## 1.7 Notation throughout this book

When mentioned in the text, the names of R packages are in bold font, e.g. ggplot2, whereas functions, objects, and variables are printed with mono-spaced font, e.g filter(), mean(), lifeExp. Functions are always followed by brackets: (), whereas data objects or variables are not.

Otherwise, R code lives in the grey areas known as ‘code chunks’. Lines of R output start with a double ## - this will be the numbers or text that R gives us after executing code. R also adds a counter at the beginning of every new line, look at the numbers in the square brackets [] below:

# colon between two numbers creates a sequence
1001:1017
##  [1] 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015
## [16] 1016 1017

Remember, lines of R code that start with # are called comments. We already introduced comments as notes about the R code earlier in this chapter (Section 1.1 “Help, what’s a script?”), however, there is a second use case for comments.

When you make R code a comment, by adding a # in front of it, it gets ‘commented out’. For example, let’s say your R script does two things, prints number from 1 to 4, and then numbers from 1001 to 1004:

# Let's print small numbers:
1:4
## [1] 1 2 3 4
# Now we're printing bigger numbers:
1001:1004
## [1] 1001 1002 1003 1004

If you decide to ‘comment out’ the printing of big numbers, the code will look like this:

# Let's print small numbers:
1:4

# Now we're printing bigger numbers:
# 1001:1004
## [1] 1 2 3 4

You may even want to add another real comment to explain why the latter was commented out:

# Now commented out as not required any more
# Now we're printing bigger numbers:
# 1001:1004

You could of course delete the line altogether, but commenting out is useful as you might want include the lines later by removing the # from the beginning of the line.

Keyboard Shortcut for commenting out/commenting in multiple lines at a time: Control+Shift+C
(On a Mac, both Control or Command work)