Question

I would like to write a function that takes any user-provided mathematical function (e.g., x^2) and do different things with it, for example:

#-----------------nonworking code---------------------
foo <- function(FUN, var){
      math_fun <- function(x){
           FUN
      }
   curve(math_fun, -5, 5)     #plot the mathematical function
   y = math_func(var)         #compute the function based on a user provided x value.
   points(x=var, y=y)         #plot the value from the last step.
}

#A user can use the function defined above in a way as shown below:
Function <- x^2 + x
foo(FUN=Function, var = 2)

But obviously this function doesn’t work:
First of all, if I run this function, I get Error in math_fun(x) : object 'x' not found.

Second of all, even if the function did work, I am assuming that the variable is x, but the user can make use of any letter.

For this second problem, one potential solution is to also ask the user to specify the letter they use as the variable.

foo <- function(FUN, var, variable){
      math_fun <- function(variable){
           FUN
      }
   curve(math_fun, -5, 5) 
   y = math_func(var)     
   points(x=var, y=y)     
}

But I am at loss as to how exactly I can implement this… If someone can help me solve at least part of the problem, that would be great. Thanks!

Answer 1

It is a lot simpler than that. The user defined function should contain the arguments in its definition, e.g. function(x) x^2 + x instead of x^2 + x. Then it can be passed and called directly:

foo <- function(math_fun, var){

   curve(math_fun, -5, 5)  #plot the mathematical function
   y = math_fun(var)       #compute the function based on a user provided x value
   points(x=var, y=y)      #plot the value from the last step.
}

#A user can use the function defined above in a way as shown below:
Function <- function(x) x^2 + x
foo(Function, var = 2)