[R] Subsetting/modifying a symbolic formula

[Bryan Hanson]

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