   http://vgoulet.act.ulaval.ca
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel

```