R is full of functions, it is after all a functional programming language, but sometimes the precise function you need isn't provided in the Base resources. You could conceivably install a package containing the function, but maybe your requirements are just so specific that no pre-made function fits the bill? Then you're left with the option of making your own.

A function can be very simple, to the point of being being pretty much pointless. It doesn't even need to take an argument:

one <- function() { 1 }
one()
[1] 1

two <- function() { 1 + 1 }
two()
[1] 2

What's between the curly braces { } is the function proper. As long as you can fit everything on a single line they aren't strictly needed, but can be useful to keep things organized.

A function can be very simple, yet highly specific. This function takes as input a vector (vec in this example) and outputs the same vector with the vector's length (6 in this case) subtracted from each of the vector's elements.

vec <- 4:9
subtract.length <- function(x) { x - length(x) }
subtract.length(vec)
[1] -2 -1  0  1  2  3

Notice that length() is in itself a pre-supplied (i.e. Base) function. You can of course use a previously self-made function within another self-made function, as well as assign variables and perform other operations while spanning several lines:

vec2 <- (4:7)/2

msdf <- function(x, multiplier=4) {
    mult <- x * multiplier
    subl <- subtract.length(x)
    data.frame(mult, subl)
}

msdf(vec2, 5)
  mult subl
1 10.0 -2.0
2 12.5 -1.5
3 15.0 -1.0
4 17.5 -0.5

multiplier=4 makes sure that 4 is the default value of the argument multiplier, if no value is given when calling the function 4 is what will be used.

The above are all examples of named functions, so called simply because they have been given names (one, two, subtract.length etc.)

An anonymous function is, as the name implies, not assigned a name. This can be useful when the function is a part of a larger operation, but in itself does not take much place. One frequent use-case for anonymous functions is within the *apply family of Base functions.

Calculate the root mean square for each column in a data.frame:

df <- data.frame(first=5:9, second=(0:4)^2, third=-1:3)

apply(df, 2, function(x) { sqrt(sum(x^2)) })
    first    second     third 
15.968719 18.814888  3.872983 

Create a sequence of step-length one from the smallest to the largest value for each row in a matrix.

x <- sample(1:6, 12, replace=TRUE)
mat <- matrix(x, nrow=3)

apply(mat, 1, function(x) { seq(min(x), max(x)) })

An anonymous function can also stand on its own:

(function() { 1 })()
[1] 1

is equivalent to

f <- function() { 1 })
f()
[1] 1

This is just a small hack for those who use self-defined functions often.
Type "fun" RStudio IDE and hit TAB.

The result will be a skeleton of a new function.

name <- function(variables) {
        
}

---

One can easily define their own snippet template, i.e. like the one below

name <- function(df, x, y) {
        require(tidyverse)
        out <- 
        return(out)
}

The option is Edit Snippets in the Global Options -> Code menu.

Sometimes one would like to pass names of columns from a data frame to a function. They may be provided as strings and used in a function using [[. Let's take a look at the following example, which prints to R console basic stats of selected variables:

basic.stats <- function(dset, vars){
    for(i in 1:length(vars)){
        print(vars[i])
        print(summary(dset[[vars[i]]]))
    }
}

basic.stats(iris, c("Sepal.Length", "Petal.Width"))

As a result of running above given code, names of selected variables and their basic summary statistics (minima, first quantiles, medians, means, third quantiles and maxima) are printed in R console. The code dset[[vars[i]]] selects i-th element from the argument vars and selects a corresponding column in declared input data set dset. For example, declaring iris[["Sepal.Length"]] alone would print the Sepal.Length column from the iris data set as a vector.