[R] Subsetting/modifying a symbolic formula

[Gabor Grothendieck]

Here is one way:

> fo <- y ~ f1 * f2
> one.x <- lapply(all.vars(fo[[3]]), function(x) { fo[[3]] <- as.name(x); fo })
> one.x
[[1]]
y ~ f1

[[2]]
y ~ f2


On Wed, Oct 21, 2009 at 11:29 AM, Bryan Hanson <hanson at depauw.edu> wrote:
> Hello All..
>
> Please consider the following:
>
> y <- rnorm(20, mean = 10)
> f1 <- as.factor(rep(c("A", "B", "B", "A"), 5))
> f2 <- as.factor(rep(c("C", "D"), 10))
> testdata <- data.frame(y, f1, f2)
>
> testFunc <- function(formula, data, ...) {
> #    mf <- model.frame(formula, data)
>    kw.res <- kruskal.test(formula, data)
>    }
>
> res <- testFunc(y ~ f1 * f2, testdata)
>
> Which works perfectly well.  Now, I would like to do some other tests which
> allow only one factor, for instance wilcox.test.  So, I would like to modify
> my formula so that it reads y ~ f1 or y ~ f2, then conduct more tests.  What
> is the smart way to subset or modify an existing symbolic formula?  I have
> been staring at the attributes of res and mf (commented out currently) but
> these are nested in a way I find difficult to extract.  ?terms,
> ?terms.formula, ?formula etc all discuss these attributes but as I said, it
> seems a bit impenetrable.  RSiteSearch("symbolic formula") returns too many
> answers.  No doubt I am missing the obvious, as this is my first foray into
> using formulas in my own functions.
>
> TIA, Bryan
> *************
> Bryan Hanson
> Acting Chair
> Professor of Chemistry & Biochemistry
> DePauw University, Greencastle IN USA
>
>
>> sessionInfo()
> R version 2.10.0 RC (2009-10-19 r50172)
> i386-apple-darwin8.11.1
>
> locale:
> [1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8
>
> attached base packages:
> [1] datasets  tools     grid      utils     stats     graphics  grDevices
> methods
> [9] base
>
> other attached packages:
>  [1] ggplot2_0.8.3      reshape_0.8.3      proto_0.3-8        mvbutils_2.2.0
>  [5] ChemoSpec_1.2      lattice_0.17-25    mvoutlier_1.4      plyr_0.1.8
>  [9] RColorBrewer_1.0-2 chemometrics_0.4   som_0.3-4
> robustbase_0.4-5
> [13] rpart_3.1-45       pls_2.1-0          pcaPP_1.7          mvtnorm_0.9-7
> [17] nnet_7.2-48        mclust_3.2         MASS_7.2-48        lars_0.9-7
> [21] e1071_1.5-19       class_7.2-48
>
> ______________________________________________
> 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.
>