Chapter 2 Writing R documentation files

2.1 Rd format

R objects are documented in files written in “R documentation” (Rd) format, a simple markup language much of which closely resembles (La)TeX, which can be processed into a variety of formats, including LaTeX, HTML and plain text. The translation is carried out by functions in the tools package called by the script Rdconv in R_HOME/bin and by the installation scripts for packages.

The R distribution contains more than 1300 such files which can be found in the src/library/pkg/man directories of the R source tree, where pkg stands for one of the standard packages which are included in the R distribution.

As an example, let us look at a simplified version of src/library/base/man/load.Rd which documents the R function load.

% File src/library/base/man/load.Rd
\name{load}
\alias{load}
\title{Reload Saved Datasets}
\description{
  Reload the datasets written to a file with the function
  \code{save}.
}
\usage{
load(file, envir = parent.frame())
}
\arguments{
  \item{file}{a connection or a character string giving the
    name of the file to load.}
  \item{envir}{the environment where the data should be
    loaded.}
}
\seealso{
  \code{\link{save}}.
}
\examples{
## save all data
save(list = ls(), file= "all.RData")

## restore the saved values to the current environment
load("all.RData")

## restore the saved values to the workspace
load("all.RData", .GlobalEnv)
}
\keyword{file}

An Rd file consists of three parts. The header gives basic information about the name of the file, the topics documented, a title, a short textual description and R usage information for the objects documented. The body gives further information (for example, on the function’s arguments and return value, as in the above example). Finally, there is an optional footer with keyword information. The header is mandatory.

Information is given within a series of sections with standard names (and user-defined sections are also allowed). Unless otherwise specified⁹¹ these should occur only once in an Rd file (in any order), and the processing software will retain only the first occurrence of a standard section in the file, with a warning.

See “Guidelines for Rd files” for guidelines for writing documentation in Rd format which should be useful for package writers. The R generic function prompt is used to construct a bare-bones Rd file ready for manual editing. Methods are defined for documenting functions (which fill in the proper function and argument names) and data frames. There are also functions promptData, promptPackage, promptClass, and promptMethods for other types of Rd file.

The general syntax of Rd files is summarized below. For a detailed technical discussion of current Rd syntax, see “Parsing Rd files”.

Rd files consist of four types of text input. The most common is LaTeX-like, with the backslash used as a prefix on markup (e.g. \alias), and braces used to indicate arguments (e.g. {load}). The least common type of text is ‘verbatim’ text, where no markup other than the comment marker (%) is processed. There is also a rare variant of ‘verbatim’ text (used in \eqn, \deqn, \figure, and \newcommand) where comment markers need not be escaped. The final type is R-like, intended for R code, but allowing some embedded macros. Quoted strings within R-like text are handled specially: regular character escapes such as \n may be entered as-is. Only markup starting with \l (e.g. \link) or \v (e.g. \var) will be recognized within quoted strings. The rarely used vertical tab \v must be entered as \\v.

Each macro defines the input type for its argument. For example, the file initially uses LaTeX-like syntax, and this is also used in the \description section, but the \usage section uses R-like syntax, and the \alias macro uses ‘verbatim’ syntax. Comments run from a percent symbol % to the end of the line in all types of text except the rare ‘verbatim’ variant (as on the first line of the load example).

Because backslashes, braces and percent symbols have special meaning, to enter them into text sometimes requires escapes using a backslash. In general balanced braces do not need to be escaped, but percent symbols always do, except in the ‘verbatim’ variant. For the complete list of macros and rules for escapes, see “Parsing Rd files”.

2.1.1 Documenting functions

The basic markup commands used for documenting R objects (in particular, functions) are given in this subsection.

\name{name}

name typically⁹² is the basename of the Rd file containing the documentation. It is the “name” of the Rd object represented by the file and has to be unique in a package. To avoid problems with indexing the package manual, it may not contain ‘!’ ‘|’ nor ‘@’, and to avoid possible problems with the HTML help system it should not contain ‘/’ nor a space. (LaTeX special characters are allowed, but may not be collated correctly in the index.) There can only be one \name entry in a file, and it must not contain any markup. Entries in the package manual will be in alphabetic⁹³ order of the \name entries.

\alias{topic}

The \alias sections specify all “topics” the file documents. This information is collected into index data bases for lookup by the on-line (plain text and HTML) help systems. The topic can contain spaces, but (for historical reasons) leading and trailing spaces will be stripped. Percent and left brace need to be escaped by a backslash.

There may be several \alias entries. Quite often it is convenient to document several R objects in one file. For example, file Normal.Rd documents the density, distribution function, quantile function and generation of random variates for the normal distribution, and hence starts with

\name{Normal}
\alias{Normal}
\alias{dnorm}
\alias{pnorm}
\alias{qnorm}
\alias{rnorm}

Also, it is often convenient to have several different ways to refer to an R object, and an \alias does not need to be the name of an object.

Note that the \name is not necessarily a topic documented, and if so desired it needs to have an explicit \alias entry (as in this example).

\title{Title}

Title information for the Rd file. This should be capitalized and not end in a period; try to limit its length to at most 65 characters for widest compatibility.

Markup is supported in the text, but use of characters other than English text and punctuation (e.g., ‘<’) may limit portability.

There must be one (and only one) \title section in a help file.

\description{…}

A short description of what the function(s) do(es) (one paragraph, a few lines only). (If a description is too long and cannot easily be shortened, the file probably tries to document too much at once.) This is mandatory except for package-overview files.

\usage{fun(arg1, arg2, …)}

One or more lines showing the synopsis of the function(s) and variables documented in the file. These are set in typewriter font. This is an R-like command.

The usage information specified should match the function definition exactly (such that automatic checking for consistency between code and documentation is possible).

It is no longer advisable to use \synopsis for the actual synopsis and show modified synopses in the \usage. Support for \synopsis will be removed in \R 3.1.0. To indicate that a function can be used in several different ways, depending on the named arguments specified, use section \details. E.g., abline.Rd contains

\details{
  Typical usages are
\preformatted{abline(a, b, untf = FALSE, \dots)
......
}

Use \method{generic}{class} to indicate the name of an S3 method for the generic function generic for objects inheriting from class “class”. In the printed versions, this will come out as generic (reflecting the understanding that methods should not be invoked directly but via method dispatch), but codoc() and other QC tools always have access to the full name.

For example, print.ts.Rd contains

\usage{
\method{print}{ts}(x, calendar, \dots)
}

which will print as

Usage:

     ## S3 method for class ‘ts’:
     print(x, calendar, ...)

Usage for replacement functions should be given in the style of dim(x) <- value rather than explicitly indicating the name of the replacement function (“dim<-” in the above). Similarly, one can use \method{generic}{class}(arglist) <- value to indicate the usage of an S3 replacement method for the generic replacement function “generic<-” for objects inheriting from class “class”.

Usage for S3 methods for extracting or replacing parts of an object, S3 methods for members of the Ops group, and S3 methods for user-defined (binary) infix operators (‘%xxx%’) follows the above rules, using the appropriate function names. E.g., Extract.factor.Rd contains

\usage{
\method{[}{factor}(x, \dots, drop = FALSE)
\method{[[}{factor}(x, \dots)
\method{[}{factor}(x, \dots) <- value
}

which will print as

Usage:

     ## S3 method for class ‘factor’:
     x[..., drop = FALSE]
     ## S3 method for class ‘factor’:
     x[[...]]
     ## S3 replacement method for class ‘factor’:
     x[...] <- value

\S3method is accepted as an alternative to \method.

\arguments{…}

Description of the function’s arguments, using an entry of the form

\item{arg_i}{Description of arg_i.}

for each element of the argument list. (Note that there is no whitespace between the three parts of the entry.) There may be optional text outside the \item entries, for example to give general information about groups of parameters.

\details{…}

A detailed if possible precise description of the functionality provided, extending the basic information in the \description slot.

\value{…}

Description of the function’s return value.

If a list with multiple values is returned, you can use entries of the form

\item{comp_i}{Description of comp_i.}

for each component of the list returned. Optional text may precede⁹⁴ this list (see for example the help for rle). Note that \value is implicitly a \describe environment, so that environment should not be used for listing components, just individual \item{}{} entries.

\references{…}

A section with references to the literature. Use \url{} or \href{}{} for web pointers.

\note{…}

Use this for a special note you want to have pointed out. Multiple \note sections are allowed, but might be confusing to the end users.

For example, pie.Rd contains

\note{
  Pie charts are a very bad way of displaying information.
  The eye is good at judging linear measures and bad at
  judging relative areas.
  ......
}

\author{…}

Information about the author(s) of the Rd file. Use \email{} without extra delimiters (such as ‘( )’ or ‘< >’) to specify email addresses, or \url{} or \href{}{} for web pointers.

\seealso{…}

Pointers to related R objects, using \code{\link{…}} to refer to them (\code is the correct markup for R object names, and \link produces hyperlinks in output formats which support this. See Marking text, and Cross-references).

\examples{…}

Examples of how to use the function. Code in this section is set in typewriter font without reformatting and is run by example() unless marked otherwise (see below).

Examples are not only useful for documentation purposes, but also provide test code used for diagnostic checking of R code. By default, text inside \examples{} will be displayed in the output of the help page and run by example() and by R CMD check. You can use \dontrun{} for text that should only be shown, but not run, and \dontshow{} for extra commands for testing that should not be shown to users, but will be run by example(). (Previously this was called \testonly, and that is still accepted.)

Text inside \dontrun{} is ‘verbatim’, but the other parts of the \examples section are R-like text.

For example,

x <- runif(10)       # Shown and run.
\dontrun{plot(x)}    # Only shown.
\dontshow{log(x)}    # Only run.

Thus, example code not included in \dontrun must be executable! In addition, it should not use any system-specific features or require special facilities (such as Internet access or write permission to specific directories). Text included in \dontrun is indicated by comments in the processed help files: it need not be valid R code but the escapes must still be used for %, \ and unpaired braces as in other ‘verbatim’ text.

Example code must be capable of being run by example, which uses source. This means that it should not access stdin, e.g. to scan() data from the example file.

Data needed for making the examples executable can be obtained by random number generation (for example, x <- rnorm(100)), or by using standard data sets listed by data() (see ?data for more info).

Finally, there is \donttest, used (at the beginning of a separate line) to mark code that should be run by example() but not by R CMD check (by default: the option –run-donttest can be used). This should be needed only occasionally but can be used for code which might fail in circumstances that are hard to test for, for example in some locales. (Use e.g. capabilities() or nzchar(Sys.which(“someprogram”)) to test for features needed in the examples wherever possible, and you can also use try() or tryCatch(). Use interactive() to condition examples which need someone to interact with.) Note that code included in \donttest must be correct R code, and any packages used should be declared in the DESCRIPTION file. It is good practice to include a comment in the \donttest section explaining why it is needed.

As from R 3.4.0, output from code between comments

## IGNORE_RDIFF_BEGIN
## IGNORE_RDIFF_END

is ignored when comparing check output to reference output (a -Ex.Rout.save file).

\keyword{key}

There can be zero or more \keyword sections per file. Each \keyword section should specify a single keyword, preferably one of the standard keywords as listed in file KEYWORDS in the R documentation directory (default R_HOME/doc). Use e.g. RShowDoc(“KEYWORDS”) to inspect the standard keywords from within R. There can be more than one \keyword entry if the R object being documented falls into more than one category, or none.

Do strongly consider using \concept (see Indices) instead of \keyword if you are about to use more than very few non-standard keywords.

The special keyword ‘internal’ marks a page of internal objects that are not part of the package’s API. If the help page for object foo has keyword ‘internal’, then help(foo) gives this help page, but foo is excluded from several object indices, including the alphabetical list of objects in the HTML help system.

help.search() can search by keyword, including user-defined values: however the ‘Search Engine & Keywords’ HTML page accessed via help.start() provides single-click access only to a pre-defined list of keywords.

2.1.2 Documenting data sets

The structure of Rd files which document R data sets is slightly different. Sections such as \arguments and \value are not needed but the format and source of the data should be explained.

As an example, let us look at src/library/datasets/man/rivers.Rd which documents the standard R data set rivers.

\name{rivers}
\docType{data}
\alias{rivers}
\title{Lengths of Major North American Rivers}
\description{
  This data set gives the lengths (in miles) of 141 \dQuote{major}
  rivers in North America, as compiled by the US Geological
  Survey.
}
\usage{rivers}
\format{A vector containing 141 observations.}
\source{World Almanac and Book of Facts, 1975, page 406.}
\references{
  McNeil, D. R. (1977) \emph{Interactive Data Analysis}.
  New York: Wiley.
}
\keyword{datasets}

This uses the following additional markup commands.

\docType{…}

Indicates the “type” of the documentation object. Always ‘data’ for data sets, and ‘package’ for pkg-package.Rd overview files. Documentation for S4 methods and classes uses ‘methods’ (from promptMethods()) and ‘class’ (from promptClass()).

\format{…}

A description of the format of the data set (as a vector, matrix, data frame, time series, …). For matrices and data frames this should give a description of each column, preferably as a list or table. See Lists and tables, for more information.

\source{…}

Details of the original source (a reference or URL, see Specifying URLs). In addition, section \references could give secondary sources and usages.

Note also that when documenting data set bar,

The \usage entry is always bar or (for packages which do not use lazy-loading of data) data(bar). (In particular, only document a single data object per Rd file.)
The \keyword entry should always be ‘datasets’.

If bar is a data frame, documenting it as a data set can be initiated via prompt(bar). Otherwise, the promptData function may be used.

2.1.3 Documenting S4 classes and methods

There are special ways to use the ‘?’ operator, namely ‘class?topic’ and ‘methods?topic’, to access documentation for S4 classes and methods, respectively. This mechanism depends on conventions for the topic names used in \alias entries. The topic names for S4 classes and methods respectively are of the form

class-class
generic,signature_list-method

where signature_list contains the names of the classes in the signature of the method (without quotes) separated by ‘,’ (without whitespace), with ‘ANY’ used for arguments without an explicit specification. E.g., ‘genericFunction-class’ is the topic name for documentation for the S4 class “genericFunction”, and ‘coerce,ANY,NULL-method’ is the topic name for documentation for the S4 method for coerce for signature c(“ANY”, “NULL”).

Skeletons of documentation for S4 classes and methods can be generated by using the functions promptClass() and promptMethods() from package methods. If it is necessary or desired to provide an explicit function declaration (in a \usage section) for an S4 method (e.g., if it has “surprising arguments” to be mentioned explicitly), one can use the special markup

\S4method{generic}{signature_list}(argument_list)

(e.g., ‘\S4method{coerce}{ANY,NULL}(from, to)’).

To make full use of the potential of the on-line documentation system, all user-visible S4 classes and methods in a package should at least have a suitable \alias entry in one of the package’s Rd files. If a package has methods for a function defined originally somewhere else, and does not change the underlying default method for the function, the package is responsible for documenting the methods it creates, but not for the function itself or the default method.

An S4 replacement method is documented in the same way as an S3 one: see the description of \method in Documenting functions.

See help(“Documentation”, package = “methods”) for more information on using and creating on-line documentation for S4 classes and methods.

2.1.4 Documenting packages

Packages may have an overview help page with an \alias pkgname-package, e.g. ‘utils-package’ for the utils package, when package?pkgname will open that help page. If a topic named pkgname does not exist in another Rd file, it is helpful to use this as an additional \alias.

Skeletons of documentation for a package can be generated using the function promptPackage(). If the final = LIBS argument is used, then the Rd file will be generated in final form, containing the information that would be produced up to library(help = pkgname). Otherwise (the default) comments will be inserted giving suggestions for content.

Apart from the mandatory \name and \title and the pkgname-package alias, the only requirement for the package overview page is that it include a \docType{package} statement. All other content is optional. We suggest that it should be a short overview, to give a reader unfamiliar with the package enough information to get started. More extensive documentation is better placed into a package vignette (see Writing package vignettes) and referenced from this page, or into individual man pages for the functions, datasets, or classes.

2.2 Sectioning

To begin a new paragraph or leave a blank line in an example, just insert an empty line (as in (La)TeX). To break a line, use \cr.

In addition to the predefined sections (such as \description{}, \value{}, etc.), you can “define” arbitrary ones by \section{section_title}{…}. For example

\section{Warning}{
  You must not call this function unless …
}

For consistency with the pre-assigned sections, the section name (the first argument to \section) should be capitalized (but not all upper case). Whitespace between the first and second braced expressions is not allowed. Markup (e.g. \code) within the section title may cause problems with the latex conversion (depending on the version of macro packages such as ‘hyperref’) and so should be avoided.

The \subsection macro takes arguments in the same format as \section, but is used within a section, so it may be used to nest subsections within sections or other subsections. There is no predefined limit on the nesting level, but formatting is not designed for more than 3 levels (i.e. subsections within subsections within sections).

Note that additional named sections are always inserted at a fixed position in the output (before \note, \seealso and the examples), no matter where they appear in the input (but in the same order amongst themselves as in the input).

2.3 Marking text

The following logical markup commands are available for emphasizing or quoting text.

\emph{text}

\strong{text}

Emphasize text using italic and bold font if possible; \strong is regarded as stronger (more emphatic).

\bold{text}

Set text in bold font where possible.

\sQuote{text}

\dQuote{text}

Portably single or double quote text (without hard-wiring the characters used for quotation marks).

Each of the above commands takes LaTeX-like input, so other macros may be used within text.

The following logical markup commands are available for indicating specific kinds of text. Except as noted, these take ‘verbatim’ text input, and so other macros may not be used within them. Some characters will need to be escaped (see Insertions).

\code{text}

Indicate text that is a literal example of a piece of an R program, e.g., a fragment of R code or the name of an R object. Text is entered in R-like syntax, and displayed using typewriter font where possible. Macros \var and \link are interpreted within text.

\preformatted{text}

Indicate text that is a literal example of a piece of a program. Text is displayed using typewriter font where possible. Formatting, e.g. line breaks, is preserved. (Note that this includes a line break after the initial {, so typically text should start on the same line as the command.)

Due to limitations in LaTeX as of this writing, this macro may not be nested within other markup macros other than \dQuote and \sQuote, as errors or bad formatting may result.

\kbd{keyboard-characters}

Indicate keyboard input, using slanted typewriter font if possible, so users can distinguish the characters they are supposed to type from computer output. Text is entered ‘verbatim’.

\samp{text}

Indicate text that is a literal example of a sequence of characters, entered ‘verbatim’. No wrapping or reformatting will occur. Displayed using typewriter font where possible.

\verb{text}

Indicate text that is a literal example of a sequence of characters, with no interpretation of e.g. \var, but which will be included within word-wrapped text. Displayed using typewriter font if possible.

\pkg{package_name}

Indicate the name of an R package. LaTeX-like.

\file{file_name}

Indicate the name of a file. Text is LaTeX-like, so backslash needs to be escaped. Displayed using a distinct font where possible.

\email{email_address}

Indicate an electronic mail address. LaTeX-like, will be rendered as a hyperlink in HTML and PDF conversion. Displayed using typewriter font where possible.

\url{uniform_resource_locator}

Indicate a uniform resource locator (URL) for the World Wide Web. The argument is handled as ‘verbatim’ text (with percent and braces escaped by backslash), and rendered as a hyperlink in HTML and PDF conversion. Linefeeds are removed, and leading and trailing whitespace⁹⁵ is removed. See Specifying URLs.

Displayed using typewriter font where possible.

\href{uniform_resource_locator}{text}

Indicate a hyperlink to the World Wide Web. The first argument is handled as ‘verbatim’ text (with percent and braces escaped by backslash) and is used as the URL in the hyperlink, with the second argument of LaTeX-like text displayed to the user. Linefeeds are removed from the first argument, and leading and trailing whitespace is removed.

Note that RFC3986-encoded URLs (e.g. using ‘\%28VS.85\%29’ in place of ‘(VS.85)’) may not work correctly in versions of R before 3.1.3 and are best avoided—use URLdecode() to decode them.

\var{metasyntactic_variable}

Indicate a metasyntactic variable. In some cases this will be rendered distinctly, e.g. in italic, but not in all⁹⁶. LaTeX-like.

\env{environment_variable}

Indicate an environment variable. ‘Verbatim’. Displayed using typewriter font where possible

\option{option}

Indicate a command-line option. ‘Verbatim’. Displayed using typewriter font where possible.

\command{command_name}

Indicate the name of a command. LaTeX-like, so \var is interpreted. Displayed using typewriter font where possible.

\dfn{term}

Indicate the introductory or defining use of a term. LaTeX-like.

\cite{reference}

Indicate a reference without a direct cross-reference via \link (see Cross-references), such as the name of a book. LaTeX-like.

\acronym{acronym}

Indicate an acronym (an abbreviation written in all capital letters), such as GNU. LaTeX-like.

2.4 Lists and tables

The \itemize and \enumerate commands take a single argument, within which there may be one or more \item commands. The text following each \item is formatted as one or more paragraphs, suitably indented and with the first paragraph marked with a bullet point (\itemize) or a number (\enumerate).

Note that unlike argument lists, \item in these formats is followed by a space and the text (not enclosed in braces). For example

  \enumerate{
    \item A database consists of one or more records, each with one or
    more named fields.
    \item Regular lines start with a non-whitespace character.
    \item Records are separated by one or more empty lines.
  }

\itemize and \enumerate commands may be nested.

The \describe command is similar to \itemize but allows initial labels to be specified. Each \item takes two arguments, the label and the body of the item, in exactly the same way as an argument or value \item. \describe commands are mapped to <DL> lists in HTML and \description lists in LaTeX.

The \tabular command takes two arguments. The first gives for each of the columns the required alignment (‘l’ for left-justification, ‘r’ for right-justification or ‘c’ for centring.) The second argument consists of an arbitrary number of lines separated by \cr, and with fields separated by \tab. For example:

  \tabular{rlll}{
    [,1] \tab Ozone   \tab numeric \tab Ozone (ppb)\cr
    [,2] \tab Solar.R \tab numeric \tab Solar R (lang)\cr
    [,3] \tab Wind    \tab numeric \tab Wind (mph)\cr
    [,4] \tab Temp    \tab numeric \tab Temperature (degrees F)\cr
    [,5] \tab Month   \tab numeric \tab Month (1--12)\cr
    [,6] \tab Day     \tab numeric \tab Day of month (1--31)
  }

There must be the same number of fields on each line as there are alignments in the first argument, and they must be non-empty (but can contain only spaces). (There is no whitespace between \tabular and the first argument, nor between the two arguments.)

2.5 Cross-references

The markup \link{foo} (usually in the combination \code{\link{foo}}) produces a hyperlink to the help for foo. Here foo is a topic, that is the argument of \alias markup in another Rd file (possibly in another package). Hyperlinks are supported in some of the formats to which Rd files are converted, for example HTML and PDF, but ignored in others, e.g. the text format.

One main usage of \link is in the \seealso section of the help page, see Rd format.

Note that whereas leading and trailing spaces are stripped when extracting a topic from a \alias, they are not stripped when looking up the topic of a \link.

You can specify a link to a different topic than its name by \link[=dest]{name} which links to topic dest with name name. This can be used to refer to the documentation for S3/4 classes, for example \code{“\link[=abc-class]{abc}”} would be a way to refer to the documentation of an S4 class “abc” defined in your package, and \code{“\link[=terms.object]{terms}”} to the S3 “terms” class (in package stats). To make these easy to read in the source file, \code{“\linkS4class{abc}”} expands to the form given above.

There are two other forms of optional argument specified as \link[pkg]{foo} and \link[pkg:bar]{foo} to link to the package pkg, to files foo.html and bar.html respectively. These are rarely needed, perhaps to refer to not-yet-installed packages (but there the HTML help system will resolve the link at run time) or in the normally undesirable event that more than one package offers help on a topic⁹⁷ (in which case the present package has precedence so this is only needed to refer to other packages). They are currently only used in HTML help (and ignored for hyperlinks in LaTeX conversions of help pages), and link to the file rather than the topic (since there is no way to know which topics are in which files in an uninstalled package). The only reason to use these forms for base and recommended packages is to force a reference to a package that might be further down the search path. Because they have been frequently misused, the HTML help system looks for topic foo in package pkg if it does not find file foo.html.

2.6 Mathematics

Mathematical formulae should be set beautifully for printed documentation yet we still want something useful for text and HTML online help. To this end, the two commands \eqn{latex}{ascii} and \deqn{latex}{ascii} are used. Whereas \eqn is used for “inline” formulae (corresponding to TeX’s $…$ ), \deqn gives “displayed equations” (as in LaTeX’s displaymath environment, or TeX’s $$…$$). Both arguments are treated as ‘verbatim’ text.

Both commands can also be used as \eqn{latexascii} (only one argument) which then is used for both latex and ascii. No whitespace is allowed between command and the first argument, nor between the first and second arguments.

The following example is from Poisson.Rd:

  \deqn{p(x) = \frac{\lambda^x e^{-\lambda}}{x!}}{%
        p(x) = \lambda^x exp(-\lambda)/x!}
  for \eqn{x = 0, 1, 2, \ldots}.

For text on-line help we get

    p(x) = lambda^x exp(-lambda)/x!

for x = 0, 1, 2, ....

Greek letters (both cases) will be rendered in HTML if preceded by a backslash, \dots and \ldots will be rendered as ellipses and \sqrt, \ge and \le as mathematical symbols.

Note that only basic LaTeX can be used, there being no provision to specify LaTeX style files such as the AMS extensions.

2.7 Figures

To include figures in help pages, use the \figure markup. There are three forms.

The two commonly used simple forms are \figure{filename} and \figure{filename}{alternate text}. This will include a copy of the figure in either HTML or LaTeX output. In text output, the alternate text will be displayed instead. (When the second argument is omitted, the filename will be used.) Both the filename and the alternate text will be parsed verbatim, and should not include special characters that are significant in HTML or LaTeX.

The expert form is \figure{filename}{options: string}. (The word ‘options:’ must be typed exactly as shown and followed by at least one space.) In this form, the string is copied into the HTML img tag as attributes following the src attribute, or into the second argument of the \Figure macro in LaTeX, which by default is used as options to an \includegraphics call. As it is unlikely that any single string would suffice for both display modes, the expert form would normally be wrapped in conditionals. It is up to the author to make sure that legal HTML/LaTeX is used. For example, to include a logo in both HTML (using the simple form) and LaTeX (using the expert form), the following could be used:

\if{html}{\figure{Rlogo.svg}{options: width=100 alt="R logo"}}
\if{latex}{\figure{Rlogo.pdf}{options: width=0.5in}}

The files containing the figures should be stored in the directory man/figures. Files with extensions .jpg, .jpeg, .pdf, .png and .svg from that directory will be copied to the help/figures directory at install time. (Figures in PDF format will not display in most HTML browsers, but might be the best choice in reference manuals.) Specify the filename relative to man/figures in the \figure directive.

2.8 Insertions

Use \R for the R system itself. Use \dots for the dots in function argument lists ‘…’, and \ldots for ellipsis dots in ordinary text.⁹⁸ These can be followed by {}, and should be unless followed by whitespace.

After an unescaped ‘%’, you can put your own comments regarding the help text. The rest of the line (but not the newline at the end) will be completely disregarded. Therefore, you can also use it to make part of the “help” invisible.

You can produce a backslash (‘\’) by escaping it by another backslash. (Note that \cr is used for generating line breaks.)

The “comment” character ‘%’ and unpaired braces⁹⁹ almost always need to be escaped by ‘\’, and ‘\\’ can be used for backslash and needs to be when there are two or more adjacent backslashes. In R-like code quoted strings are handled slightly differently; see “Parsing Rd files” for details – in particular braces should not be escaped in quoted strings.

All of ‘% { } \’ should be escaped in LaTeX-like text.

Text which might need to be represented differently in different encodings should be marked by \enc, e.g. \enc{Jöreskog}{Joreskog} (with no whitespace between the braces) where the first argument will be used where encodings are allowed and the second should be ASCII (and is used for e.g. the text conversion in locales that cannot represent the encoded form). (This is intended to be used for individual words, not whole sentences or paragraphs.)

2.9 Indices

The \alias command (see Documenting functions) is used to specify the “topics” documented, which should include all R objects in a package such as functions and variables, data sets, and S4 classes and methods (see Documenting S4 classes and methods). The on-line help system searches the index data base consisting of all alias topics.

In addition, it is possible to provide “concept index entries” using \concept, which can be used for help.search() lookups. E.g., file cor.test.Rd in the standard package stats contains

\concept{Kendall correlation coefficient}
\concept{Pearson correlation coefficient}
\concept{Spearman correlation coefficient}

so that e.g. ??Spearman will succeed in finding the help page for the test for association between paired samples using Spearman’s rho.

(Note that help.search() only uses “sections” of documentation objects with no additional markup.)

If you want to cross reference such items from other help files via \link, you need to use \alias and not \concept.

2.10 Platform-specific documentation

Sometimes the documentation needs to differ by platform. Currently two OS-specific options are available, ‘unix’ and ‘windows’, and lines in the help source file can be enclosed in

#ifdef OS
   ...
#endif

#ifndef OS
   ...
#endif

for OS-specific inclusion or exclusion. Such blocks should not be nested, and should be entirely within a block (that, is between the opening and closing brace of a section or item), or at top-level contain one or more complete sections.

If the differences between platforms are extensive or the R objects documented are only relevant to one platform, platform-specific Rd files can be put in a unix or windows subdirectory.

2.11 Conditional text

Occasionally the best content for one output format is different from the best content for another. For this situation, the \if{format}{text} or \ifelse{format}{text}{alternate} markup is used. Here format is a comma separated list of formats in which the text should be rendered. The alternate will be rendered if the format does not match. Both text and alternate may be any sequence of text and markup.

Currently the following formats are recognized: example, html, latex and text. These select output for the corresponding targets. (Note that example refers to extracted example code rather than the displayed example in some other format.) Also accepted are TRUE (matching all formats) and FALSE (matching no formats). These could be the output of the \Sexpr macro (see Dynamic pages).

The \out{literal} macro would usually be used within the text part of \if{format}{text}. It causes the renderer to output the literal text exactly, with no attempt to escape special characters. For example, use the following to output the markup necessary to display the Greek letter in LaTeX or HTML, and the text string alpha in other formats:

\ifelse{latex}{\out{$\alpha$}}{\ifelse{html}{\out{&alpha;}}{alpha}}

2.12 Dynamic pages

Two macros supporting dynamically generated man pages are \Sexpr and \RdOpts. These are modelled after Sweave, and are intended to contain executable R expressions in the Rd file.

The main argument to \Sexpr must be valid R code that can be executed. It may also take options in square brackets before the main argument. Depending on the options, the code may be executed at package build time, package install time, or man page rendering time.

The options follow the same format as in Sweave, but different options are supported. Currently the allowed options and their defaults are:

eval=TRUE Whether the R code should be evaluated.
echo=FALSE Whether the R code should be echoed. If TRUE, a display will be given in a preformatted block. For example, \Sexpr[echo=TRUE]{ x <- 1 } will be displayed as
```
> x <- 1
```
keep.source=TRUE Whether to keep the author’s formatting when displaying the code, or throw it away and use a deparsed version.
results=text How should the results be displayed? The possibilities are:
- - results=text Apply as.character() to the result of the code, and insert it as a text element.
  - - results=verbatim Print the results of the code just as if it was executed at the console, and include the printed results verbatim. (Invisible results will not print.)
    - - results=rd The result is assumed to be a character vector containing markup to be passed to parse_Rd(), with the result inserted in place. This could be used to insert computed aliases, for instance. parse_Rd() is called first with fragment = FALSE to allow a single Rd section macro to be inserted. If that fails, it is called again with fragment = TRUE, the older behavior.
      - results=hide Insert no output.
      - strip.white=TRUE Remove leading and trailing white space from each line of output if strip.white=TRUE. With strip.white=all, also remove blank lines.
      - stage=install Control when this macro is run. Possible values are
        
        stage=build The macro is run when building a source tarball.
        
        stage=install The macro is run when installing from source.
        
        stage=render The macro is run when displaying the help page.
        
        Conditionals such as #ifdef (see Platform-specific sections) are applied after the build macros but before the install macros. In some situations (e.g. installing directly from a source directory without a tarball, or building a binary package) the above description is not literally accurate, but authors can rely on the sequence being build, #ifdef, install, render, with all stages executed.
        
        Code is only run once in each stage, so a \Sexpr[results=rd] macro can output an \Sexpr macro designed for a later stage, but not for the current one or any earlier stage.
        
        width, height, fig These options are currently allowed but ignored.
        
        The \RdOpts macro is used to set new defaults for options to apply to following uses of \Sexpr.
        
        For more details, see the online document “Parsing Rd files”.
        
        2.13 User-defined macros
        
        The \newcommand and \renewcommand macros allow new macros to be defined within an Rd file. These are similar but not identical to the same-named LaTeX macros.
        
        They each take two arguments which are parsed verbatim. The first is the name of the new macro including the initial backslash, and the second is the macro definition. As in LaTeX, \newcommand requires that the new macro not have been previously defined, whereas \renewcommand allows existing macros (including all built-in ones) to be replaced. (As from version 3.2.0, this test is disabled by default, but may be enabled by setting the environment variable WARN_DUPLICATE_RD_MACROS to a true value.)
        
        Also as in LaTeX, the new macro may be defined to take arguments, and numeric placeholders such as #1 are used in the macro definition. However, unlike LaTeX, the number of arguments is determined automatically from the highest placeholder number seen in the macro definition. For example, a macro definition containing #1 and #3 (but no other placeholders) will define a three argument macro (whose second argument will be ignored). As in LaTeX, at most 9 arguments may be defined. If the # character is followed by a non-digit it will have no special significance. All arguments to user-defined macros will be parsed as verbatim text, and simple text-substitution will be used to replace the place-holders, after which the replacement text will be parsed.
        
        As of R version 3.2.0, a number of macros are defined in the file share/Rd/macros/system.Rd of the R source or home directory, and these will normally be available in all .Rd files. For example, that file contains the definition
        
        \newcommand{\PR}{\Sexpr[results=rd]{tools:::Rd_expr_PR(#1)}}
        
        which defines \PR to be a single argument macro; then code (typically used in the NEWS.Rd file) like
        
        \PR{1234}
        
        will expand to
        
        \Sexpr[results=rd]{tools:::Rd_expr_PR(1234)}
        
        when parsed.
        
        Some macros that might be of general use are:
        
        \CRANpkg{pkg}
        
        A package on CRAN
        
        \sspace
        
        A single space (used after a period that does not end a sentence).
        
        \doi{numbers}
        
        A digital object identifier (DOI).
        
        See the system.Rd file in share/Rd/macros for more details and macro definitions, including macros \packageTitle, \packageDescription, \packageAuthor, \packageMaintainer, \packageDESCRIPTION and \packageIndices.
        
        Packages may also define their own common macros; these would be stored in an .Rd file in man/macros in the package source and will be installed into help/macros when the package is installed. A package may also use the macros from a different package by listing the other package in the ‘RdMacros’ field in the DESCRIPTION file.
        
        2.14 Encoding
        
        Rd files are text files and so it is impossible to deduce the encoding they are written in unless ASCII: files with 8-bit characters could be UTF-8, Latin-1, Latin-9, KOI8-R, EUC-JP, etc. So an \encoding{} section must be used to specify the encoding if it is not ASCII. (The \encoding{} section must be on a line by itself, and in particular one containing no non-ASCII characters. The encoding declared in the DESCRIPTION file will be used if none is declared in the file.) The Rd files are converted to UTF-8 before parsing and so the preferred encoding for the files themselves is now UTF-8.
        
        Wherever possible, avoid non-ASCII chars in Rd files, and even symbols such as ‘<’, ‘>’, ‘$’, ‘^’, ‘&’, ‘|’, ‘@’, ‘~’, and ‘*’ outside ‘verbatim’ environments (since they may disappear in fonts designed to render text). (Function showNonASCIIfile in package tools can help in finding non-ASCII bytes in the files.)
        
        For convenience, encoding names ‘latin1’ and ‘latin2’ are always recognized: these and ‘UTF-8’ are likely to work fairly widely. However, this does not mean that all characters in UTF-8 will be recognized, and the coverage of non-Latin characters¹⁰⁰ is fairly low. Using LaTeX inputenx (see ?Rd2pdf in R) will give greater coverage of UTF-8.
        
        The \enc command (see Insertions) can be used to provide transliterations which will be used in conversions that do not support the declared encoding.
        
        The LaTeX conversion converts the file to UTF-8 from the declared encoding, and includes a
        
        \inputencoding{utf8}
        
        command, and this needs to be matched by a suitable invocation of the \usepackage{inputenc} command. The R utility R CMD Rd2pdf looks at the converted code and includes the encodings used: it might for example use
        
        \usepackage[utf8]{inputenc}
        
        (Use of utf8 as an encoding requires LaTeX dated 2003/12/01 or later. Also, the use of Cyrillic characters in ‘UTF-8’ appears to also need ‘\usepackage[T2A]{fontenc}’, and R CMD Rd2pdf includes this conditionally on the file t2aenc.def being present and environment variable R_CYRILLIC_TEX being set.)
        
        Note that this mechanism works best with Latin letters: the coverage of UTF-8 in LaTeX is quite low.
        
        2.15 Processing documentation files
        
        There are several commands to process Rd files from the system command line.
        
        Using R CMD Rdconv one can convert R documentation format to other formats, or extract the executable examples for run-time testing. The currently supported conversions are to plain text, HTML and LaTeX as well as extraction of the examples.
        
        R CMD Rd2pdf generates PDF output from documentation in Rd files, which can be specified either explicitly or by the path to a directory with the sources of a package. In the latter case, a reference manual for all documented objects in the package is created, including the information in the DESCRIPTION files.
        
        R CMD Sweave and R CMD Stangle process vignette-like documentation files (e.g. Sweave vignettes with extension ‘.Snw’ or ‘.Rnw’, or other non-Sweave vignettes). R CMD Stangle is used to extract the R code fragments.
        
        The exact usage and a detailed list of available options for all of these commands can be obtained by running R CMD command –help, e.g., R CMD Rdconv –help. All available commands can be listed using R –help (or Rcmd –help under Windows).
        
        All of these work under Windows. You may need to have installed the the tools to build packages from source as described in the “R Installation and Administration” manual, although typically all that is needed is a LaTeX installation.
        
        2.16 Editing Rd files
        
        It can be very helpful to prepare .Rd files using a editor which knows about their syntax and will highlight commands, indent to show the structure and detect mis-matched braces, and so on.
        
        The system most commonly used for this is some version of Emacs (including XEmacs) with the ESS package (https://ESS.R-project.org/: it is often is installed with Emacs but may need to be loaded, or even installed, separately).
        
        Another is the Eclipse IDE with the Stat-ET plugin (http://www.walware.de/goto/statet), and (on Windows only) Tinn-R (http://sourceforge.net/projects/tinn-r/).
        
        People have also used LaTeX mode in a editor, as .Rd files are rather similar to LaTeX files.
        
        Some R front-ends provide editing support for .Rd files, for example RStudio (https://rstudio.org/).