[R] change some levels of a factor column in data frame according to a condi

[Francisco J. Zagmutt]

Hi Gesman

There may be more elegant ways to do this but here is one option:

d=data.frame(crit1=gl(2,5), crit2=factor(letters[1:10]), x=rnorm(10)) 
#Creates data

levels(d$crit2)=c(levels(d$crit2),"Small")#Adds the level "Small" to the 
factor crit2.

d2=d[order(d$crit1,d$x),]#Sorts x ascending, by crit1

idx=do.call("rbind",by(d2,d$crit1,head,2))#selects the 2 smallest by crit1 
and merges the results by row

d2[d2$x %in% idx$x,'crit2']="Small" #Changes the desired crit2 to "Small"


Cheers

Francisco


>From: "Gesmann, Markus" <Markus.Gesmann at lloyds.com>
>To: r-help at stat.math.ethz.ch
>Subject: [R] change some levels of a factor column in data frame according 
>to a condition
>Date: Mon, 14 Nov 2005 20:05:38 +0000
>
>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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