R help - Oct 2010 - R optimization and curve()?

I'm trying to figure out how to plot basic utility maximization results with
R, Ideally I'd like to plot the value of u through x1,x2 space, so you can
graph income / substitution effects easily... 

also, it'd be nice if I could put a linear budget constraint on the graph

 here's an example with cobb douglas utility 

u <- function(x) {
    x1 <- x[1]
    x2 <- x[2]
 (x1^alpha)*(x2^(1-alpha))
}
utility <- function(x) x[1]^(alpha)*x[2]^(1-alpha)
p <- c(2,1)
i <- -100
alpha <- .3
umax <- function(p,i,u) {
res <- constrOptim(c(.5,.5), u, grad=NULL, ui=-p, ci=i, mu = 1e-04,
control=list(fnscale=-1))
return(res)
}
curve(umax, c(c(2,1),c(2,1)), c(10,100))
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On Wed, 13 Oct 2010, jcress410 wrote:
> u <- function(x) {
> ? ? x1 <- x[1]
> ? ? x2 <- x[2]
> ?(x1^alpha)*(x2^(1-alpha))
> }
> utility <- function(x) x[1]^(alpha)*x[2]^(1-alpha)
> p <- c(2,1)
> i <- -100
> alpha <- .3
> umax <- function(p,i,u) {
> res <- constrOptim(c(.5,.5), u, grad=NULL, ui=-p, ci=i, mu = 1e-04,
> control=list(fnscale=-1))
> return(res)
> }
> curve(umax, c(c(2,1),c(2,1)), c(10,100))
I don't see any question here.

However you must RTFM.

1) The returned value of umax() is a list.  How do you expect curve() to plot a
list?

2) curve() requires that the function umax() must be vectorisable wrt its first
parameter,
i.e. you pass it a numeric vector and it returns a numeric vector of the same
length.

3) you have to realise that curve() will only graph a 1-dimensional function

HTH,
Ray