## 2.4 Reading and writing Code

### 2.4.1 Code Chunks

In this book, R code is (almost) always presented in a separate gray box like this one:

# A code chunk

# Define a vector a as the integers from 1 to 5
a <- 1:5

# Print a
a
## [1] 1 2 3 4 5

# What is the mean of a?
mean(a)
## [1] 3

This is called a code chunk. You should always be able to copy and paste code chunks directly into R. If you copy a chunk and it does not work for you, it is most likely because the code refers to a package, function, or object that I defined in a previous chunk. If so, read back and look for a previous chunk that contains the missing definition.

### 2.4.2 Comments with #

Lines that begin with # are comments. If you evaluate any code that starts with #, R will just ignore that line. In this book, comments will be either be literal comments that I write directly to explain code, or they will be output generated automatically from R. For example, in the code chunk below, you see lines starting with ##. These are the output from the previous line(s) of code. When you run the code yourself, you should see the same output in your console.

# This is a comment I wrote

1 + 2
## [1] 3

# The line above (## [1] 3) is the output from the previous code that has been 'commented out'

### 2.4.3 Element numbers in output [1]

The output you see will often start with one or more number(s) in brackets such as [1]. This is just a visual way of telling you where the numbers occur in the output. For example, in the code below, I will print a long vector containing the multiples of 2 from 0 to 100:

seq(from = 0, to = 100, by = 2)
##  [1]   0   2   4   6   8  10  12  14  16  18  20  22  24  26  28  30  32
## [18]  34  36  38  40  42  44  46  48  50  52  54  56  58  60  62  64  66
## [35]  68  70  72  74  76  78  80  82  84  86  88  90  92  94  96  98 100

As you can see, the first line of the output starts with ## [1], and the next two lines start with [18] and [35]. This is just telling you that 0 is the [1]st element, 34 is the [18]th element, and 68 is the [35]th element. Sometimes this information will be helpful, but most of the time you can just ignore it.