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 ______________________________________________ R-help@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.