12.1 Presentations from R Markdown

1. Why knit slides from code

A slide deck that is also a source file has one job: the numbers and plots on the slide come from the same code you would put in a homework file. Knit (or Render) again after a data fix and the deck updates. You are not pasting a screenshot of a plot you can no longer reproduce.

The file is still Markdown plus chunks. The output is slides instead of a scrolling report. Note 1.5 is the document system; this note is the slide formats.

2. Formats

R Markdown presentation outputs you will see:

Output What you get

ioslides_presentation HTML slides (the usual starting point)

slidy_presentation HTML slides, W3C Slidy

beamer_presentation PDF slides. Needs LaTeX

powerpoint_presentation A .pptx file

xaringan::moon_reader HTML slides (remark.js). Needs the xaringan package

Quarto’s HTML slide format is format: revealjs. Lab 12 accepts that as well.

Pick one HTML format and stay with it for the lab. Beamer is for when you already have LaTeX and want a PDF. PowerPoint is for a file someone else will click through in PowerPoint.

3. YAML and slide breaks

R Markdown uses output:. A minimal ioslides header:

The title slide is built from title and author. Every later slide starts with ##.

A slide title

A sentence.

mean(mtcars$mpg)

## [1] 20.09062

# is a section (ioslides can turn it into a divider). The slides you write are ##.

xaringan is the exception: slides are separated by a line with only ---, not by ##. If you pick xaringan, follow a xaringan template. If you pick ioslides or slidy, use ##.

Quarto reveal uses format: revealjs and the same ## slide titles.

4. What to show

Do not show package-load code. Put library() in a setup chunk with echo = FALSE (R Markdown) or #| echo: false / #| include: false (Quarto). The reader came for the table and the plot.

A chunk that computes a table can show the code (echo = TRUE). A chunk that draws the plot the audience should look at often hides the code (echo = FALSE). Both chunks still run.

HTML widgets (an interactive plotly chart, a DT table) need an HTML format: ioslides, slidy, xaringan, or Quarto reveal. They will not appear in Beamer or PowerPoint.

PowerPoint is one-way. Knit writes a .pptx. Edits you then make in PowerPoint do not come back into the .Rmd. If you need to change a number, change the source and knit again.

5. One idea per slide

A slide is not a report page. One claim, one table, or one plot. If you need a second plot, it is a second slide.

A few habits that keep a short deck readable:

• title the slide with the claim, not the function name
• show the code only when the code is the point
• give the plot room; drop theme_bw() legends you do not need
• end with a sentence the audience can repeat

Starter .Rmd files are in the Data / templates list on the course page: ioslides, slidy, Beamer, PowerPoint, xaringan, and Shiny. Download one and knit it. Do not vendor a 10 MB knitted deck.

xaringan uses --- between slides, not ##. RStudio: File → New File → R Markdown → From Template → Ninja Presentation. {xaringanExtra} adds tile view (press o). {xaringanthemer} sets colors. {plotly} and {DiagrammeR} are HTML widgets: they need an HTML format.

A Shiny presentation is not a static file. The viewer needs R running. Use it for a live demo, not for a PDF you email.

{revealjs} and {rmdshower} are other HTML decks. Quarto revealjs is the current default if you are starting fresh.

6. Practice

Outline a 3-slide deck (do not spend the hour polishing fonts):

1. Title slide (YAML title / author).
2. A slide with a small table (a summarize() of a built-in table, or mtcars / penguins).
3. A slide with a plot and no code (echo = FALSE).