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<title>Roman Numerals</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.qi.tutorials.roman_numerals"></a><a class="link" href="roman_numerals.html" title="Roman Numerals">Roman Numerals</a>
</h4></div></div></div>
<p>
This example demonstrates:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
symbol table
</li>
<li class="listitem">
rule
</li>
<li class="listitem">
grammar
</li>
</ul></div>
<a name="spirit.qi.tutorials.roman_numerals.symbol_table"></a><h6>
<a name="id806131"></a>
<a class="link" href="roman_numerals.html#spirit.qi.tutorials.roman_numerals.symbol_table">Symbol
Table</a>
</h6>
<p>
The symbol table holds a dictionary of symbols where each symbol is a sequence
of characters (a <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">wchar_t</span></code>, <code class="computeroutput"><span class="keyword">int</span></code>,
enumeration etc.) . The template class, parameterized by the character
type, can work efficiently with 8, 16, 32 and even 64 bit characters. Mutable
data of type T are associated with each symbol.
</p>
<p>
Traditionally, symbol table management is maintained separately outside
the BNF grammar through semantic actions. Contrary to standard practice,
the Spirit symbol table class <code class="computeroutput"><span class="identifier">symbols</span></code>
is a parser. An object of which may be used anywhere in the EBNF grammar
specification. It is an example of a dynamic parser. A dynamic parser is
characterized by its ability to modify its behavior at run time. Initially,
an empty symbols object matches nothing. At any time, symbols may be added
or removed, thus, dynamically altering its behavior.
</p>
<p>
Each entry in a symbol table has an associated mutable data slot. In this
regard, one can view the symbol table as an associative container (or map)
of key-value pairs where the keys are strings.
</p>
<p>
The symbols class expects two template parameters. The first parameter
specifies the character type of the symbols. The second specifies the data
type associated with each symbol: its attribute.
</p>
<p>
Here's a parser for roman hundreds (100..900) using the symbol table. Keep
in mind that the data associated with each slot is the parser's attribute
(which is passed to attached semantic actions).
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">hundreds_</span> <span class="special">:</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">symbols</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="identifier">hundreds_</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">add</span>
<span class="special">(</span><span class="string">"C"</span> <span class="special">,</span> <span class="number">100</span><span class="special">)</span>
<span class="special">(</span><span class="string">"CC"</span> <span class="special">,</span> <span class="number">200</span><span class="special">)</span>
<span class="special">(</span><span class="string">"CCC"</span> <span class="special">,</span> <span class="number">300</span><span class="special">)</span>
<span class="special">(</span><span class="string">"CD"</span> <span class="special">,</span> <span class="number">400</span><span class="special">)</span>
<span class="special">(</span><span class="string">"D"</span> <span class="special">,</span> <span class="number">500</span><span class="special">)</span>
<span class="special">(</span><span class="string">"DC"</span> <span class="special">,</span> <span class="number">600</span><span class="special">)</span>
<span class="special">(</span><span class="string">"DCC"</span> <span class="special">,</span> <span class="number">700</span><span class="special">)</span>
<span class="special">(</span><span class="string">"DCCC"</span> <span class="special">,</span> <span class="number">800</span><span class="special">)</span>
<span class="special">(</span><span class="string">"CM"</span> <span class="special">,</span> <span class="number">900</span><span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span> <span class="identifier">hundreds</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Here's a parser for roman tens (10..90):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">tens_</span> <span class="special">:</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">symbols</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="identifier">tens_</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">add</span>
<span class="special">(</span><span class="string">"X"</span> <span class="special">,</span> <span class="number">10</span><span class="special">)</span>
<span class="special">(</span><span class="string">"XX"</span> <span class="special">,</span> <span class="number">20</span><span class="special">)</span>
<span class="special">(</span><span class="string">"XXX"</span> <span class="special">,</span> <span class="number">30</span><span class="special">)</span>
<span class="special">(</span><span class="string">"XL"</span> <span class="special">,</span> <span class="number">40</span><span class="special">)</span>
<span class="special">(</span><span class="string">"L"</span> <span class="special">,</span> <span class="number">50</span><span class="special">)</span>
<span class="special">(</span><span class="string">"LX"</span> <span class="special">,</span> <span class="number">60</span><span class="special">)</span>
<span class="special">(</span><span class="string">"LXX"</span> <span class="special">,</span> <span class="number">70</span><span class="special">)</span>
<span class="special">(</span><span class="string">"LXXX"</span> <span class="special">,</span> <span class="number">80</span><span class="special">)</span>
<span class="special">(</span><span class="string">"XC"</span> <span class="special">,</span> <span class="number">90</span><span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span> <span class="identifier">tens</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
and, finally, for ones (1..9):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ones_</span> <span class="special">:</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">symbols</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="identifier">ones_</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">add</span>
<span class="special">(</span><span class="string">"I"</span> <span class="special">,</span> <span class="number">1</span><span class="special">)</span>
<span class="special">(</span><span class="string">"II"</span> <span class="special">,</span> <span class="number">2</span><span class="special">)</span>
<span class="special">(</span><span class="string">"III"</span> <span class="special">,</span> <span class="number">3</span><span class="special">)</span>
<span class="special">(</span><span class="string">"IV"</span> <span class="special">,</span> <span class="number">4</span><span class="special">)</span>
<span class="special">(</span><span class="string">"V"</span> <span class="special">,</span> <span class="number">5</span><span class="special">)</span>
<span class="special">(</span><span class="string">"VI"</span> <span class="special">,</span> <span class="number">6</span><span class="special">)</span>
<span class="special">(</span><span class="string">"VII"</span> <span class="special">,</span> <span class="number">7</span><span class="special">)</span>
<span class="special">(</span><span class="string">"VIII"</span> <span class="special">,</span> <span class="number">8</span><span class="special">)</span>
<span class="special">(</span><span class="string">"IX"</span> <span class="special">,</span> <span class="number">9</span><span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span> <span class="identifier">ones</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Now we can use <code class="computeroutput"><span class="identifier">hundreds</span></code>,
<code class="computeroutput"><span class="identifier">tens</span></code> and <code class="computeroutput"><span class="identifier">ones</span></code> anywhere in our parser expressions.
They are all parsers.
</p>
<a name="spirit.qi.tutorials.roman_numerals.rules"></a><h6>
<a name="id807800"></a>
<a class="link" href="roman_numerals.html#spirit.qi.tutorials.roman_numerals.rules">Rules</a>
</h6>
<p>
Up until now, we've been inlining our parser expressions, passing them
directly to the <code class="computeroutput"><span class="identifier">phrase_parse</span></code>
function. The expression evaluates into a temporary, unnamed parser which
is passed into the <code class="computeroutput"><span class="identifier">phrase_parse</span></code>
function, used, and then destroyed. This is fine for small parsers. When
the expressions get complicated, you'd want to break the expressions into
smaller easier-to-understand pieces, name them, and refer to them from
other parser expressions by name.
</p>
<p>
A parser expression can be assigned to what is called a "rule".
There are various ways to declare rules. The simplest form is:
</p>
<pre class="programlisting"><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">;</span>
</pre>
<p>
At the very least, the rule needs to know the iterator type it will be
working on. This rule cannot be used with <code class="computeroutput"><span class="identifier">phrase_parse</span></code>.
It can only be used with the <code class="computeroutput"><span class="identifier">parse</span></code>
function -- a version that does not do white space skipping (does not have
the skipper argument). If you want to have it skip white spaces, you need
to pass in the type skip parser, as in the next form:
</p>
<pre class="programlisting"><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Skipper</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">;</span>
</pre>
<p>
Example:
</p>
<pre class="programlisting"><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span><span class="special">,</span> <span class="identifier">space_type</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">;</span>
</pre>
<p>
This type of rule can be used for both <code class="computeroutput"><span class="identifier">phrase_parse</span></code>
and <code class="computeroutput"><span class="identifier">parse</span></code>.
</p>
<p>
For our next example, there's one more rule form you should know about:
</p>
<pre class="programlisting"><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Signature</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">;</span>
</pre>
<p>
or
</p>
<pre class="programlisting"><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Signature</span><span class="special">,</span> <span class="identifier">Skipper</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">;</span>
</pre>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
All rule template arguments after Iterator can be supplied in any order.
</p></td></tr>
</table></div>
<p>
The Signature specifies the attributes of the rule. You've seen that our
parsers can have an attribute. Recall that the <code class="computeroutput"><span class="identifier">double_</span></code>
parser has an attribute of <code class="computeroutput"><span class="keyword">double</span></code>.
To be precise, these are <span class="emphasis"><em>synthesized</em></span> attributes. The
parser "synthesizes" the attribute value. Think of them as function
return values.
</p>
<p>
There's another type of attribute called "inherited" attribute.
We won't need them for now, but it's good that you be aware of such attributes.
You can think of them as function arguments. And, rightly so, the rule
signature is a function signature of the form:
</p>
<pre class="programlisting"><span class="identifier">result</span><span class="special">(</span><span class="identifier">argN</span><span class="special">,</span> <span class="identifier">argN</span><span class="special">,...,</span> <span class="identifier">argN</span><span class="special">)</span>
</pre>
<p>
After having declared a rule, you can now assign any parser expression
to it. Example:
</p>
<pre class="programlisting"><span class="identifier">r</span> <span class="special">=</span> <span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="special">*(</span><span class="char">','</span> <span class="special">&gt;&gt;</span> <span class="identifier">double_</span><span class="special">);</span>
</pre>
<a name="spirit.qi.tutorials.roman_numerals.grammars"></a><h6>
<a name="id808192"></a>
<a class="link" href="roman_numerals.html#spirit.qi.tutorials.roman_numerals.grammars">Grammars</a>
</h6>
<p>
A grammar encapsulates one or more rules. It has the same template parameters
as the rule. You declare a grammar by:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
deriving a struct (or class) from the <code class="computeroutput"><span class="identifier">grammar</span></code>
class template
</li>
<li class="listitem">
declare one or more rules as member variables
</li>
<li class="listitem">
initialize the base grammar class by giving it the start rule (its
the first rule that gets called when the grammar starts parsing)
</li>
<li class="listitem">
initialize your rules in your constructor
</li>
</ol></div>
<p>
The roman numeral grammar is a very nice and simple example of a grammar:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">roman</span> <span class="special">:</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">grammar</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">()&gt;</span>
<span class="special">{</span>
<span class="identifier">roman</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">roman</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">(</span><span class="identifier">start</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">eps</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">lit</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">_val</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">;</span>
<span class="identifier">start</span> <span class="special">=</span> <span class="identifier">eps</span> <span class="special">[</span><span class="identifier">_val</span> <span class="special">=</span> <span class="number">0</span><span class="special">]</span> <span class="special">&gt;&gt;</span>
<span class="special">(</span>
<span class="special">+</span><span class="identifier">lit</span><span class="special">(</span><span class="char">'M'</span><span class="special">)</span> <span class="special">[</span><span class="identifier">_val</span> <span class="special">+=</span> <span class="number">1000</span><span class="special">]</span>
<span class="special">||</span> <span class="identifier">hundreds</span> <span class="special">[</span><span class="identifier">_val</span> <span class="special">+=</span> <span class="identifier">_1</span><span class="special">]</span>
<span class="special">||</span> <span class="identifier">tens</span> <span class="special">[</span><span class="identifier">_val</span> <span class="special">+=</span> <span class="identifier">_1</span><span class="special">]</span>
<span class="special">||</span> <span class="identifier">ones</span> <span class="special">[</span><span class="identifier">_val</span> <span class="special">+=</span> <span class="identifier">_1</span><span class="special">]</span>
<span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">unsigned</span><span class="special">()&gt;</span> <span class="identifier">start</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Things to take notice of:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
The grammar and start rule signature is <code class="computeroutput"><span class="keyword">unsigned</span><span class="special">()</span></code>. It has a synthesized attribute (return
value) of type <code class="computeroutput"><span class="keyword">unsigned</span></code>
with no inherited attributes (arguments).
</li>
<li class="listitem">
We did not specify a skip-parser. We don't want to skip in between
the numerals.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">roman</span><span class="special">::</span><span class="identifier">base_type</span></code> is a typedef for <code class="computeroutput"><span class="identifier">grammar</span><span class="special">&lt;</span><span class="identifier">Iterator</span><span class="special">,</span>
<span class="keyword">unsigned</span><span class="special">()&gt;</span></code>.
If <code class="computeroutput"><span class="identifier">roman</span></code> was not a
template, you could simply write: base_type(start)
</li>
<li class="listitem">
It's best to make your grammar templates such that they can be reused
for different iterator types.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">_val</span></code> is another <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> placeholder
representing the rule's synthesized attribute.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">eps</span></code> is a special spirit
parser that consumes no input but is always successful. We use it to
initialize <code class="computeroutput"><span class="identifier">_val</span></code>, the
rule's synthesized attribute, to zero before anything else. The actual
parser starts at <code class="computeroutput"><span class="special">+</span><span class="identifier">char_</span><span class="special">(</span><span class="char">'M'</span><span class="special">)</span></code>,
parsing roman thousands. Using <code class="computeroutput"><span class="identifier">eps</span></code>
this way is good for doing pre and post initializations.
</li>
<li class="listitem">
The expression <code class="computeroutput"><span class="identifier">a</span> <span class="special">||</span>
<span class="identifier">b</span></code> reads: match a or b and
in sequence. That is, if both <code class="computeroutput"><span class="identifier">a</span></code>
and <code class="computeroutput"><span class="identifier">b</span></code> match, it must
be in sequence; this is equivalent to <code class="computeroutput"><span class="identifier">a</span>
<span class="special">&gt;&gt;</span> <span class="special">-</span><span class="identifier">b</span> <span class="special">|</span> <span class="identifier">b</span></code>, but more efficient.
</li>
</ul></div>
<a name="spirit.qi.tutorials.roman_numerals.let_s_parse_"></a><h6>
<a name="id808955"></a>
<a class="link" href="roman_numerals.html#spirit.qi.tutorials.roman_numerals.let_s_parse_">Let's Parse!</a>
</h6>
<p>
</p>
<pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">parse</span><span class="special">(</span><span class="identifier">iter</span><span class="special">,</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">roman_parser</span><span class="special">,</span> <span class="identifier">result</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">r</span> <span class="special">&amp;&amp;</span> <span class="identifier">iter</span> <span class="special">==</span> <span class="identifier">end</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"-------------------------\n"</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Parsing succeeded\n"</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"result = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"-------------------------\n"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">rest</span><span class="special">(</span><span class="identifier">iter</span><span class="special">,</span> <span class="identifier">end</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"-------------------------\n"</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Parsing failed\n"</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"stopped at: \": "</span> <span class="special">&lt;&lt;</span> <span class="identifier">rest</span> <span class="special">&lt;&lt;</span> <span class="string">"\"\n"</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"-------------------------\n"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
<code class="computeroutput"><span class="identifier">roman_parser</span></code> is an object
of type <code class="computeroutput"><span class="identifier">roman</span></code>, our roman
numeral parser. This time around we are using the no-skipping version of
the parse functions. We do not want to skip any spaces! We are also passing
in an attribute, <code class="computeroutput"><span class="keyword">unsigned</span> <span class="identifier">result</span></code>, which will receive the parsed
value.
</p>
<p>
The full cpp file for this example can be found here: <a href="../../../../../example/qi/roman.cpp" target="_top">../../example/qi/roman.cpp</a>
</p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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