On 2/22/2008 8:01 PM, Robert Walters wrote: > R folks, > As an R novice, I struggle with the mystery of subsetting. Textbook and > online examples of this seem quite straightforward yet I cannot get my > mind around it. For practice, I'm using the code in MASS Ch. 6, > "whiteside data" to analyze a different data set with similar variables > and structure. > Here is my data frame: > > ###subset one of three cases for the variable 'position' > >data.b<-data.a[data.a$position=="inrow",] > > print(data.b) > position porosity x y > 1 inrow macro 1.40 16.5 > 2 inrow macro . . > . . . . > . . . . > 7 inrow micro > 8 inrow micro > > Now I want to do separate lm's for each case of porosity, macro and > micro. The code as given in MASS, p.141, slightly modified would be: > > fit1 <- lm(y ~ x, data=data.b, subset = porosity == "macro") > fit2 <- update(fit1, subset = porosity == "micro") > > ###simplest code with subscripting > fit1 <- lm(y ~ x, data.b[porosity=="macro"])
Assuming data.b has two dimensions, you need a comma after porosity=="macro" to indicate that you are selecting a subset of rows of the data frame: fit1 <- lm(y ~ x, data.b[porosity=="macro",]) > ###following example in ?subset > fit1 <- lm(y ~ x, data.b, subset(data.b, porosity, select=macro)) The select argument to subset is meant to select variables (i.e., it indicates "columns to select from a data frame") and you are misusing it by specifying the level of a factor. If you make your call to subset by itself (a good idea when you are learning how a function works), you should get an error like this: > subset(whiteside, Insul, select=Before) Error in subset.data.frame(whiteside, Insul, select = Before) : 'subset' must evaluate to logical What I think you intended was this: subset(data.b, porosity == "macro") Even with the correct call to subset, you also don't want both data.b and the subset piece, because subset returns a data frame. In other words, you would be passing lm() two different data frames. So try this instead: fit1 <- lm(y ~ x, subset(data.b, porosity == "macro")) > None of th above, plus many permutations thereof, works. > Can anyone educate me? > > Thanks, > > Robert Walters > > ______________________________________________ > 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. -- Chuck Cleland, Ph.D. NDRI, Inc. 71 West 23rd Street, 8th floor New York, NY 10010 tel: (212) 845-4495 (Tu, Th) tel: (732) 512-0171 (M, W, F) fax: (917) 438-0894 ______________________________________________ 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.