How can I customize an object of the class "expression"? I need to write a function for which the parameters change

Question

I am trying to write an expression for which I need to find the parameters, but once I define the parameters to come from another variable, the expression does not recognize them. For example:

This works fine:

expression(2*x*exp(-3*t))

I get:

expression(2 * x * exp(-3 * t))

But the issue is that I don't know if 2 and 3 are the right values (I'm trying to find them). So I tried to put this into a function like this:

 exp.fx <- function(params){
   u         <- params[1]
   D         <- params[2]
   expr1     <- expression(u*x*exp(-D*t))
   
   return(expr1)
}

And this is what I get:

> exp.fx(c(2,3))
u * x * exp(-D * t)

I need to get instead

2 * x * exp(-3 * t)

Bottom line, I need to put these two parameters into an optim so I can try to find them and that's why I need a function that changes the expression each time accordingly.

Answer 1

What you are looking for is interpolation or injection. There are lots of different ways of achieving it. My favourite way is to use bquote instead of quote/expression:

exp_fx <- function (params) {
    u <- params[1L]
    D <- params[2L]
    bquote(.(u) * x * exp(-.(D) * t))
}

(Note that there’s no need for the expr variable, or for the return() function call; I’ve also replaced the . in the function name by _ since . can lead to confusion with S3 methods, and is therefore potentially problematic.)

An alternative is to use substitute:

exp_fx <- function (params) {
    substitute(
        u * x * exp(-D * t),
        list(u = params[1L], D = params[2L])
    )
}