try this:

# create data

x.by <http://x.by> <- data.frame(crit1=rep(c(1,2),c(10,10)),
crit2=sample(letters[1:4],20,T), val=runif(20))
levels(x.by$crit2) <- c(levels(x.by$crit2), 'small') # add 'small' to the
levels
y <- by(x.by <http://x.by>, x.by$crit1, function(.grp){
.small <- order(.grp$val) # find the smallest values
.grp$crit2[.small[1:min(2,length(.small))]] <- 'small' # make sure we don't
exceed vector
.grp
})
do.call('rbind', y) # put it back together


 On 11/14/05, Gesmann, Markus <Markus.Gesmann@lloyds.com> wrote:
>
> Dear R-users,
>
> I am looking for an elegant way to change some levels of a factor column
> in data frame according to a condition.
> Lets look at the following data frame:
>
> > data.frame(crit1=gl(2,5), crit2=factor(letters[1:10]), x=rnorm(10))
> crit1 crit2 x
> 1 1 a -1.06957692
> 2 1 b 0.24368402
> 3 1 c -0.24958322
> 4 1 d -1.37577955
> 5 1 e -0.01713288
> 6 2 f -1.25203573
> 7 2 g -1.94348533
> 8 2 h -0.16041719
> 9 2 i -1.91572616
> 10 2 j -0.20256478
>
> Now I would like to find for each level in crit1 the two smallest values
> of x and change the levels of crit2 to "small", so the result would look
> like this:
>
> crit1 crit2 x
> 1 1 small -1.06957692
> 2 1 b 0.24368402
> 3 1 c -0.24958322
> 4 1 small -1.37577955
> 5 1 e -0.01713288
> 6 2 f -1.25203573
> 7 2 small -1.94348533
> 8 2 h -0.16041719
> 9 2 small -1.91572616
> 10 2 j -0.20256478
>
> Thank you for advice!
>
> Markus Gesmann
>
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