[R] a question of substitute
Prof Brian Ripley
ripley at stats.ox.ac.uk
Tue Jan 9 14:41:09 CET 2007
oneway.test expects a literal formula, not a variable containing a
formula. The help page says
formula: a formula of the form 'lhs ~ rhs' where 'lhs' gives the
sample values and 'rhs' the corresponding groups.
Furthermore, if you had
foo.2 <- function() oneway.test(value ~ group)
it would still not work, as
data: an optional matrix or data frame (or similar: see
'model.frame') containing the variables in the formula
'formula'. By default the variables are taken from
'environment(formula)'.
I could show you several complicated workarounds, but why do you want to
do this?
On Tue, 9 Jan 2007, Adrian Dusa wrote:
>
> Hi all,
>
> I want to write a wrapper for an analysis of variance and I face a curious
> problem. Here are two different wrappers:
>
> fun.1 <- function(formula) {
> summary(aov(formula))
> }
>
> fun.2 <- function(formula) {
> oneway.test(formula)
> }
>
> values <- c(15, 8, 17, 7, 26, 12, 8, 11, 16, 9, 16,
> 24, 20, 19, 9, 17, 11, 8, 15, 6, 14)
> group <- rep(1:3, each=7)
>
> # While the first wrapper works just fine:
> fun.1(values ~ group)
>
> # the second throws an error:
> fun.2(values ~ group)
> Error in substitute(formula)[[2]] : object is not subsettable
>
> ###
>
> I also tried binding the two vectors in a data.frame, with no avail.
> I did find a hack, creating two new vectors inside the function and creating a
> fresh formula, so I presume this has something to do with environments.
>
> Could anybody give me a hint on this?
> Thank you,
> Adrian
>
>
--
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
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