[R] How to create as.numeric.xxx

[Prof Brian Ripley]

On 16/11/2012 10:01, Marc Girondot wrote:
> I would like to create a function to convert data based on a class using
> as.numeric
>
> Similarly, using a plot, I do:
> plot.essai <- function(x, ...) {return(x*2)}
> d <- 10
> class(d) <- "essai"
>
> plot(d)
>
> It works:
>
> [1] 20
> attr(,"class")
> [1] "essai"
>
> Now same with as.numeric:
> as.numeric.essai <- function(x, ...) {return(x*2)}
> as.numeric(d)

You need to read the help (as the posting guide asked you to do before 
posting):

      ‘as.numeric’ is a generic function, but S3 methods must be written
      for ‘as.double’.  It is identical to ‘as.double’ (and ‘as.real’).

The rest of the help page makes clear why using 'numeric' will confuse 
others as well as confusing yourself.

> It does not work:
>
> [1] 10
>
> Is it possible ?
>
> Thanks a lot
> Marc Girondot
>
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.


-- 
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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