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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>

    
    <h2>Summary: R Style Rules</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>
        (or <code>variableName</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="#else">else</a>: Surround else with braces </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>Summary: R Language Rules</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>

    <h3>Notation and Naming</h3>

    <h4 id="filenames">File Names</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">Identifiers</h4>
          <p>
            Don't use underscores ( <code>_</code> ) or hyphens
            ( <code>-</code> ) in identifiers.
            Identifiers should be named according to the following conventions.
            The preferred form for variable names is all lower case
            letters and words separated with dots
            (<code>variable.name</code>), but <code>variableName</code>
            is also accepted;
            function names have initial capital letters and no dots
            (<code>FunctionName</code>);
            constants are named like functions but with an initial
            <code>k</code>.
          </p>
          <ul>
            <li><code>variable.name</code> is preferred, <code>variableName</code> is accepted
              <br/> GOOD: <code>avg.clicks</code>
              <br/> OK: <code>avgClicks</code>
              <br/> BAD: <code><span style="color:red">avg_Clicks</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>
            <li><code>kConstantName </code></li>
          </ul>


      <h3>Syntax</h3>

      <h4 id="linelength">Line Length</h4>
<p>
  The maximum line length is 80 characters.
</p>

          <h4 id="indentation">Indentation</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">Spacing</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:
          </p>
<pre>tab.prior &lt;- table(df[df$days.from.opt &lt; 0, "campaign.id"])
total &lt;- sum(x[, 1])
total &lt;- sum(x[1, ])</pre>
          <p>
            BAD:
          </p>
<pre><span style="color:red">tab.prior &lt;- table(df[df$days.from.opt&lt;0, "campaign.id"])  # Needs spaces around '&lt;'
tab.prior &lt;- table(df[df$days.from.opt &lt; 0,"campaign.id"])  # Needs a space after the comma
tab.prior&lt;- table(df[df$days.from.opt &lt; 0, "campaign.id"])  # Needs a space before &lt;-
tab.prior&lt;-table(df[df$days.from.opt &lt; 0, "campaign.id"])  # 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>
</pre>
          <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>&lt;-</code>).
          </p>
<pre>plot(x    = x.coord,
     y    = data.mat[, MakeColName(metric, ptiles[1], "roiOpt")],
     ylim = ylim,
     xlab = "dates",
     ylab = metric,
     main = (paste(metric, " for 3 samples ", sep = "")))
</pre>
          <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:</p><pre>if (debug)
x[1, ]</pre>
          <p>
            BAD:</p><pre><span style="color:red">if ( debug )  # No spaces around debug
x[1,]  # Needs a space after the comma </span></pre>

          <h4 id="curlybraces">Curly Braces</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>
          <pre>
if (is.null(ylim)) {
  ylim &lt;- c(0, 0.06)
}</pre>
          <p>
            xor (but not both)
          </p>
          <pre>
if (is.null(ylim))
  ylim &lt;- c(0, 0.06)</pre>
          <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="else">Surround else with braces</h4>
    <p>
            An <code>else</code> statement should always be surrounded on the
            same line by curly braces.</p>
          <pre>
if (condition) {
  one or more lines
} else {
  one or more lines
}
</pre>
          <p>
            BAD:<br/>
          </p>
          <pre style="color:red">
if (condition) {
  one or more lines
}
else {
  one or more lines
}
</pre>
          <p>
            BAD:<br/>
          </p>
          <pre style="color:red">
if (condition)
  one line
else
  one line
</pre>

        <h4 id="assignment">Assignment</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">Semicolons</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>


    <h3> Organization </h3>
        <h4 id="generallayout">General Layout and Ordering</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>
            <li>Author comment</li>
            <li>File description comment, including purpose of
              program, inputs, and outputs</li>
            <li><code>source()</code> and <code>library()</code> statements</li>
            <li>Function definitions</li>
            <li>Executed statements, if applicable (e.g.,
              <code> print</code>, <code>plot</code>)</li>
          </ol>
          <p>
            Unit tests should go in a separate file named
            <code>originalfilename_test.R</code>.
          </p>
        <h4 id="comments">Commenting Guidelines</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># Create histogram of frequency of campaigns by pct budget spent.
hist(df$pct.spent,
     breaks = "scott",  # method for choosing number of buckets
     main   = "Histogram: fraction budget spent by campaignid",
     xlab   = "Fraction of budget spent",
     ylab   = "Frequency (count of campaignids)")
</pre>
        <h4 id="functiondefinition">Function Definitions and
          Calls</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:
          </p>
<pre>PredictCTR &lt;- function(query, property, num.days,
                       show.plot = TRUE)
</pre>
           BAD:
<pre><span style="color:red">PredictCTR &lt;- function(query, property, num.days, show.plot =
                       TRUE)
</span></pre>
          <p> Ideally, unit tests should serve as sample function calls (for
            shared library routines).
          </p>
        <h4 id="functiondocumentation">Function Documentation</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.
          </p>

        <h4 id="examplefunction">Example Function</h4>
<pre>
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("Arguments x and y have different lengths: ",
         length(x), " and ", length(y), ".")
  }
  if (TRUE %in% is.na(x) || TRUE %in% is.na(y)) {
    stop(" Arguments x and y must not have missing values.")
  }
  covariance &lt;- var(x, y)
  if (verbose)
    cat("Covariance = ", round(covariance, 4), ".\n", sep = "")
  return(covariance)
}
</pre>

<h4 id="todo">TODO Style</h4>

<p>
  Use a consistent style for TODOs throughout your code.
  <br/><code>TODO(username): Explicit description of action to
    be taken</code>
</p>


      <h3> Language </h3>

        <h4 id="attach">Attach</h4>
        <p> The possibilities for creating errors when using
          <code>attach</code> are numerous. Avoid it.</p>
        <h4 id="functionlanguage">Functions</h4>
        <p> Errors should be raised using <code>stop()</code>.</p>
        <h4 id="object">Objects and Methods</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>
          <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>
          <p>Avoid mixing S3 and S4: S4 methods ignore S3 inheritance and
            vice-versa.
          </p>


     <h3>Exceptions</h3>

<p>
  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.
</p>


<h3>Parting Words</h3>

<p>
  Use common sense and BE CONSISTENT.
</p>
     <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>
     <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.
     </p>
     
     <p>
     OK, enough writing about writing code; the code itself is much more
     interesting. Have fun!
     </p>


     <h3>References</h3>

     <p>
     <a href="http://www.maths.lth.se/help/R/RCC/">
     http://www.maths.lth.se/help/R/RCC/</a> - R Coding Conventions
     </p>
     <p>
     <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.
     </p>

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