[R] how to write assignment form of function

[Prof Brian Ripley]

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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