[R] how to write assignment form of function

[Prof Brian Ripley]

On Wed, 10 Aug 2005, Heinz Tuechler wrote:

> 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

https://svn.r-project.org/R/trunk/src/library/base/R/factor.R

> 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.

Yes, that's true.

>
> Thanks again
>
> Heinz Tüchler
>
> 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
>>
>>
>
>
>

-- 
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