Alternatively,  you can make a separate S3 class for a data frame with this
attribute -- and then have appropriate, print, plot, etc. methods that
access it. That way, your messy syntax has to be written only once in the
method functions.

-- Bert

On Mon, Oct 3, 2011 at 8:15 AM, Joshua Wiley <jwiley.psych@gmail.com> wrote:

> Hi Bruno,
>
> It sounds like what you want is really a separate class, one that has
> stores information about units for each variable.  This is far from an
> elegant example, but depending on your situation may be useful.  I
> create a new class inheriting from the data frame class.  This is
> likely fraught with problems because a formal S4 class is inheriting
> from an informal S3.  Then a data frame can be stored in the .Data
> slot (special---I did not make it), but character data can also be
> stored in the units slot (which I did define).  You could get fancier
> imposing constraints that the length of units be equal to the number
> of columns in the data frame or the like.  S3 methods for data frames
> should still mostly work, but you also have the ability to access the
> new units slot.  You could define special S4 methods to do the
> extraction then, if you wanted, so that your ultimate syntax to get
> the units of a particular variable would be shorter.
>
> setOldClass("data.frame")
>
> setClass("mydf", representation(units = "character"),
>  contains = "data.frame", S3methods = TRUE)
>
> tmp <- new("mydf")
>
> tmp@.Data <- mtcars
> tmp@row.names <- rownames(mtcars)
> tmp@units <- c("x", "y")
>
> ## data frameish
> colMeans(tmp)
> tmp + 10
>
> # but
> tmp@units
>
> Cheers,
>
> Josh
>
> N.B. I've read once and skimmeda gain Chambers' book, but I still do
> not have a solid grasp on S4 so I may have made some fundamental
> blunder in the example.
>
>
>
> On Mon, Oct 3, 2011 at 7:35 AM, bruno Piguet <bruno.piguet@gmail.com>
> wrote:
> > Dear all,
> >
> >  I'd like to have a dataframe store information about the units of
> > the data it contains.
> >
> >  You'll find below a minimal exemple of the way I do, so far. I add a
> > "units" attribute to the dataframe. But  I dont' like the long syntax
> > needed to access to the unit of a given variable (namely, something
> > like :
> >   var_unit <- attr(my_frame, "units")[[match(var_name, attr(my_frame,
> > "names"))]]
> >
> >  Can anybody point me to a better solution ?
> >
> > Thanks in advance,
> >
> > Bruno.
> >
> >
> > # Dataframe creation
> > x <- c(1:10)
> > y <- c(11:20)
> > z <- c(101:110)
> > my_frame <- data.frame(x, y, z)
> > attr(my_frame, "units") <- c("x_unit", "y_unit")
> >
> > #
> > # later on, using dataframe
> > for (var_name in c("x", "y")) {
> >   idx <- match(var_name, attr(my_frame, "names"))
> >   var_unit <- attr(my_frame, "units")[[idx]]
> >   print (paste("max ", var_name, ": ", max(my_frame[[var_name]]),
> var_unit))
> > }
> >
> > ______________________________________________
> > R-help@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.
> >
>
>
>
> --
> Joshua Wiley
> Ph.D. Student, Health Psychology
> Programmer Analyst II, ATS Statistical Consulting Group
> University of California, Los Angeles
> https://joshuawiley.com/
>
> ______________________________________________
> R-help@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.
>



-- 
"Men by nature long to get on to the ultimate truths, and will often be
impatient with elementary studies or fight shy of them. If it were possible
to reach the ultimate truths without the elementary studies usually prefixed
to them, these would not be preparatory studies but superfluous diversions."

-- Maimonides (1135-1204)

Bert Gunter
Genentech Nonclinical Biostatistics
467-7374
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

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