Stylistic Conventions

R code that is evaluated

When R expressions are typed at the R console prompt > and evaluated they appear like this:

> x <- matrix(1:6, nrow = 2, ncol = 3,
+             byrow = TRUE) # This is a comment
> #### This is also a comment
> x

     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6

The > symbol is the command prompt. Notice that when R evaluates an expression that continues onto the next line(s), a + sign appears to the left. This means that this line is a continuation of the previous line, and the R expression is not complete. The output appears without the prompt symbol.

In this book however, the above expressions will appear like this:

x <- matrix(1:6, nrow = 2, ncol = 3,
            byrow = TRUE) # This is a comment
#### This is also a comment
x

##      [,1] [,2] [,3]
## [1,]    1    2    3
## [2,]    4    5    6

The R prompt (>) and continuation (+) symbols have been removed, and output is preceded by double comment (##) symbols. Again, R output is preceded by double comment (##) symbols. Actual comments will be preceded by one, three, or more comment symbols.

These stylistic conventions (double comment symbols preceding the output lines) is for convenience only. Removing the prompt (>) and continuation (+) symbols, and preceding output with comment symbols allows a reader to copy and test code from PDF or ebook versions of this book. Removing extraneous symbols also improves readability of R expressions.

R code in a script file (not evaluated)

The same R code above can also be saved as a R script for later evaluation. A R script is a collection of R expressions and saved as an ASCII text file with a .R extension. The R script can be edited using a text editor or RStudio. R script snippets are displayed without evaluation like this:

x <- matrix(1:6, nrow = 2, ncol = 3,
            byrow = TRUE) # This is a comment
#### This is also a comment
x

If the R script is evaluated it will appears as in above examples.