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lecture-03-functions-part-1.md

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Functions - Part 1

Functions in R are created using the function() directive and are stored as R objects just like anything else. In particular, they are R objects of class "function".

f <- function(<arguments>) {
	## Do something here
}

Functions in R are "first class objects", which means that they can be treated much like any other R object. Importantly,

  • Function can be passed as arguments to other function
  • Functions can be nested, so that you can define a function inside of another function. the return value of a function is the last express in the function body to be evaluated.

Function Arguments

Functions have named arguments which potentially have default values.

  • The formal arguments are the arguments included in the function definition
  • The formals function return a list of all the formal arguments of a function
  • Not every function call in R makes use of all the formal arguments
  • Function arguments can be missing or might have default values

Argument Matching

R function arguments can be matched positionally or by name. So the following calls to sd are all equivalent.

> mydata < -rnorm(100)
> sd(mydata)
> sd(x = mydata)
> sd(x = mydata, na.rm = FALSE)
> sd(na.rm = FALSE, x = mydata)
> sd(na.rm = FALSE, mydata)

Even though it's legal, I don't recommend messing around with the order of the arguments too much since it can lead to some confusion.

You can mix positional matching with matching by name. When an argument is matched by name, it is "taken out" of the argument list and the remaining unnamed arguments are matched in the order that they are listed in the function definition.

> args(lm)
function(formula, data, subset, weights, na.action,
	method = 'qr', model = TRUE, x = FALSE,
	y = FALSE, qr = TRUE, x = FALSE,
	contrasts = NULL, offset, …)

The following two calls are equivalent.

lm(data = mydata, y - x, model = FALSE, 1:100)
lm(y - x, mydata, 1:100, model = FALSE)
  • Most of the time, names arguments are useful on the command line when you have a long argument list and you want to use the defaults for everything except for an argument near the end of the list
  • Named arguments also help if you cannot remember the name of the argument and its position on the argument list (plotting is a good example)

Function arguments can also be potentially matched, which is useful for interactive work. the order of operations when given an argument is

  1. Check for an exact match for a named argument
  2. Check for a potential match (type part of the name)
  3. Check for a positional match