[R] Problems with factor

[ripley@stats.ox.ac.uk]

On Wed, 31 Jul 2002, kjetil halvorsen wrote:

> Why does I get only the underlying representation, and not the level
> names, in the following code to recode a factor? Seems like a bug to me.

You have coercion going on there.  Here is a simpler example:

> x <- factor(letters)
> ifelse(x=="a", "a", x)
 [1] "a"  "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9"  "10" "11" "12" "13" "14" "15"
[16] "16" "17" "18" "19" "20" "21" "22" "23" "24" "25" "26"

S does the same.

>From the help page

  `ifelse' returns a value with the same shape as `test' which is
  filled with elements selected from either `yes' or `no' depending
  on whether the element of `test' is `TRUE' or `FALSE'.

Note that there is no mention of carrying attributes across from yes or no.

I think you want something like

Malaria$TratUl <- Malaria$Trat
Malaria$TratUl[Malaria$DiaC>0] <-
   ifelse(Malaria$Trat=="QT", "SP", "QT")[Malaria$DiaC>0]

since that only works with character vectors on the RHS.

>
> > attach(Malaria)
> > Malaria$TratUl <- factor(ifelse(Malaria$DiaC>0,
> +                          ifelse(Malaria$Trat=="QT", "SP", "QT"),
> +                          Malaria$Trat))
> > Malaria$TratUl
>  [1] 1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  QT 2  2  2  2  2  QT
> QT 2
> [25] 2  2  QT 2  2  2
> Levels:  1 2 QT
> > Malaria$Trat
>  [1] QT QT QT QT QT QT QT QT QT QT QT QT QT QT QT SP SP SP SP SP SP SP
> SP SP
> [25] SP SP SP SP SP SP
> Levels:  QT SP

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._