I'm attempting to refactor a script by splitting it into multiple functions, having a main function and "help functions". Here I stumbled upon a problem which can be reduced to the following example:
g <- function(a,b){ # help function
a^2 + b^2 - c
}
f <- function(a,b,c,d){ # main function
g(a,b)
}
The problem here is, that f cannot be computed because g does not know what c is, but why is that the case?
I've read here that if R doesn't know a variable/argument in a function, it searches for that missing variable/argument in the outer function/environment.
Why does g still not know the value c which gets declared by f?
R uses lexical scoping which means that if a function needs to reference an object not defined in that function it looks at the environment in which the function was defined, not the caller. In the question g is defined in the global environment so that is where g looks for c.
Also note that in R we would not call the functions in the question nested functions. Rather what is nested is the calls, not the functions. In (3) below we show nested functions.
1) We can reset a function's environment in which case it will think it was defined in that environment.
g <- function(a,b){ # help function
a^2 + b^2 - c
}
f <- function(a,b,c,d){ # main function
environment(g) <- environment()
g(a,b)
}
f(1, 2, 3, 4)
## [1] 2
2) Another possibility is to explicitly tell it which environment to search using envir$c (where envir is the desired environment) or get("c", envir) or with(envir, c) . envir$c will look into envir. The other two will look there and if not found will look into ancestor environments. (Each environment has a parent or the emptyenv(). This is distinct from the call stack.)
g <- function(a, b, envir = parent.frame()){ # help function
a^2 + b^2 - envir$c
}
f <- function(a,b,c,d){ # main function
g(a,b)
}
f(1, 2, 3, 4)
## [1] 2
3) We can nest the functions so that g is defined in f.
f <- function(a,b,c,d){ # main function
g <- function(a,b){ # help function
a^2 + b^2 - c
}
g(a,b)
}
f(1, 2, 3, 4)
## [1] 2
4) Of course you could just pass c and avoid all these problems.
g <- function(a, b, c) {
a^2 + b^2 - c
}
f <- function(a, b, c, d) { # main function
g(a, b, c)
}
f(1, 2, 3, 4)
## [1] 2