On Dec 5, 2009, at 11:30 AM, baptiste auguie wrote:
Hi,
Try this,
cor(pollute[ ,c("Pollution","Temp","Industry")])
and ?"[" in particular,
"Character vectors will be matched to the names of the object "
John-Paul;
In the time it took me to compose this, I see that others have already
pointed out all of what I had written so it only remains to offer yet-
another-R-method for ranges of column names.
You could have defined a "targets" vector of names if you know the
starting and ending position:
?Extract # or equivalently ?"["
targets <- names(pollute)[1:3] # colnames is an equivalent function
for dataframe objects
targets
pollute[ , targets]
--
Best;
David.
HTH,
baptiste
2009/12/5 John-Paul Ferguson <ferguson_john-p...@gsb.stanford.edu>:
I apologize for how basic a question this is. I am a Stata user who
has begun using R, and the syntax differences still trip me up. The
most basic questions, involving as they do general terms, can be the
hardest to find solutions for through search.
Assume for the moment that I have a dataset that contains seven
variables: Pollution, Temp, Industry, Population, Wind, Rain and
Wet.days. (This actual dataset is taken from Michael Crawley's
"Statistics: An Introduction Using R" and is available as
"pollute.txt" in
http://www.bio.ic.ac.uk/research/crawley/statistics/data/zipped.zip.)
Assume I have attached pollute. Then
cor(pollute)
will give me the correlation table for these seven variables. If I
would prefer only to see the correlations between, say, Pollution,
Temp and Industry, I can get that with
cor(pollute[,1:3])
or with
cor(pollute[1:3])
Similarly, I can see the correlations between Temp, Population and
Rain with
cor(pollute[,c(2,4,6)])
or with
cor(pollute[c(2,4,6)])
This is fine for a seven-variable dataset. When I have 250 variables,
though, I start to pale at looking up column indexes over and over. I
know from reading the list archives that I can extract the column
index of Industry, for example, by typing
which("Industry"==names(pollute))
but doing that before each command seems dire. Trained to using Stata
as I am, I am inclined to check the correlation of the first three or
the second, fourth and sixth columns by substituting the column names
for the column indexes--something like the following:
cor(pollute[Pollution:Industry])
cor(pollute[c(Temp,Population,Rain)])
These however throw errors.
I know that many commands in R are perfectly happy to take variable
names--the regression models, for example--but that some do not. And
so I ask you two general questions:
1. Is there a syntax for referring to variable names rather than
column indexes in situations like these?
2. Is there something that I should look for in a command's help file
that often indicates whether it can take column names rather than
indexes?
Again, apologies for asking something that has likely been asked
before. I would appreciate any suggestions that you have.
Best,
John-Paul Ferguson
Assistant Professor of Organizational Behavior
Stanford University Graduate School of Business
518 Memorial Way, K313
Stanford, CA 94305
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
______________________________________________
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