Chapter 6 Functions

6.1 Introduction

Most of the time with R, you will need to use functions to do what you want.

Functions are tools that are there to simplify your life. They allow you to quickly execute operations on objects without having to write every mathematical step. Functions are usually pre-existing R code that are executed when used. They remove the need to create this code and to rewrite it every time you need it.

To execute a function, you will need to call it. A function call is essentially a shortcut to the code of the function.

To perform the function call you will need entry values called arguments (or sometimes parameters). After performing its operations, the function will then give you a return value. The command also must be structured properly, following the “grammar rules” of the R language (syntax).

A function call is structured as follow: the name of the function, followed by parenthesis ( ). Inside the parenthesis, you insert all your arguments separated with commas.

function_name(arg1, arg2, ...)

Here is an example:

sum(1, 2)

Arguments are values and the instructions the function needs to run. Objects can be passed into functions:

a <- 3
b <- 5
sum(a, b)

## [1] 8

On the last line, the output that appears is the return value of the function. In this case, it is the sum of a and b, 8.

6.1.1 CHALLENGE 11

Create a vector a that contains all numbers from 1 to 5
Create an object b with value of 2
Add a and b together using the basic + operator and save the result in an object called result_add
Add a and b together using the sum function and save the result in an object called result_sum
Compare result_add and result_sum. Are they different?
Add 5 to result_sum using the sum function.

Solution

Create a vector a that contains all numbers from 1 to 5

a <- 1:5

Create an object b with value of 2

b <- 2

Add a and b together using the basic + operator and save the result in an object called result_add

result_add <- a + b

Add a and b together using the sum function and save the result in an object called result_sum

result_sum <- sum(a, b)

Compare result_add and result_sum. Are they different?

result_add

## [1] 3 4 5 6 7

The operation on the vector adds 2 to each element. The result is a vector.

result_sum

## [1] 17

The function sum() adds all values of a and b. It is the same as doing 1 + 2 + 3 + 4 + 5 + 2. The result is a number.

Add 5 to result_sum using the sum function.

sum(result_sum, 5)

## [1] 22

6.2 Arguments

Arguments each have a name that can be provided during a function call.

To provide an argument name during a function call, just enter it in the form name=value.

log(8, base = 2)

If the name of the argument is not provided, the order of the arguments does matter.

log(8, 2)

## [1] 3

log(2, 8)

## [1] 0.3333333

If the name argument is provided, the order of the arguments does not matter.

log(x = 8, base = 2)

## [1] 3

log(base = 2, x = 8)

## [1] 3

6.2.1 CHALLENGE 12

plot(x, y) is a function that draws a graph of y as a function of x. It requires two arguments named x and y.
What are the differences between the following lines?

a <- 1:100
b <- a^2
plot(a, b)
plot(b, a)
plot(x = a, y = b)
plot(y = b, x = a)

Solution

plot(a, b)

Plots the graph of b as a function of a.

plot(b, a)

Plots the graph of a as a function of b.The argument names are not provided, the order of the arguments matters!

plot(x = a, y = b)

Plots the graph of b as a function of a, same as plot(a, b).

plot(y = b, x = a)

Plots the graph of b as a function of a. The argument names are provided, the order of the arguments does not matter!

As a reference, here is a list of some of the most common R functions:

sqrt, log, exp, max, min, sum, mean, sd, var, summary, plot, par, paste, format,
head, length, str, names, typeof, class, attributes, library, ls, rm, setwd, getwd, 
file.choose, c, seq, rep, tapply, lapply, aggregate, merge, cbind, rbind, unique,
help (or ?), help.search (or ??), help.start