On Aug 13, 2010, at 1:22 PM, TGS wrote:
To clarify, I'd like to create a column of indicators for the
respective letters so that I could maybe do regression on
indicators, etc.
You can just enter that column name in a regression formula. No need
to create a separate variable. Try:
lm(count ~ spray, data=InsectSprays)
For instance, "A" gets "1", "B" gets "2", and so on.
That happens to be exactly the manner in which factor variables are
stored internally. Try this:
str(InsectSprays)
If for some better reason, other than what you have so far stated, you
still needed to get the at the internal values of the factor
variables, you can just use:
as.numeric(InsectSprays$spray)
This question is making me think you have not yet worked through much
of "Introduction to R".
http://cran.r-project.org/doc/manuals/R-intro.pdf
Admittedly it is long but I think you said you were strong on CS and
weaker in statistics? If you are in a real hurry and had a solid stats
background, you could look at other contributed introductions. One
that kept me up at night when I was starting R (about 5 years ago) was
Faraway's "Practical Regression and ANOVA Using R:
http://cran.r-project.org/doc/contrib/Faraway-PRA.pdf
I also though that Kuhnert and Venables' offering was scintillating:
http://cran.r-project.org/doc/contrib/Kuhnert+Venables-R_Course_Notes.zip
Others:
http://cran.r-project.org/other-docs.html
Faraway gets to factor object types by page 11, whereas you would need
to be several chapters into the "Introduction to R" to get that
information.
--
David.
On Aug 13, 2010, at 10:19 AM, David Winsemius wrote:
On Aug 13, 2010, at 1:03 PM, TGS wrote:
# how would I code in R to look at the letter of the alphabet
# in the second column and create a indicator column for the
# corresponding letter?
data(InsectSprays)
InsectSprays$spray
It's already what most people mean when they say "indicator column",
i.e., a factor variable (and not a character vector) .... so, what
do _you_ mean?
--
David Winsemius, MD
West Hartford, CT
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