[R] Subsetting/modifying a symbolic formula
Bryan Hanson
hanson at depauw.edu
Wed Oct 21 17:29:51 CEST 2009
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
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