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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
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<head>
<title>Bison 3.0.2: Actions and Locations</title>

<meta name="description" content="Bison 3.0.2: Actions and Locations">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Tracking-Locations.html#Tracking-Locations" rel="up" title="Tracking Locations">
<link href="Location-Default-Action.html#Location-Default-Action" rel="next" title="Location Default Action">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Actions-and-Locations"></a>
<div class="header">
<p>
Next: <a href="Location-Default-Action.html#Location-Default-Action" accesskey="n" rel="next">Location Default Action</a>, Previous: <a href="Location-Type.html#Location-Type" accesskey="p" rel="prev">Location Type</a>, Up: <a href="Tracking-Locations.html#Tracking-Locations" accesskey="u" rel="up">Tracking Locations</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Terms.html#Index-of-Terms" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Actions-and-Locations-1"></a>
<h4 class="subsection">3.5.2 Actions and Locations</h4>
<a name="index-location-actions"></a>
<a name="index-actions_002c-location"></a>
<a name="index-_0040_0024"></a>
<a name="index-_0040n-2"></a>
<a name="index-_0040name"></a>
<a name="index-_0040_005bname_005d"></a>

<p>Actions are not only useful for defining language semantics, but also for
describing the behavior of the output parser with locations.
</p>
<p>The most obvious way for building locations of syntactic groupings is very
similar to the way semantic values are computed.  In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the <var>n</var>th component of the right hand side is
<code>@<var>n</var></code>, while the location of the left hand side grouping is
<code>@$</code>.
</p>
<p>In addition, the named references construct <code>@<var>name</var></code> and
<code>@[<var>name</var>]</code> may also be used to address the symbol locations.
See <a href="Named-References.html#Named-References">Named References</a>, for more information about using the named
references construct.
</p>
<p>Here is a basic example using the default data type for locations:
</p>
<div class="example">
<pre class="example">exp:
  &hellip;
| exp '/' exp
    {
      @$.first_column = @1.first_column;
      @$.first_line = @1.first_line;
      @$.last_column = @3.last_column;
      @$.last_line = @3.last_line;
      if ($3)
        $$ = $1 / $3;
      else
        {
          $$ = 1;
          fprintf (stderr, &quot;%d.%d-%d.%d: division by zero&quot;,
                   @3.first_line, @3.first_column,
                   @3.last_line, @3.last_column);
        }
    }
</pre></div>

<p>As for semantic values, there is a default action for locations that is
run each time a rule is matched.  It sets the beginning of <code>@$</code> to the
beginning of the first symbol, and the end of <code>@$</code> to the end of the
last symbol.
</p>
<p>With this default action, the location tracking can be fully automatic.  The
example above simply rewrites this way:
</p>
<div class="example">
<pre class="example">exp:
  &hellip;
| exp '/' exp
    {
      if ($3)
        $$ = $1 / $3;
      else
        {
          $$ = 1;
          fprintf (stderr, &quot;%d.%d-%d.%d: division by zero&quot;,
                   @3.first_line, @3.first_column,
                   @3.last_line, @3.last_column);
        }
    }
</pre></div>

<a name="index-yylloc-3"></a>
<p>It is also possible to access the location of the lookahead token, if any,
from a semantic action.
This location is stored in <code>yylloc</code>.
See <a href="Action-Features.html#Action-Features">Special Features for Use in Actions</a>.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Location-Default-Action.html#Location-Default-Action" accesskey="n" rel="next">Location Default Action</a>, Previous: <a href="Location-Type.html#Location-Type" accesskey="p" rel="prev">Location Type</a>, Up: <a href="Tracking-Locations.html#Tracking-Locations" accesskey="u" rel="up">Tracking Locations</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Terms.html#Index-of-Terms" title="Index" rel="index">Index</a>]</p>
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