Background

Writing documents in R Markdown is relatively straightforward once you know some of the basics. You can create many different formats, including .pdf, .html, and .doc, and customize them in a variety of ways.

Templates

There are a number of existing templates available in the rticles package that you can use for knitting your document to a specific style in .pdf format. For .html formats, there are a variety of “themes” available as well. For example, this document was created with the readable theme.

Equations

You can write nicely formatted equations in R Markdown using LaTeX language constructs. Although the initial leap into LaTeX can be a bit daunting, it gets much easier with some practice and help from the many guides available online (eg, math and document).

Here are a few things to help you get started (with examples below):

The default typeface is italic, which is commonly used to indicate a scalar; vectors and matrices are typically denoted with bold face, but you’ll need special coding to indicate them as such;
Sub- and superscripts are denoted by _ and ^, respectively; any text/number longer than one character/digit must be enclosed in curly braces {}

Inline

You can include equations inline by enabling the “inline math mode” through the use of a single dollar sign ($) at the beginning and end of the equation. For example,

The errors are normally distributed, such that $e_{i,j} \sim \text{N}(0, \sigma^2)$.

will render as

“The errors are normally distributed, such that $e_{i,j} \sim \text{N}(0, \sigma^2)$.”

Stand alone

In many cases, you may want to have your equations set apart from the text on lines by themselves. To do this in Markdown, begin and end the equation block with 2 dollar signs ($$). If you want equations on multiple lines, use \\ to indicate a line break. For example,

$$
y_{i,j} = \alpha + \beta x_{i,j} + e_{i,j} \\
e_{i,j} \sim \text{N}(0, \sigma^2)
$$

will render to

\[ y_{i,j} = \alpha + \beta x_{i,j} + e_{i,j} \\ e_{i,j} \sim \text{N}(0, \sigma^2) \]

and

$$
\mathbf{x}_t = \mathbf{B} \mathbf{x}_{t-1} + \mathbf{C} \mathbf{c}_t + \mathbf{w}_t \\
\mathbf{y}_t = \mathbf{Z} \mathbf{x}_t + \mathbf{a} + \mathbf{v}_t
$$

would give

\[ \mathbf{x}_t = \mathbf{B} \mathbf{x}_{t-1} + \mathbf{C} \mathbf{c}_t + \mathbf{w}_t \\ \mathbf{y}_t = \mathbf{Z} \mathbf{x}_t + \mathbf{a} + \mathbf{v}_t \]

You can also use some additional LaTeX options inside the equation block for additional formatting options. For example, the aligned command can be used with an ampersand & to align multiple equations, such that

\begin{aligned}
  y &= 2x + x^2 + 3 \\
    &= x(2 + x) + 3
\end{aligned}

yields

\[ \begin{aligned} y &= 2x + x^2 + 3 \\ &= x(2 + x) + 3 \end{aligned} \]

Multiline equations in pdf

It turns out that pandoc won’t properly render multiline equations in .pdf format when demarking them with double dollar signs ($$). Instead, you’ll need to use the LaTeX commands \begin{equation} and \end{equation} to set them apart. For example,

\begin{equation}
y_{i,j} = \alpha + \beta x_{i,j} + e_{i,j} \\
e_{i,j} \sim \text{N}(0, \sigma^2)
\begin{equation}

will render to

\[\begin{equation} y_{i,j} = \alpha + \beta x_{i,j} + e_{i,j} \\ e_{i,j} \sim \text{N}(0, \sigma^2) \end{equation}\]

It turns out that this format works with .html formats as well, so you may just want to get in the habit of using this form.

Tables

There are several ways to create tables in R Markdown documents, but they vary in format based on your chosen output format. For .html documents, you can create simple tables with pipes (|) and colons (:). For example,

| Tables        | Are           | Cool  |
| ------------- |:-------------:| -----:|
| col 3 is      | right-aligned | $1600 |
| col 2 is      | centered      |   $12 |
| col 1 is      | left-aligned  |    $1 |

will result in

Tables	Are	Cool
col 3 is	right-aligned	$1600
col 2 is	centered	$12
col 1 is	left-aligned	$1

Note that the above code has been formatted for easier reading, but you wouldn’t have to do that. For example,

| Tables | Are | Cool |
| --- |:---:| ---:|
| col 3 is | right-aligned | $1600 |
| col 2 is | centered | $12 |
| col 1 is | left-aligned | $1 |

will give the same result

Tables	Are	Cool
col 3 is	right-aligned	$1600
col 2 is	centered	$12
col 1 is	left-aligned	$1

kableExtra

You can create much fancier tables with the kableExtra package (click here for the vignette with lots of examples). For example, the following code will create a table with striped rows, custom column widths, and a scroll box.

## load package
library(kableExtra)

## create exanple data frame
tbl_txt <- palmerpenguins::penguins[1:20, c(1, 2, 8, 7, 6)]
colnames(tbl_txt) <- c("Species", "Island", "Year", "Sex", "Body mass")

## generate table
kable(tbl_txt, format = "html",
      caption = "Table 1. An example of a `kableExtra` table.",
      align = "ccccr", escape = FALSE) %>%
  kable_styling(bootstrap_options = "striped",
                full_width = FALSE,
                position = "left") %>%
  column_spec(c(1,3,4,5), width = "6em") %>%
  column_spec(2, width = "12em") %>%
  scroll_box(height = "300px", extra_css = "border-style: none;")

Table 1. An example of a `kableExtra` table.
Species	Island	Year	Sex	Body mass
Adelie	Torgersen	2007	male	3750
Adelie	Torgersen	2007	female	3800
Adelie	Torgersen	2007	female	3250
Adelie	Torgersen	2007	NA	NA
Adelie	Torgersen	2007	female	3450
Adelie	Torgersen	2007	male	3650
Adelie	Torgersen	2007	female	3625
Adelie	Torgersen	2007	male	4675
Adelie	Torgersen	2007	NA	3475
Adelie	Torgersen	2007	NA	4250
Adelie	Torgersen	2007	NA	3300
Adelie	Torgersen	2007	NA	3700
Adelie	Torgersen	2007	female	3200
Adelie	Torgersen	2007	male	3800
Adelie	Torgersen	2007	male	4400
Adelie	Torgersen	2007	female	3700
Adelie	Torgersen	2007	female	3450
Adelie	Torgersen	2007	male	4500
Adelie	Torgersen	2007	female	3325
Adelie	Torgersen	2007	male	4200

Figures

Another of Markdown’s strengths is its ability to render high-quality figures. This can be done by either inserting links to saved figures/images as shown above, or executing R code to do so. For example, this code

```{r xy_plot}
set.seeed(123)
n <- 100
x <- runif(nn, 0, 10)
e <- rnorm(nn)
y <- 1 + 0.5 * x + e
plot(x, y)
```

would produce the following plot

You can also add chunk options to change the figure’s size, location, etc. For example,

```{r xy_plot, fig.height = 4, fig.width = 4, fig.align = "center"}
set.seed(497)
n <- 100
x <- runif(n, 0, 10)
e <- rnorm(n)
y <- 1 + 0.5 * x + e
plot(x, y)
```

would instead yield

References

R Markdown is set up to use BibTeX-style references and format the citations accordingly. To do so, you’ll need to have your references in a BibTeX library with a .bib extension.

Creating a `.bib` file

One option is to manually create reference entries, but there are several methods for exporting references from reference management software, such as EndNote or Zotero.

Export from Zotero

To export your citations from Zotero, do the following

Select the library you wish to export
From the menu, choose File > Export Library
On the next screen, choose BibTeX from the dropdown menu & click OK
Navigate to the directory where you want to save your file & save it with a .bib suffix

Export from EndNote

To export a reference library from EndNote, do the following

From the main menu, select Edit > Output Styles > Open Style Manager...

Enter “bibtex” into the search bar at the top of the pop-up window and check the box next to BibTeX Export
Close the window when you’re finished

From the main menu, select Edit > Output Styles and check the BibTeX Export option

From the main menu, select File > Export...
Make sure to give your filename a .bib suffix and verify that the options Text only and BibTeX Export are selected
Click Save when you’re finished

Export from Mendeley

To export a reference library from Mendeley, do the following

Go to the Mendeley Preferences menu
Click on the BibTeX tab
Check the box next to Enable BibTeX syncing
Click the radio button next to Create one BibTeX file per group
Change the Path to your desired location for the library files
Click OK when you are finished

Citations

YAML

The first step in getting properly formatted citations and references in R Markdown documents is to specify two pieces of information in the documents YAML:

the name of of the .bib file containing the references
the name of the style file that specifies how the references will be formatted in the printed docuement.

So, for example, the information for this document is

bibliography: "references.bib"
csl: "ecology.csl"

You can download just about any journal’s style file here.

In-text

There are two options for citations in R Markdown documents, both based upon the standard Author Year format. BibTeX entries have a “citekey” that begins with @ by which they can be referenced. So, for example, if we had this reference in our .bib library

@Article{Smith_2021,
  year = {2021},
  publisher = {The Ecological Society of America},
  volume = {123},
  number = {2},
  pages = {1-10},
  author = {Sarah Smith and Joe Johnson},
  title = {Probably the best paper ever written},
  journal = {Ecology},
}

we could use [@Smith_2021] to format the in-text reference as “(Smith and Johnson 2021),” or we could use @Smith_2021 to format the in-text reference as “Smith and Johnson (2021).”

`knitcitations` package

Carl Boettiger and colleagues have created a really neat package called knitcitations for creating and citing BibTeX-style references via several mechanisms. To begin, you load the package and then use the cleanbib() function to remove any old .bib files.

## load the package
library("knitcitations")

## clean out any existing .bib files
cleanbib()

## set pandoc for formatting
options("citation_format" = "pandoc")

Cite by DOI

You can cite an article by its DOI, using either citep() or citet(), and the full citation information will be gathered automatically. For example, the inline command

`r citep("10.1890/11-0011.1")`

will do two things:

create an (Author[s] Year) inline citation that looks like (Abrams et al. 2012)
create the following BibTeX reference that can be cited elsewhere using [@Abrams_2012] or @Abrams_2012:

@Article{Abrams_2012,
  doi = {10.1890/11-0011.1},
  url = {https://doi.org/10.1890/11-0011.1},
  year = {2012},
  month = {feb},
  publisher = {Wiley},
  volume = {93},
  number = {2},
  pages = {281--293},
  author = {Peter A. Abrams and Lasse Ruokolainen and Brian J. Shuter and Kevin S. McCann},
  title = {Harvesting creates ecological traps: consequences of invisible mortality risks in predator{\textendash}prey metacommunities},
  journal = {Ecology},
}

On the other hand, the inline command

`r citet("10.1098/rspb.2013.1372")`

will do two things:

create an Author[s] (Year) inline citation that looks like Boettiger and Hastings (2013)
create the following BibTeX reference that can be cited elsewhere using [@Boettiger_2013] or @Boettiger_2013:

@Article{Boettiger_2013,
  doi = {10.1098/rspb.2013.1372},
  url = {https://doi.org/10.1098/rspb.2013.1372},
  year = {2013},
  month = {sep},
  publisher = {The Royal Society},
  volume = {280},
  number = {1766},
  pages = {20131372},
  author = {Carl Boettiger and Alan Hastings},
  title = {No early warning signals for stochastic transitions: insights from large deviation theory},
  journal = {Proceedings of the Royal Society B: Biological Sciences},
}

Cite by URL

Not all the literature we may wish to cite includes DOIs, such as arXiv preprints, Wikipedia pages, or other academic blogs. {knitcitations} can produce citations from any URL using the Greycite API. For example, we can use the call

`r citep("http://knowledgeblog.org/greycite")`

to generate the citation to the Greycite tool, which will render as (Lord 2012) and produce the following BibTeX reference that can be cited elsewhere using [@greycite32194] or @greycite32194.

@Misc{greycite32194,
  url = {http://knowledgeblog.org/greycite},
  title = {Greycite},
  journal = {Knowledge Blog},
  year = {2012},
  howpublished = {\url{http://knowledgeblog.org/greycite}},
  author = {Phillip Lord},
  archived = {http://greycite.knowledgeblog.org/?uri=http%3A%2F%2Fknowledgeblog.org%2Fgreycite, http://wayback.archive.org/web/http://knowledgeblog.org/greycite},
}

Cite BibTeX objects directly

We can also cite BibTeX objects directly, such as those that R provides for citing packages using the citation() function. So, for example, here are the citations for the {knitr} package

citation("knitr")

## 
## To cite the 'knitr' package in publications use:
## 
##   Yihui Xie (2020). knitr: A General-Purpose Package for Dynamic Report
##   Generation in R. R package version 1.30.
## 
##   Yihui Xie (2015) Dynamic Documents with R and knitr. 2nd edition.
##   Chapman and Hall/CRC. ISBN 978-1498716963
## 
##   Yihui Xie (2014) knitr: A Comprehensive Tool for Reproducible
##   Research in R. In Victoria Stodden, Friedrich Leisch and Roger D.
##   Peng, editors, Implementing Reproducible Computational Research.
##   Chapman and Hall/CRC. ISBN 978-1466561595
## 
## To see these entries in BibTeX format, use 'print(<citation>,
## bibtex=TRUE)', 'toBibtex(.)', or set
## 'options(citation.bibtex.max=999)'.

We can use the following inline command

`r citep(citation("knitr"))`

to generate the 3 BibTeX objects as

@InCollection{Xie_2014,
  booktitle = {Implementing Reproducible Computational Research},
  editor = {Victoria Stodden and Friedrich Leisch and Roger D. Peng},
  title = {knitr: A Comprehensive Tool for Reproducible Research in {R}},
  author = {Yihui Xie},
  publisher = {Chapman and Hall/CRC},
  year = {2014},
  note = {ISBN 978-1466561595},
  url = {http://www.crcpress.com/product/isbn/9781466561595},
}

@Book{Xie_2015,
  title = {Dynamic Documents with {R} and knitr},
  author = {Yihui Xie},
  publisher = {Chapman and Hall/CRC},
  address = {Boca Raton, Florida},
  year = {2015},
  edition = {2nd},
  note = {ISBN 978-1498716963},
  url = {https://yihui.org/knitr/},
}

@Manual{Xie_2020,
  title = {knitr: A General-Purpose Package for Dynamic Report Generation in R},
  author = {Yihui Xie},
  year = {2020},
  note = {R package version 1.30},
  url = {https://yihui.org/knitr/},
}

and also create the citation

(Xie 2014, 2015, 2020)

We can now cite the package with normal BibTeX entries and pandoc will correctly avoid duplicating the author’s name. For example,

[@Xie_2014; @Xie_2015; @Xie_2020]

will produce

(Xie 2014, 2015, 2020)

Similarly, we can cite references that are already included in our .bib file using the standard notation, such that

[@Boettiger_2013] or @Boettiger_2013

will render as

(Boettiger and Hastings 2013) or Boettiger and Hastings (2013)

Re-using BibTeX keys

When citep() or citet() are called, they automatically generate a BibTeX key in the format AuthorLastName_Year. We can then use this key to cite a reference without remembering its DOI. For example,

`r citep("Abrams_2012")`

will simply create the citation

(Abrams et al. 2012)

Creating the references cited section

Creating the final references cited section is straightforward. All we have to do is call the function write.bibtex() with the name of the .bib file and the references will be cited with the formatting specified in the .cls file supplied in the document’s YAML. For example, we can generate the references cited section from the citations in this tutorial with the following:

## create ref cited section
write.bibtex(file = "references.bib")

References

Abrams, P. A., L. Ruokolainen, B. J. Shuter, and K. S. McCann. 2012. Harvesting creates ecological traps: Consequences of invisible mortality risks in predatorprey metacommunities. Ecology 93:281–293.

Boettiger, C., and A. Hastings. 2013. No early warning signals for stochastic transitions: Insights from large deviation theory. Proceedings of the Royal Society B: Biological Sciences 280:20131372.

Lord, P. 2012. Greycite. http://knowledgeblog.org/greycite.

Xie, Y. 2014. Knitr: A comprehensive tool for reproducible research in R. in V. Stodden, F. Leisch, and R. D. Peng, editors. Implementing reproducible computational research. Chapman; Hall/CRC.

Xie, Y. 2015. Dynamic documents with R and knitr. 2nd edition. Chapman; Hall/CRC, Boca Raton, Florida.

Xie, Y. 2020. Knitr: A general-purpose package for dynamic report generation in r.

Using R Markdown for publications

Part 1

Mark Scheuerell

17 February 2021

Background

Templates

Equations

Inline

Stand alone

Multiline equations in pdf

Tables

kableExtra

Figures

References

Creating a `.bib` file

Export from Zotero

Export from EndNote

Export from Mendeley

Citations

YAML

In-text

`knitcitations` package

Cite by DOI

Cite by URL

Cite BibTeX objects directly

Re-using BibTeX keys

Creating the references cited section

References

Using R Markdown for publications

Part 1

Mark Scheuerell

17 February 2021

Background

Templates

Equations

Inline

Stand alone

Multiline equations in pdf

Tables

kableExtra

Figures

References

Creating a .bib file

Export from Zotero

Export from EndNote

Export from Mendeley

Citations

YAML

In-text

knitcitations package

Cite by DOI

Cite by URL

Cite BibTeX objects directly

Re-using BibTeX keys

Creating the references cited section

References

Creating a `.bib` file

`knitcitations` package