[Rd] problem assigning an array to a variable in a data frame

John Fox
Thu, 08 Jan 2004 16:58:03 -0000

Dear Brian,

At 08:36 AM 1/8/2004 +0000, Prof Brian Ripley wrote:
>The problem is not in $ but in printing the resulting object (try
>traceback()).  You assigned a column which is a one-dimensional array:
>
> > unlist(sapply(sample20, class))
>   school1   school2       ses   mathach   meanses
>"ordered"  "factor" "numeric"
"numeric"   "array"
>
>I presume you did not actually intend to do this?

Right -- I understood that meanses is a one-dimensional array, though I
didn't notice that when I first wrote the command.

>The `problem' is in the use of tapply, which does something different
here
>from its S counterpart: in R you need to surround its result by drop().
>In R <= 1.8.0 subsetting a 1-d array dropped the dimensions, which was a
>bug, and you were saved by a fortuitous subsetting.
>
>But what $ does in R is compatible with S:
>
>X <- data.frame(x=1:10)
>y <- array(1:10, dim=10)
>X$y <- y
>sapply(X, class)
>
>for which both give integer, array.
>
>However, R's print method is deficient and so the error message is in
>precisely the right place.  (S does have an array method for
>as.data.frame, and since S4 classes were introduced into R, R needs one.)
>
>Brian

Thanks for the explanation.

John

>On Wed, 7 Jan 2004, John Fox wrote:
>
> > Dear r-devel list members,
> >
> > Dirk Eddelbuettel brought the following problem to my attention. The
code
> > is abstracted from the appendix on mixed models from my R and S-PLUS
> Companion:
> >
> >  > set.seed(12345)  # for reproducibility
> >  > library(nlme)
> > Loading required package: lattice
> >  > data(MathAchieve)
> >  > data(MathAchSchool)
> >  > attach(MathAchieve)
> >  > mses <- tapply(SES, School, mean)
> >  > detach(MathAchieve)
> >  > Bryk <- as.data.frame(MathAchieve[, c("School",
"SES", "MathAch")])
> >  > names(Bryk) <- c("school", "ses",
"mathach")
> >  > sample20 <- sort(sample(7185, 20))
> >  > Bryk$meanses <- mses[as.character(Bryk$school)]
> >  > Bryk[sample20,]
> > Error in as.data.frame.default(x[[i]], optional = TRUE) :
> >          can't coerce array into a data.frame
> >  >
> >
> > This is with R 1.8.1 under Windows 2000. The same code works fine with
R
> > 1.8.0. As Dirk discovered, the problem disappears if the assignment
> >
> > Bryk$meanses <- mses[as.character(Bryk$school)]
> >
> > is changed to
> >
> > Bryk$meanses <- as.vector(mses[as.character(Bryk$school)])
> >
> > It seems to me that it if $ doesn't want to coerce the array
> > mses[as.character(Bryk$school)] to a vector (as I guess was previously
the
> > case),
>
>Don't guess, check!
>
> > then the error should have been produced at that point.
> >
> > Thanks,
> >   John
> > -----------------------------------------------------
> > John Fox
> > Department of Sociology
> > McMaster University
> > Hamilton, Ontario, Canada L8S 4M4
> > email: jfox@mcmaster.ca
> > phone: 905-525-9140x23604
> > web: www.socsci.mcmaster.ca/jfox
> >
> > ______________________________________________
> > R-devel@stat.math.ethz.ch mailing list
> > https://www.stat.math.ethz.ch/mailman/listinfo/r-devel
> >
> >
>
>--
>Brian D. Ripley,                  ripley@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

-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox@mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox