Using global variables as input to R function

Question

There are plenty of examples about updating global variables from the inside of R functions by using <<- or assign() (here is just one example), but I can't find any examples on how to use such variables as input for functions. For instance, consider this MATLAB function, which is just a complicated way of adding pi to a number:

function final = parentFun(why)
    global pie
    pie = pi;
    final = childFun(why);
end

function y = childFun(x)
    global pie
    y = x + pie;
end

So that

>> parentFun(100)

ans =

  103.1416

I would like to recreate this in R. The closest I got was by nesting childFun into parentFun like this:

parentFun <- function(why) {
    pie <- pi
    childFun <- function(x) {
        x + pie
    }
    childFun(why)
}

And indeed

> parentFun(100)
[1] 103.1416

The problem is that in my real case parentFun is a couple hundred lines long and it contains several children which can be just as long. In other words, nesting is basically off the table.

Also, these are functions in a package, so pie is in fact acquired from within a function and isn't started by a script.

Of course, separating both functions doesn't work because childFun() can't see pie:

> parentFun <- function(why) {
        pie <- pi
        childFun(why)
}
> childFun <- function(x) {
        x + pie
}
> parentFun(100)
Error in x + pie : non-numeric argument to binary operator

Answer 1

One possibility would be to change the environment of childfun

childFun <- function(x) {
        x + pie
}

parentFun <- function(why) {
        pie <- pi
        environment(childFun) <- environment()
        childFun(why)
}

parentFun(100)
[1] 103.1416

This has to do with lexical scoping of R. When you call a function R looks for the variables inside the function environment first. If it is not sucessful R continues the search in parent environments (in your case the global environment).

By writing environment(childFun) <- environment() you tell the function to look in the environment of parentFun for variables.

I recommend this book if you want to learn more about environments.