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<title>Sum - adding numbers</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="spirit_x3.tutorials.sum___adding_numbers"></a><a class="link" href="sum___adding_numbers.html" title="Sum - adding numbers">Sum - adding
      numbers</a>
</h3></div></div></div>
<p>
        Here's a parser that sums a comma-separated list of numbers.
      </p>
<p>
        Ok we've glossed over some details in our previous examples. First, our includes:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">home</span><span class="special">/</span><span class="identifier">x3</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
</pre>
<p>
        Then some using directives:
      </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">x3</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">x3</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">ascii</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">x3</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">;</span>

<span class="keyword">using</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">double_</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">space</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">_attr</span><span class="special">;</span>
</pre>
<p>
        Now the actual parser:
      </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">bool</span> <span class="identifier">adder</span><span class="special">(</span><span class="identifier">Iterator</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">Iterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&amp;</span> <span class="identifier">n</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">auto</span> <span class="identifier">assign</span> <span class="special">=</span> <span class="special">[&amp;](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">){</span> <span class="identifier">n</span> <span class="special">=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">);</span> <span class="special">};</span>
    <span class="keyword">auto</span> <span class="identifier">add</span> <span class="special">=</span> <span class="special">[&amp;](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">){</span> <span class="identifier">n</span> <span class="special">+=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">);</span> <span class="special">};</span>

    <span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span>

        <span class="comment">//  Begin grammar</span>
        <span class="special">(</span>
            <span class="identifier">double_</span><span class="special">[</span><span class="identifier">assign</span><span class="special">]</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><span class="identifier">add</span><span class="special">])</span>
        <span class="special">)</span>
        <span class="special">,</span>
        <span class="comment">//  End grammar</span>

        <span class="identifier">space</span><span class="special">);</span>

    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">first</span> <span class="special">!=</span> <span class="identifier">last</span><span class="special">)</span> <span class="comment">// fail if we did not get a full match</span>
        <span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
    <span class="keyword">return</span> <span class="identifier">r</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        The full cpp file for this example can be found here: <a href="../../../../../example/x3/sum.cpp" target="_top">sum.cpp</a>
      </p>
<p>
        This is almost like our original numbers list example. We're incrementally
        building on top of our examples. This time though, like in the complex number
        example, we'll be adding the smarts. There's an accumulator (<code class="computeroutput"><span class="keyword">double</span><span class="special">&amp;</span> <span class="identifier">n</span></code>) that adds the numbers parsed. On a successful
        parse, this number is the sum of all the parsed numbers.
      </p>
<p>
        The first <code class="computeroutput"><span class="identifier">double_</span></code> parser
        attaches this action:
      </p>
<pre class="programlisting"><span class="special">[&amp;](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">){</span> <span class="identifier">n</span> <span class="special">=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">);</span> <span class="special">}</span>
</pre>
<p>
        This assigns the parsed result (actually, the attribute of <code class="computeroutput"><span class="identifier">double_</span></code>) to <code class="computeroutput"><span class="identifier">n</span></code>.
        The second <code class="computeroutput"><span class="identifier">double_</span></code> parser
        attaches this action:
      </p>
<pre class="programlisting"><span class="special">[&amp;](</span><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">ctx</span><span class="special">){</span> <span class="identifier">n</span> <span class="special">+=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">);</span> <span class="special">}</span>
</pre>
<p>
        So, subsequent numbers add into <code class="computeroutput"><span class="identifier">n</span></code>.
      </p>
<p>
        That wasn't too bad, was it :-) ?
      </p>
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