Is the problem that you want to use the formula as if it were a function? In that case:
1. nls2 in the nls2 package is like nls but its extensions include a brute force algorithm which simply computes the formula at the starting values: > my.formula <- y ~ a+b*x^2 > y <- x <- 1:10 > library(nls2) > fitted(nls2(my.formula, start = c(a = 2, b = 3), alg = "brute")) [1] 5 14 29 50 77 110 149 194 245 302 attr(,"label") [1] "Fitted values" 2. Also fn$ in the gsubfn package can be used to convert a formula to a function. If there is no left hand side, LHS, then it is assumed that the arguments are the free variables in the right hand side. See http://gsubfn.googlecode.com > library(gsubfn) Loading required package: proto > fo <- my.formula > fo[[2]] <- NULL # zap LHS > fn$force(fo) function (a, b, x) a + b * x^2 > fn$force(fo)(a = 2, b = 3, x = x) [1] 5 14 29 50 77 110 149 194 245 302 On Tue, May 11, 2010 at 8:35 AM, ivo welch <ivo...@gmail.com> wrote: > dear R wizards--- > > I am looking for a reference that explains how to work with formula > objects. for example, say, I have a formula which I want to use in an > NLS. I want to test what this formula to see if my function was > defined correctly. Is there a way to apply a formula to data? For > example, > > my.formula = (y ~ a+b*x^2) > x= 1:10; a=2; b=3; > computed.y= apply.a.formula( my.formula, x, a, b ) > > I know that this has issues, not the least of which is that a formula > may not return a y. and that an lm() specification would require me > to infer that I have an implicit coefficient vector that is not even > specified (i.e., y~x really is y~a+b*x). so, is there a reference to > formula objects somewhere? > > sincerely, > > /iaw > ---- > Ivo Welch (ivo.we...@brown.edu, ivo.we...@gmail.com) > > ______________________________________________ > 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.