# 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 these two characters (#>). Again, R output is preceded by the (#>) characters. Actual comments will be preceded by one or more comment (#) characters without >.

These stylistic conventions (#> characters preceding the output lines) is for convenience only. This enables readers 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.