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<title>Bison 3.0.4: Rpcalc Input</title>

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<body lang="en">
<a name="Rpcalc-Input"></a>
<div class="header">
<p>
Next: <a href="Rpcalc-Line.html#Rpcalc-Line" accesskey="n" rel="next">Rpcalc Line</a>, Up: <a href="Rpcalc-Rules.html#Rpcalc-Rules" accesskey="u" rel="up">Rpcalc Rules</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="Explanation-of-input"></a>
<h4 class="subsubsection">2.1.2.1 Explanation of <code>input</code></h4>

<p>Consider the definition of <code>input</code>:
</p>
<div class="example">
<pre class="example">input:
  %empty
| input line
;
</pre></div>

<p>This definition reads as follows: &ldquo;A complete input is either an empty
string, or a complete input followed by an input line&rdquo;.  Notice that
&ldquo;complete input&rdquo; is defined in terms of itself.  This definition is said
to be <em>left recursive</em> since <code>input</code> appears always as the
leftmost symbol in the sequence.  See <a href="Recursion.html#Recursion">Recursive Rules</a>.
</p>
<p>The first alternative is empty because there are no symbols between the
colon and the first &lsquo;<samp>|</samp>&rsquo;; this means that <code>input</code> can match an
empty string of input (no tokens).  We write the rules this way because it
is legitimate to type <kbd>Ctrl-d</kbd> right after you start the calculator.
It&rsquo;s conventional to put an empty alternative first and to use the
(optional) <code>%empty</code> directive, or to write the comment &lsquo;<samp>/* empty
*/</samp>&rsquo; in it (see <a href="Empty-Rules.html#Empty-Rules">Empty Rules</a>).
</p>
<p>The second alternate rule (<code>input line</code>) handles all nontrivial input.
It means, &ldquo;After reading any number of lines, read one more line if
possible.&rdquo;  The left recursion makes this rule into a loop.  Since the
first alternative matches empty input, the loop can be executed zero or
more times.
</p>
<p>The parser function <code>yyparse</code> continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.
</p>



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