Comments

Commenting is a way to describe the contents of an R script. Commenting is very important for reproducibility because it helps make sense of code to others and to a future you.

Hash comments

The scripts used in these materials include comments. Any text that follows a has symbol (#) becomes an R comment. Anything can be in a comment. It is ignored by R. You can comment an entire line or just the end of a line, like:

2 + 2  # This is a comment. The code before the `#` is still evaluated by R.

## [1] 4

Some languages provide mutli-line comments. R doesn't have these. Every line has to be commented individually. Most script editors provide the ability to comment out multiple lines at once. This can be helpful if you change your mind about some code:

a <- 1:10

b <- 1:10

b <- 10:1

In the above example, we comment out the line we don't want to run.

Ignoring Code blocks

If there are large blocks of valid R code that you decide you don't want to run, you can wrap them in an if statement:

a <- 10
b <- 10
c <- 10
if (FALSE) {
    a <- 1
    b <- 2
    c <- 3
}
a

## [1] 10

b

## [1] 10

c

## [1] 10

The lines inside the if(FALSE){...} block are not run. If you decide you want to run them after all, you can just change FALSE to TRUE.

R comment function

R also provides a quite useful function called comment that stores a hidden description of an object. This can be useful in interactive sessions for keeping track of a large number of objects. It also has other uses in modelling and plotting that are discussed elsewhere. To add a comment to an object, we simply assign something to the object and then assign it a comment:

d <- 1:10
d

##  [1]  1  2  3  4  5  6  7  8  9 10

comment(d) <- "This is my first vector"
d

##  [1]  1  2  3  4  5  6  7  8  9 10

Adding a comment is similar to adding a names attribute to an object, but the comment is not printed when we call d. To see a comment for an object, we need to use the comment function again:

comment(d)

## [1] "This is my first vector"

If an object has no comment, we receive a NULL result:

e <- 1:5
comment(e)

## NULL

Note: Comments must be valid character vectors. It is not possible to store a numeric value as a comment, but one can have multiple comments:

comment(e) <- c("hi", "bye")
comment(e)

## [1] "hi"  "bye"

And this means that they can be indexed:

comment(e)[2]

## [1] "bye"

And that we can add additional comments:

comment(e)[length(comment(e)) + 1] <- "hello again"
comment(e)

## [1] "hi"          "bye"         "hello again"

Because comments are not printed by default, it is easy to forget about them. But they can be quite useful.