# Re: [R] how to write assignment form of function

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Thu 11 Aug 2005 - 00:18:33 EST

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@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

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