how to write assignment form of function

take a look at e.g., get("levels<-.factor"); you should return "x" in 
your function, i.e.,

y <- x <- factor(c(1,1,NA,2,3,4,4,4,1,2))
############
'levels.simple<-' <- function(x, value){
     attr(x, 'levels') <- value
     x
}

levels.simple(y) <- c('a', 'b', 'c', 'd')
y


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.be/biostat/
    http://www.student.kuleuven.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Heinz Tuechler" <tuechler at gmx.at>
To: <r-help at stat.math.ethz.ch>
Sent: Wednesday, August 10, 2005 2:24 PM
Subject: [R] how to write assignment form of function


Dear All,

where can I find information about how to write an assigment form of a
function?
For curiosity I tried to write a different form of the 
levels()-function,
since the original method for factor deletes all other attributes of a 
factor.
Of course, the simple method would be to use instead of levels(x) <-
newlevels, attr(x, 'levels') <- newlevels.

I tried the following:
## example
x <- factor(c(1,1,NA,2,3,4,4,4,1,2)); y <- x
attr(x, 'levels') <- c('a', 'b', 'c', 'd')     # does what I want
x
[1] a    a    <NA> b    c    d    d    d    a    b
Levels: a b c d

'levels.simple<-' <- function (x, value)
{
     attr(x, 'levels') <- value
}

levels.simple(y) <- c('a', 'b', 'c', 'd')     # does not what I want
y
[1] "a" "b" "c" "d"

Thanks,
Heinz T??chler