using the value of a variable from one function within a second function in R

Question

This is probably a very simple answer but can't seem to find a solution. I have a function which gives a set of parameters:

theta <-
function(
e = 0.2,l= 0.01,p= 0.05)
return(c(e=e,l=l,p=p))

So I can return from it a set of parameters, whilst changing one or more of them, e.g. by using

theta(e=0.1) #or
theta(l=0.1)

My problem is I want to call this function inside another function, where an input for that function is one of the variables.

So for example a function such as:

randFunc<-function(parameter,value){
s<-theta(parameter=value)
return(s)
}

Then use

randFunc("e",0.1) #or
randFunc("l",0.3)

However i'll get the error "Error in theta(parameter = value) : unused argument (parameter = value)"

I've tried a few things but can't seem to get the parameter "value" to be used within the theta function.

Answer 1

Another way is to use do.call:

randFunc <- function(parameter, value){
    L = list(value)
    names(L) <- parameter
    do.call(theta, L)
}

> randFunc('e', 0.1)
   e    l    p 
0.10 0.01 0.05 
> randFunc('l', 0.3)
   e    l    p 
0.20 0.30 0.05 

Answer 2

You need to use a string in the call to randFunc, because the parameter you put in does not exist. Then, within the function you can use eval(parse(text = "something")) to use it as a non-string input for the theta function.

randFunc<-function(parameter,value){
  s<-theta(eval(parse(text = parameter)) = value)
  return(s)
}

and then call it with

randFunc("e", 0.1)

---

@Cath provided a solution without the use of eval(parse()):

Change your randFunc to:

randFunc<-function(parameter,value){
  s <- theta()
  s[parameter] <- value
  return(s)
}

This is pretty elegant and will definitely find its way into future functions of my own (or even into current functions when it is time for revision).