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<title>Google's R Style Guide</title>
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<!---------------------------------------------------------------------------->
<h1>Google's R Style Guide</h1>
<p>
R is a high-level programming language used primarily for statistical
computing and graphics. The goal of the R Programming Style Guide
is to make our R code easier to read, share, and verify. The rules
below were designed in collaboration with the entire R user community
at Google.
</p>
<!---------------------------------------------------------------------------->
<ul class="NoBullet">
<h2><li>Summary: R Style Rules</li></h2>
<ol>
<li><a href=#filenames>File Names</a>: end in <code>.R</code></li>
<li><a href=#identifiers>Identifiers</a>: <code>variable.name</code>,
<code>FunctionName</code>, <code>kConstantName</code></li>
<li><a href=#linelength>Line Length</a>: maximum 80 characters</li>
<li><a href=#indentation>Indentation</a>: two spaces, no tabs</li>
<li><a href=#spacing>Spacing</a></li>
<li><a href=#curlybraces>Curly Braces</a>: first on same line, last on
own line</li>
<li><a href=#assignment>Assignment</a>: use <code>&lt;-</code>, not
<code>=</code></li>
<li><a href=#semicolons>Semicolons</a>: don't use them</li>
<li><a href=#generallayout> General Layout and Ordering</a></li>
<li><a href=#comments> Commenting Guidelines</a>: all comments begin
with <code>#</code> followed by a space; inline comments need two
spaces before the <code>#</code></li>
<li><a href=#functiondefinition>Function Definitions and Calls</a></li>
<li><a href=#functiondocumentation> Function Documentation</a></li>
<li><a href=#examplefunction> Example Function</a></li>
<li><a href=#todo> TODO Style</a>: <code>TODO(username)</code></li>
</ol>
<h2><li>Summary: R Language Rules</li></h2>
<ol>
<li><a href=#attach> <code>attach</code></a>: avoid using it</li>
<li><a href=#functionlanguage> Functions</a>:
errors should be raised using <code>stop()</code></li>
<li><a href=#object> Objects and Methods</a>: avoid S4 objects and
methods when possible; never mix S3 and S4 </li>
</ol>
</ul>
<br>
<ol>
<!---------------------------------------------------------------------------->
<h3><li>Notation and Naming</li></h3>
<ul class="NoBullet">
<h4 id="filenames"><li>File Names</li></h4>
<p>
File names should end in <code>.R</code> and, of course, be
meaningful.
<br> GOOD: <code>predict_ad_revenue.R</code>
<br> BAD: <code><span style="color:red">foo.R</span></code>
</p>
<h4 id="identifiers"><li>Identifiers</li></h4>
<p>
Don't use underscores ( <code>_</code> ) or hyphens
( <code>-</code> ) in identifiers.
Identifiers should be named according to the following conventions.
Variable names should have all lower case letters and words
separated with dots (<code>.</code>);
function names have initial capital letters and no dots
(CapWords);
constants are named like functions but with an initial
<code>k</code>.
</p>
<ul>
<li><code>variable.name </code>
<br> GOOD: <code>avg.clicks</code>
<br> BAD: <code><span style="color:red">avg_Clicks
</span></code>, <code><span style="color:red">avgClicks
</span></code>
</li>
<li><code>FunctionName </code>
<br> GOOD: <code>CalculateAvgClicks</code>
<br> BAD: <code><span style="color:red">calculate_avg_clicks
</span></code>,
<code><span style="color:red">calculateAvgClicks</span></code>
<br> Make function names verbs.
<br><em>Exception: When creating a classed object, the function
name (constructor) and class should match (e.g., lm).</em>
<li><code>kConstantName </code></li>
</ul>
</ul>
<!---------------------------------------------------------------------------->
<h3><li>Syntax</li></h3>
<ul class="NoBullet">
<h4 id="linelength"><li>Line Length</li></h4>
<p>
The maximum line length is 80 characters.
</p>
<h4 id="indentation"><li>Indentation</li></h4>
<p>
When indenting your code, use two spaces. Never use tabs or mix
tabs and spaces.
<br><em>Exception: When a line break occurs inside parentheses,
align the wrapped line with the first character inside the
parenthesis.</em>
<p>
<h4 id="spacing"><li>Spacing</li></h4>
<p>
Place spaces around all binary operators (<code>=</code>,
<code>+</code>, <code>-</code>, <code>&lt;-</code>, etc.).
<br><em> Exception: Spaces around <code>=</code>'s are
optional when passing parameters in a function call.</em>
</p>
<p>
Do not place a space before a comma, but always place one after a
comma.
<br><br> GOOD:<pre><code>tabPrior &lt;- table(df[df$daysFromOpt &lt; 0, &quot;campaignid&quot;])
total &lt;- sum(x[, 1])
total &lt;- sum(x[1, ])</code></pre>
</p>
<p>
BAD:<pre><code><span style="color:red">tabPrior &lt;- table(df[df$daysFromOpt&lt;0, &quot;campaignid&quot;]) # Needs spaces around '<'
tabPrior &lt;- table(df[df$daysFromOpt &lt; 0,&quot;campaignid&quot;]) # Needs a space after the comma
tabPrior&lt;- table(df[df$daysFromOpt &lt; 0, &quot;campaignid&quot;]) # Needs a space before &lt;-
tabPrior<-table(df[df$daysFromOpt &lt; 0, &quot;campaignid&quot;]) # Needs spaces around &lt;-
total &lt;- sum(x[,1]) # Needs a space after the comma
total &lt;- sum(x[ ,1]) # Needs a space after the comma, not before</span></code></pre>
</p>
<p>
Place a space before left parenthesis, except in a function call.
</p>
<p>
GOOD:
<br><code>if (debug)</code>
</p>
<p>
BAD:
<br><code><span style="color:red">if(debug)</span></code>
</p>
<p>
Extra spacing (i.e., more than one space in a row) is okay if it
improves alignment of equals signs or arrows (<code><-</code>).
</p>
<pre><code>plot(x = xCoord,
y = dataMat[, makeColName(metric, ptiles[1], &quot;roiOpt&quot;)],
ylim = ylim,
xlab = &quot;dates&quot;,
ylab = metric,
main = (paste(metric, &quot; for 3 samples &quot;, sep=&quot;&quot;)))
</pre></code>
<p>
Do not place spaces around code in parentheses or square brackets.
<br><em> Exception: Always place a space after a comma.</em>
</p>
<p>
GOOD: <pre><code>if (debug)
x[1, ]</code></pre>
</p>
<p>
BAD:<pre><code><span style="color:red">if ( debug ) # No spaces around debug
x[1,] # Needs a space after the comma </span></code></pre>
</p>
<h4 id="curlybraces"><li>Curly Braces</li></h4>
<p>
An opening curly brace should never go on its own line; a closing
curly brace should always go on its own line. You may omit curly
braces when a block consists of a single statement; however, you
must <em>consistently</em> either use or not use curly braces for
single statement blocks.
</p>
<code><pre>
if (is.null(ylim)) {
ylim &lt;- c(0, 0.06)
}</pre></code>
<p>
xor (but not both)
</p>
<code><pre>
if (is.null(ylim))
ylim &lt;- c(0, 0.06)</pre></code>
<p>
Always begin the body of a block on a new line.
</p>
<p>
BAD:
<br><code><span style="color:red"> if (is.null(ylim))
ylim &lt;- c(0, 0.06)</span></code>
<br><code><span style="color:red"> if (is.null(ylim))
{ylim &lt;- c(0, 0.06)} </span></code>
</p>
<h4 id="assignment"><li>Assignment</li></h4>
<p>
Use <code>&lt;-</code>, not <code>=</code>, for assignment.
</p>
<p>
GOOD:
<br><code> x &lt;- 5 </code>
</p>
<p>
BAD:
<br><code><span style="color:red"> x = 5</span></code>
</p>
<h4 id="semicolons"><li>Semicolons</li></h4>
<p>
Do not terminate your lines with semicolons or use semicolons to
put more than one command on the same line. (Semicolons are not
necessary, and are omitted for consistency with other Google style
guides.)
</p>
</ul>
<!---------------------------------------------------------------------------->
<h3><li> Organization </li></h3>
<ul class="NoBullet">
<h4 id="generallayout"><li>General Layout and Ordering</li></h4>
<p>
If everyone uses the same general ordering, we'll be able to
read and understand each other's scripts faster and more easily.
</p>
<ol>
<li>Copyright statement comment
<li>Author comment
<li>File description comment, including purpose of
program, inputs, and outputs
<li><code>source()</code> and <code>library()</code> statements
<li>Function definitions
<li>Executed statements, if applicable (e.g.,
<code> print</code>, <code>plot</code>)
</ol>
<p>
Unit tests should go in a separate file named
<code>originalfilename_unittest.R</code>.
<p>
<h4 id="comments"><li>Commenting Guidelines</li></h4>
<p>
Comment your code. Entire commented lines should begin with
<code>#</code> and one space.
</p>
<p>
Short comments can be placed after code preceded by two spaces,
<code>#</code>, and then one space.
</p>
<pre><code># Create histogram of frequency of campaigns by pct budget spent.
hist(df$pctSpent,
breaks = &quot;scott&quot;, # method for choosing number of buckets
main = &quot;Histogram: fraction budget spent by campaignid&quot;,
xlab = &quot;Fraction of budget spent&quot;,
ylab = &quot;Frequency (count of campaignids)&quot;)
</code></pre>
<h4 id="functiondefinition"><li> Function Definitions and
Calls</li></h4>
<p>
Function definitions should first list arguments without default
values, followed by those with default values.
</p>
<p>
In both function definitions and function calls, multiple
arguments per line are allowed; line breaks are only allowed
between assignments.
<br>GOOD:
<pre><code>PredictCTR &lt;- function(query, property, numDays,
showPlot = TRUE)
</code></pre>
BAD:
<pre><code><span style="color:red">PredictCTR &lt;- function(query, property, numDays, showPlot =
TRUE)
</span></code></pre>
<p> Ideally, unit tests should serve as sample function calls (for
shared library routines).
<h4 id="functiondocumentation"><li> Function Documentation </li></h4>
<p> Functions should contain a comments section immediately below
the function definition line. These comments should consist of a
one-sentence description of the function; a list of the function's
arguments, denoted by <code>Args:</code>, with a description of
each (including the data type); and a description of the return
value, denoted by <code>Returns:</code>. The comments should be
descriptive enough that a caller can use the function without
reading any of the function's code.
<h4 id="examplefunction"><li> Example Function </li></h4><pre><code>
CalculateSampleCovariance &lt;- function(x, y, verbose = TRUE) {
# Computes the sample covariance between two vectors.
#
# Args:
# x: One of two vectors whose sample covariance is to be calculated.
# y: The other vector. x and y must have the same length, greater than one,
# with no missing values.
# verbose: If TRUE, prints sample covariance; if not, not. Default is TRUE.
#
# Returns:
# The sample covariance between x and y.
n &lt;- length(x)
# Error handling
if (n &lt;= 1 || n != length(y)) {
stop(&quot;Arguments x and y have invalid lengths: &quot;,
length(x), &quot; and &quot;, length(y), &quot;.&quot;)
}
if (TRUE %in% is.na(x) || TRUE %in% is.na(y)) {
stop(&quot; Arguments x and y must not have missing values.&quot;)
}
covariance &lt;- var(x, y)
if (verbose)
cat(&quot;Covariance = &quot;, round(covariance, 4), &quot;.\n&quot;, sep = &quot;&quot;)
return(covariance)
}
</code></pre>
<h4 id="todo"><li> TODO Style </li></h4>
<p> Use a consistent style for TODOs throughout your code.
<br><CODE>TODO(username): Explicit description of action to
be taken</CODE>
</ul>
<!---------------------------------------------------------------------------->
<h3><li> Language </li></h3>
<ul class="NoBullet">
<h4 id="attach"><li> Attach </li></h4>
<p> The possibilities for creating errors when using
<code>attach</code> are numerous. Avoid it.
<h4 id="functionlanguage"><li>Functions </li></h4>
<p> Errors should be raised using <code>stop()</code>.
<h4 id="object"><li>Objects and Methods</li></h4>
<p> The S language has two object systems, S3 and S4, both of which
are available in R. S3 methods are more interactive and flexible,
whereas S4 methods are more formal and rigorous. (For an illustration
of the two systems, see Thomas Lumley's
"Programmer's Niche: A Simple
Class, in S3 and S4" in R News 4/1, 2004, pgs. 33 - 36:
<a href="http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pdf">
http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pdf</a>.)
<p>Use S3 objects and methods unless there is a strong reason to use
S4 objects or methods. A primary justification for an S4 object
would be to use objects directly in C++ code. A primary
justification for an S4 generic/method would be to dispatch on two
arguments.
<p>Avoid mixing S3 and S4: S4 methods ignore S3 inheritance and
vice-versa.
</ul>
<!---------------------------------------------------------------------------->
<h3><li> Exceptions </li></h3>
The coding conventions described above should be followed, unless
there is good reason to do otherwise. Exceptions include
legacy code and modifying third-party code.
<!---------------------------------------------------------------------------->
<h3><li> Parting Words </li></h3>
Use common sense and BE CONSISTENT.
<p>
If you are editing code, take a few minutes to look at the code around
you and determine its style. If others use spaces around their
<code>if </code>
clauses, you should, too. If their comments have little boxes of stars
around them, make your comments have little boxes of stars around them,
too.
<p>
The point of having style guidelines is to have a common vocabulary of
coding so people can concentrate on <em>what</em> you are saying,
rather than on <em>how</em> you are saying it. We present global style
rules here so people
know the vocabulary. But local style is also important. If code you add
to a file looks drastically different from the existing code around it,
the discontinuity will throw readers out of their rhythm when they go to
read it. Try to avoid this.
OK, enough writing about writing code; the code itself is much more
interesting. Have fun!
<!---------------------------------------------------------------------------->
<h3><li> References </li></h3>
<a href="http://www.maths.lth.se/help/R/RCC/">
http://www.maths.lth.se/help/R/RCC/</a> - R Coding Conventions
<br>
<a href="http://ess.r-project.org/">http://ess.r-project.org/</a> - For
emacs users. This runs R in your emacs and has an emacs mode.
</ol>
<!---------------------------------------------------------------------------->
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