### Introduction to Formal Arguments in R

Formal arguments in R are essentially variables you define within a function’s code block. These arguments act as placeholders for the data that are provided when a user uses the function. Formal are the formal arguments of function returned as an object of class `pairlist`

which can be thought of as something similar to a list with an important difference:

is.null(pairlist()) is.null(list())

That is a pairlist of length zero is NULL while a list is not.

### Specifygin Formal Arguments Positions

Formal arguments in R can be specified by position or by name and we can mix positional matching with matching by name. The following are equivalent.

mean(x = 1:5, trim = 0.1) mean(1:5, trim = 0.1) mean(x = 1:5, 0.1) mean(1:5, 0.1) mean(trim = 0.1, x = 1:5)

### Functions Formals Default Values

Functions formals may also have the construct `symbol=default`

, which unless differently specified, forces any argument to be used with its default value. Specifically, functions `mean()`

also have a third argument `na.rm`

that defaults to `FALSE`

and as a result, passing vectors with `NA`

values to `mean()`

returns `NA`

.

mean(c(1, 2, NA))

while by specifying na.rm=TRUE we get the mean of all non-missing elements of vector x.

mean(c(1, 2, NA), na.rm = TRUE)

we can redefine `mean()`

function that defaults `na.rm`

to `TRUE`

by simply

mean(c(1, 2, NA))

Now we have a copy of `mean.default()`

in our `globalenv`

:

exists("mean.default", envir = globaenv())

also, notice

environment(mean.default)

### The … Argument in a Function

The `â€¦`

argument of a function is special and can contain any number of `symbol=value`

arguments. The `â€¦`

argument is transformed by R into a list that is simply added to the formal list:

h<-function(x, â€¦){ Â Â Â Â Â Â Â Â Â Â Â 0 } formals(h)

The `â€¦`

argument can be used if the number of arguments is unknown. Suppose one wants to define a function that counts the number of rows of any given number of data frames. One can write:

count_rows<-function(â€¦){ list<-list(â€¦) lapply(list, nrow) } count_rows(airquality, cars)

By effectively using formal arguments in R Language, one can create reusable and adaptable functions that make the R code more concise and efficient.