# Generate a gradient function from a nonlinear model expression and a vector of named parameters. Given a nonlinear model expressed as an expression of the form lhs ~ formula_for_rhs and a start vector where parameters used in the model formula are named, attempts to build the the `R` function for the gradient of the residual sum of squares. As a side effect, a text file with the program code is generated. ```r model2grfun(modelformula, pvec, funname="mygr", filename=NULL) ``` ## Arguments - `modelformula`: This is a modeling formula of the form (as in `nls`) lhsvar ~ rhsexpression for example, y ~ b1/(1+b2*exp(-b3*tt)) You may also give this as a string. - `pvec`: A named parameter vector. For our example, we could use start=c(b1=1, b2=2.345, b3=0.123) WARNING: the parameters in the output function will be used in the order presented in this vector. Names are NOT respected in the output function. - `funname`: The (optional) name for the function that is generated in the file named in the next argument. The default name is 'mygr'. - `filename`: The (optional) name of a file that is written containing the (text) program code for the function. If NULL, no file is written. ## Details None. ## Returns An `R` function object that computes the gradient of the sum of squared residuals of a nonlinear model at a set of parameters. ## References Nash, J. C. (1979, 1990) _Compact Numerical Methods for Computers. Linear Algebra and Function Minimisation._ Adam Hilger./Institute of Physics Publications ## Author(s) John C Nash ## See Also Function `nls()`, packages `optim` and `optimx`. ## Examples ```r cat("See also examples in nlmrt-package.Rd\n") require(numDeriv) y <- c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443, 38.558, 50.156, 62.948, 75.995, 91.972) # for testing tt <- seq_along(y) # for testing f <- y ~ b1/(1 + b2 * exp(-1 * b3 * tt)) p <- c(b1 = 1, b2 = 1, b3 = 1) mygr <- model2grfun(f, p) myss <- model2ssfun(f, p) # for check cat("mygr:\n") print(mygr) ans <- mygr(p, tt = tt, y = y) print(ans) gnum <- grad(myss, p, tt = tt, y = y) cat("Max(abs(ans-gnum)) = ",max(abs(ans-gnum)),"\n") bnew <- c(b1 = 200, b2 = 50, b3 = 0.3) ans <- mygr(prm = bnew, tt = tt, y = y) print(ans) gnum <- grad(myss, bnew, tt = tt, y = y) cat("Max(abs(ans-gnum)) = ",max(abs(ans-gnum)),"\n") cat("Test with un-named vector\n") # At 20120424 should fail bthree <- c(100, 40, 0.1) ans <- mygr(prm = bthree, tt = tt, y = y) print(ans) gnum <- grad(myss, bthree, tt = tt, y = y) cat("Max(abs(ans-gnum)) = ",max(abs(ans-gnum)),"\n") ```

model2grfun function