how to write assignment form of function

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?chler

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)
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