Comments - how to

1 How to spefify comments in R code and Quarto MarkDown

One of the core ideas of R and Quarto MarkDown is to make the code human readable – a concept known as literal programming.

R+Quarto code is certainly more readable than classic programming languages, but it is unlikely that you are able to write code that is entirely self-explanatory to a human reader … you will need to add comments for future readers (probably yourself!) to understand the code.

R code

In R code, you can add a comment using the hashtag #, like this:

# This is a single-line comment in R code
# ...and there are no multi-line comments: You must add a # for each line
x <- 2 + 2 # ...but inline comments are okay

Quarto

<!-- This is a single-line comment in markdown -->
<!--
This is a multi-line 
comment in markdown
-->
..and inline <!-- comments are okay too -->

Important! … the hashtag # has different meaning in R code and in quarto code!

As you have just seen, the hashtag # is used to add comments in R code, but in quarto markdown, the hashtag # is used to add level 1 headings, like this:

---
title: "My Quarto Markdown document"
---

# This is a level 1 heading

## This is a level 2 heading

Exercise #1

Look at this, admittedly ugly and pointless Quarto MarkDown document:

---
title: "My ugly Quarto Markdown document"
author: S. O'Neill
---

# E <- MC^2

data <- c(2,4,7,12,4)

<!-- c <- sqrt(a^2 + b^2) -->

# # if (3>2) then print("two too small")

```{r my chunk}
# data <- rsample(1:1000, 100, replace=TRUE)
<!-- data <- NULL ->>

```

# if (pi > 3) then <!-- print("Pi is bigger than 3") -->

Discuss the following points with your colleague, before looking at the solution below:

• Can you identify the comments in the document?
• What would you expect the output to look like?

Exercise #1 Solution

If you typed the MarkDown document above into Rstudio, you might have noticed red squiggly lines and a the warning symbol in the left margin of line 16? .

This should tell you, there is a problem with the R code and indeed the code will not run to produce output – can you tell what the problem is? …now click below for the full answer.

Exercise #1 ..the full answer.

Okay so. Line 16 was not valid R code. The line was actually a single-line markdown comment and it belongs in markdown and not inside an R code chunk.

If we comment out that line with a hashtag # as appropriate for valid R code, we get the following output:

In other words, only lines 10 and 15 were valid comments.

Bonus question: Explain how line 20 worked to produce the output it did.