Try this: library(alr3) library(gsubfn) example(delta.method) # defines m1 num <- 2 fn$delta.method(m1, "-b1/($num*b2)")
See gsubfn home page at: http://gsubfn.googlecode.com On Thu, Nov 6, 2008 at 12:13 PM, Christoph Scherber <[EMAIL PROTECTED]> wrote: > Dear all, Dear Keith, > > Well, of course in fact the problem is more complicated than that. The > example was just for illustration. > > I have several statistical models for which I want to retrieve predictions > using delta.method (from library(alr3)). > > Now for this I need a character string such as e.g. > delta.method(model, "a + exp(c * x)") # model could for example be an > exponential nls fit > > where the "x" shall be replaced by a numeric value at which the predictions > plus s.e. shall be returned: > > delta.method(model, "a + exp(c * 10)") > > ## > > Because there are many models for which this shall be done, I would like to > have a function that does something like the following: > > extract.estimates=function(model.list,xval) > for (i in 1:length(model.list)){ > mod=model.list[i] > > - extract the model formula using formula(mod) > - every time the variable "x" is present, it shall be replaced by xval > - create a pasted character string "pastestring" to be used by delta.method > > result=list(delta.method(mod,pastestring)) > return(result) > } > > I hope this has helped illustrating my point. > > All the best > Christoph > > > > > Keith Jewell schrieb: >> >> Hi, >> >> Firstly, I'd recommend using '<-' for assignment, rather than '='; it >> saves confusion >> Secondly, I don't think you want 'a*x+b' as a formula, I think you want an >> expression. >> Thirdly, your 'y' has only one term, a 9 character constant = "a * x + b" >> >> Consider instead, >> y <- expression(a*x+b) >> a <- 2 >> b <- 3 >> x <- 1:10 >> y >> eval(y) >> >> Now, how to replace 'x' by 'w'? >> I'm not an expert, but this is the kind of thing I need to do, so I'd >> welcome criticism of my approach. >> I would view the expression as a list: >> as.list(y) >> as.list(y[[1]]) >> >> So y is an expression containing a sub-expression; that is y is '(a*x) + >> b' >> You want to access the sub-expression 'a*x' >> y[[1]][[2]] >> as.list(y[[1]][[2]]) >> >> Now you want to replace the third item in that sub-expression with the >> name (not the character) w >> y[[1]][[2]][[3]] <- as.name("w") >> w <- 11:20 >> y >> eval(y) >> >> hth >> >> Keith J >> >> P.S. Perhaps you really do want a formula; y ~ a*x+b ?? >> In that case I'd still probably manipulate it as a list. >> ------------------------------------------- >> "Christoph Scherber" <[EMAIL PROTECTED]> wrote in >> message news:[EMAIL PROTECTED] >>> >>> Dear all, >>> >>> How can I replace text in objects that are of class "formula"? >>> >>> y="a * x + b" >>> class(y)="formula" >>> grep("x",y) >>> y[1] >>> >>> Suppose I would like to replace the "x" by "w" in the formula object "y". >>> >>> How can this be done? Somehow, the methods that can be used in character >>> objects do not work 1:1 in formula objects... >>> >>> Many thanks and best wishes >>> Christoph >>> >>> >>> >>> -- >>> Dr. rer.nat. Christoph Scherber >>> University of Goettingen >>> DNPW, Agroecology >>> Waldweg 26 >>> D-37073 Goettingen >>> Germany >>> >>> phone +49 (0)551 39 8807 >>> fax +49 (0)551 39 8806 >>> >>> Homepage http://www.gwdg.de/~cscherb1 >> >> ______________________________________________ >> 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. >> >> . >> > > -- > Dr. rer.nat. Christoph Scherber > University of Goettingen > DNPW, Agroecology > Waldweg 26 > D-37073 Goettingen > Germany > > phone +49 (0)551 39 8807 > fax +49 (0)551 39 8806 > > Homepage http://www.gwdg.de/~cscherb1 > > ______________________________________________ > 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. > ______________________________________________ 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.
