[R] how to write assignment form of function

[Heinz Tuechler]

Dear Professor Ripley,

thank you for your answer. Adding a return value, as also Dimitris
Rizopoulos suggested the function does what I need, that is to rename
factor levels.
I tried to look at levels<-.factor in R-devel but I have to admit that I do
not know exactly where to look and searching I did not find it. For the
moment my problem is solved and I interpret your hint that way that in the
future levels<-.factor will not more drop all other attributes.

Thanks again

Heinz Tüchler

At 15:18 10.08.2005 +0100, Prof Brian Ripley wrote:
>On Wed, 10 Aug 2005, Heinz Tuechler wrote:
>
>> where can I find information about how to write an assigment form of a
>> function?
>
>In all good books on S programming, or by studying examples.  But in this 
>case the problem is actually about defining functions with the return 
>value you expect.
>
>> 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.
>
>And that would not do what the current function does, which is to merge 
>levels as required.
>
>I suggest you look at levels<-.factor in R-devel, which does not drop 
>attributes.
>
>
>> 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
>> }
>
>This did not return anything!  Try returning 'x'.
>
>> levels.simple(y) <- c('a', 'b', 'c', 'd')     # does not what I want
>> y
>> [1] "a" "b" "c" "d"
>
>
>-- 
>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 272866 (PA)
>Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>
>