[R] factor always have type integer

[Prof Brian Ripley]

On Wed, 8 Sep 2004, Erich Neuwirth wrote:

> The function I need is
> valtype<-function(x)
> 	typeof(ifelse(is.factor(x),levels(x),x))
> 
> It is easy enough to write.
> Are there any other special cases where the values
> and the storage mode differ?

All classed objects are more than the internal type recorded by typeof().

> The background for all this is that I am transferring data
> from R to Excel with VBA and I have to handle numbers
> and strings differently. So I need to check for the type
> of values I will get returned before I do the transfer.
> 
> Therefore, I would like to know if there are other types of variables
> (besides factors) which give a misunderstandable answer about the type 
> of their values when asked typeof.

All of them.  Some people are capable of misunderstanding anything!

I believe you should be using class() not typeof().

> Prof Brian Ripley wrote:
> 
> > On Wed, 8 Sep 2004, Erich Neuwirth wrote:
> > 
> > 
> >>typeof applied to a factor always seems to return "integer",
> >>independently of the type of the levels.
> > 
> > 
> > typeof is telling you the internal structure. From ?factor
> > 
> >      'factor' returns an object of class '"factor"' which has a set of
> >      integer codes the length of 'x' with a '"levels"' attribute of
> >      mode 'character'. 
> > 
> > (Despite that, we don't enforce this and people have managed to create 
> > factors with non-integer numeric codes.)
> > 
> > Now ?typeof says
> > 
> >      'typeof' determines the (R internal) type or storage mode of any
> >      object
> > 
> > and that is the "integer" as the codes are stored in an INTSXP.
> > 
> > BTW, factors were an internal type long ago, and were one of the two
> > unnamed types which appear in output from memory.profile().
> > 
> > 
> >>This has a strange side effect.
> > 
> > 
> > It's a very well documented feature of data.frame, as others have 
> > pointed out.
> > 
> > 
> >>When a variable is "imported" into a data frame,
> >>its type changes.
> >>character variables automatically are converted
> >>to factors when imported into data frames.
> >>
> >>Here is an example:
> >>
> >> > v1<-1:3
> >> > v2<-c("a","b","c")
> >> > df<-data.frame(v1,v2)
> >> > typeof(v2)
> >>[1] "character"
> >> > typeof(df$v2)
> >>[1] "integer"
> >>
> >>It is somewhat surprising that
> >>the types of v2 and df$v2 are different.
> >>
> >>the answer is to do
> >>levels(df$v2)[df$v2]
> >>but that is somewhat involved.
> >>
> >>Should the types not be identical, and typeof applied to factors
> >>return the type of the levels?
> >>
> >>
> >>
> > 
> > 
> 
> 
> 

-- 
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