## Notes:
##
## 1. Because of the difference in scoping rules between R and S it is
##    not necessary (and indeed an error) to assign the function in
##    frame 1.  The function f.check can see the function f because it
##    exists in the environment where f is defined.
##
## 2. It is also not necessary (although permissible) to wrap a "list"
##    around the function f.check in the .C call.  R passes such
##    functions through to the underlying C code in "undigested" form.
##    Correspondingly, the underlying C code does not need to extract
##    the function from the passed "list".

dyn.load(file.path(R.home(), "demos", "dynload", "zero.dll"))
##-- you may need to change this to
## dyn.load("<R_HOME>/demos/dynload/zero.so")
##-- substituting the proper path for <R_HOME>,
## unless you are executing R from the directory containing zero.so

zero <- function(f, guesses, tol = 1e-7) {
    f.check <- function(x) {
	x <- f(x)
	if(!is.numeric(x)) stop("Need a numeric result")
	as.double(x)
    }
    z <- .C("zero_find",
	    f.check,
	    ans = as.double(guesses),
	    as.double(tol))
    z$ans[1]
}

cube1 <- function(x) (x^2 + 1) * (x - 1.5)
x0 <- zero(cube1, c(0, 5))
print(x0)
print(x0, 15)