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] 3This 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.
2.4.3 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 100As 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.
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.