CODEWITHSUNDEEP
rfunctionglobal-variables
Theoden
Theoden

Reputation: 297

What is the rationale behind this type of data input?

The example is from the rootsolve package:

We have this function:

gradient(f, x, centered = FALSE, pert = 1e-8, ...)

in which f is a function and x is the data input in for of a vector

Now the following is an instance of the code being run:

logistic <- function (x, times) {
with (as.list(x),
{
N <- K / (1+(K-N0)/N0*exp(-r*times))
return(c(N = N))
})
}
# parameters for the US population from 1900
x <- c(N0 = 76.1, r = 0.02, K = 500)
# Sensitivity function: SF: dfi/dxj at
# output intervals from 1900 to 1950
SF <- gradient(f = logistic, x, times = 0:50)

My question is how does the code understand to use times in its routine. It's not defined globally and it is not part of the function input list either. Is it possible to pass inputs to a function when it is not defined in its structure? Does ... play a role here?

Upvotes: 0

Views: 55

Answers (1)

rbm
rbm

Reputation: 3253

... is just a way of getting an extra arguments and passing them on to another function.

Simple example:

power.function <- function(x,power) { x^power }
apply.function <- function(f, data, ...) { f(data, ...) }
sample <- c(1,2,3)
apply.function (power.function, sample, power = 3)

# which is the same as

apply.function (power.function, sample, 3)

produces

> apply.function (power.function, sample, 3)
[1]  1  8 27

EDIT

To make it crystal clear, if you look at the source of the rootSolve::gradient you'll see the definition as

function (f, x, centered = FALSE, pert = 1e-08, ...)

and further down the call to

reff <- f(x, ...)

which is the same as described above in the example.

Upvotes: 3

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