How to write a (first-order) derivative as a function in R?

Question

I need the derivative of p-t*(p^g)*(1-p)^d) as a function of p. t,d and g are all defined. I was trying:

firstder<-D(expression(p-t*(p^g)*(1-p)^d), "p")
firstderivative<-function(p){
firstder
}

However, calling

firstderivative(p=0.1)

gives me nothing more than the expression of the first derivative. Thanks in advance!

Answer 1

firstder is an object of class "call".

class(firstder)
#[1] "call"

You are forgetting to evaluate the call.

firstder <- D(expression(p-t*(p^g)*(1-p)^d), "p")

firstderivative <- function(p){
    eval(firstder)
}

g <- 1
d <- 1
t <- 1
firstderivative(p=0.1)
#[1] 0.2