[R] Best method to add unit information to dataframe ?

[Joshua Wiley]

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 at .Data <- mtcars
tmp at row.names <- rownames(mtcars)
tmp at units <- c("x", "y")

## data frameish
colMeans(tmp)
tmp + 10

# but
tmp at 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 at 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))
> }
>
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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
Programmer Analyst II, ATS Statistical Consulting Group
University of California, Los Angeles
https://joshuawiley.com/