[R] Problem with a non-factor, non-numeric variable in a data.frame

[Alun Pope]

Unfortunately it is not quite as simple as this.  It seems (to me) that
the change to the behaviour of as.numeric() in the latest release means
that the advice given in FAQ 7.10 (and the help) is incorrect when the
factor levels are integers.  The following example illustrates:

> x1 <- c(1:4,4,4,4)
> f <- factor(x1)
> levels(f)
[1] "1" "2" "3" "4"
> as.numeric(levels(f))[as.integer(f)] #this is pasted from FAQ 7.10
Error in UseMethod("as.double") : no applicable method for "as.double"
> as.integer(levels(f))[as.integer(f)]
[1] 1 2 3 4 4 4 4

And (not surprisingly) the other FAQ 7.10 recommendation behaves the
same way.

Even more unfortunate is the fact that packages may also be affected.
Yhis seems to be the case for example with spdep, in which the function
nb2listw() contains statements of the form

    mode(x) <- "numeric" 

which I assume is what is causing that function to fail with the same
error message as above.  I shall post separately to the list for spdep.

Alun

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Matthew Keller
Sent: Wednesday, 7 November 2007 8:04 AM
To: Alexandre Santos
Cc: r-help at r-project.org
Subject: Re: [R] Problem with a non-factor,non-numeric variable in a
data.frame

Alexandre,

Try rereading FAX 7.10, it explains why as.numeric() won't do it:

"In any case, do not call as.numeric() or their likes directly for the
task at hand as as.numeric() or unclass() give the internal codes"

I.e., the INTERNAL CODE of the factor is what as.numeric() is working
on rather than the numeric representation that you see.



On 11/6/07, Alexandre Santos <alexandre.santos at ochipepe.org> wrote:
> I tested
>
> as.numeric(as.character(Ratio))
>
> and it works perfectly!
>
> I still don't get why as.numeric(Ratio) was not enough, but at least
> now I know how to deal with it.
>
> Thanks for the tip, and sorry for missing the R-FAQ issue 7.10.
>
> Cheers,
> Alexandre Santos
>
>
> 2007/11/6, John Kane <jrkrideau at yahoo.ca>:
> > Have a look at the R-FAQ issue 7.10.   It's a standard
> > problem
> >
> > For more information about your variable try
> >
> > str(variable).
> >
> >
> > --- Alexandre Santos <alexandre.santos at ochipepe.org>
> > wrote:
> >
> > > Dear R list,
> > >
> > > I would like to perform an ANOVA in a set of
> > > measurements, but I have
> > > problems formatting the data.
> > >
> > > The data is a two dimensional array containing two
> > > columns:
> > > - "Stim" : the type of stimulation (string)
> > > - "Ratio" : a ratio of two numeric values
> > >
> > > Now, because some values are missing in the data
> > > (defaulting to zero),
> > > part of this array will be populated with NA ratios.
> > > Maybe this is
> > > important later.
> > >
> > > In order to make the ANOVA analysis, I need to turn
> > > my vector into a data.frame.
> > >
> > > I tried vector.table=as.data.frame(vector)
> > >
> > > But I realized that
> > > is.numeric(Ratio) gives FALSE
> > > is.factor(Ratio) gives TRUE
> > >
> > > After reading the documentation, I tried
> > >
> > > vector.table=as.data.frame(vector, stringsAsFactors
> > > = FALSE)
> > >
> > > This time
> > >
> > > is.numeric(Ratio) gives FALSE
> > > is.factor(Ratio) gives FALSE
> > >
> > > So I don't even know what is Ratio, but it's not yet
> > > numeric (is this
> > > due to the NA values?).
> > >
> > > How can I get R to understand that Ratio is numeric?
> > > Checking the
> > > documentation it seems you can do it with I(x), but
> > > the details are
> > > not explained. I also tried as.numeric(Ratio), and
> > > everything was
> > > turned into zeros.
> > >
> > > Any suggestions?
> > >
> > > Cheers,
> > >
> > > Alexandre Santos
> > >
> > > Neuro-MPI, Martinsread, Germany
> > >
> > > ______________________________________________
> > > R-help at r-project.org mailing list
> > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide
> > > http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained,
> > > reproducible code.
> > >
> >
> >
> >
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> ______________________________________________
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>


-- 
Matthew C Keller
Asst. Professor of Psychology
University of Colorado at Boulder
www.matthewckeller.com

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