R - Change return values of existing function

Question

I am using the approxfun() function to get linear interpolation. I want to write a function which takes the results of approxfun() then shifts and scales it by an amount that I specify. I need to be able to call this new function just like I would call any other function.

Simplified version of my attempt:

set.seed(42)
x = rnorm(50)
y = rnorm(50, 5, 2)
fhat = approxfun(x, y, rule = 2)

new_function = function(fhat, a, b){

  new_fhat <- (function(fhat, a, b) a * fhat() + b)()

  return(new_fhat)

}

I expect the results to be the same as

2 * fhat(1) + 3

but instead when I run my function

new_function(fhat, a = 2, b = 3)

I get an error message:

*Error in (function(fhat, a, b) a * fhat() + b)() : argument "a" is missing, with no default*

James · Accepted Answer · 2017-12-05 19:48:43Z

2

You have four problems:

new_fhat isn't passed the values of a and b from the new_function call, and can't see them because you are creating new ones in your function definition. This is actually a bit of a red herring because...
The function you return should only have a single argument - the point at which you want to evaluate it.
You are attempting to evaluate new_fhat immediately.
You are trying to call fhat without an argument.

The solution is:

new_function = function(fhat, a, b){

  new_fhat <- function(v) a * fhat(v) + b

  return(new_fhat)

}

Results:

fhat(1)
[1] 5.31933
new_function(fhat,a=2,b=3)(1)
[1] 13.63866
2 * fhat(1) + 3
[1] 13.63866

answered Dec 5, 2017 at 19:48

James

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Comments

janos · Accepted Answer · 2017-12-05 19:45:34Z

1

This is equivalent to the code in the question, but simplified:

new_function = function(fhat, a, b) {
  a * fhat() + b
}

But this is not correct, because in the posted code fhat requires an argument. For example, to make new_function(fhat, a = 2, b = 3) return the same results as 2 * fhat(1) + 3, you would have to add the parameter 1 inside the function:

new_function = function(fhat, a, b) {
  a * fhat(1) + b
}

answered Dec 5, 2017 at 19:45

janos

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