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29<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">phoenix</span><span class="special">/</span><span class="identifier">statement</span><span class="special">/</span><span class="keyword">for</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
30</pre>
31<p>
32          The syntax is:
33        </p>
34<pre class="programlisting"><span class="identifier">for_</span><span class="special">(</span><span class="identifier">init_statement</span><span class="special">,</span> <span class="identifier">conditional_expression</span><span class="special">,</span> <span class="identifier">step_statement</span><span class="special">)</span>
35<span class="special">[</span>
36    <span class="identifier">sequenced_statements</span>
37<span class="special">]</span>
38</pre>
39<p>
40          It is again very similar to the C++ for statement. Take note that the init_statement,
41          conditional_expression and step_statement are separated by the comma instead of the semi-colon
42          and each must be present (i.e. <code class="computeroutput"><span class="identifier">for_</span><span class="special">(,,)</span></code> is invalid). This is a case where the
43          <a class="link" href="../core/nothing.html" title="Nothing"><code class="computeroutput"><span class="identifier">nothing</span></code></a>
44          actor can be useful.
45        </p>
46<p>
47          Example: This code prints each element N times where N is the element's
48          value. A newline terminates the printout of each value.
49        </p>
50<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">iii</span><span class="special">;</span>
51<span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span>
52    <span class="special">(</span>
53        <span class="identifier">for_</span><span class="special">(</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">iii</span><span class="special">)</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">iii</span><span class="special">)</span> <span class="special">&lt;</span> <span class="identifier">arg1</span><span class="special">,</span> <span class="special">++</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">iii</span><span class="special">))</span>
54        <span class="special">[</span>
55            <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">arg1</span> <span class="special">&lt;&lt;</span> <span class="string">", "</span>
56        <span class="special">],</span>
57        <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">(</span><span class="string">"\n"</span><span class="special">)</span>
58    <span class="special">)</span>
59<span class="special">);</span>
60</pre>
61<p>
62          As before, all these are lazily evaluated. The result of such statements
63          are in fact expressions that are passed on to STL's for_each function.
64          In the viewpoint of <code class="computeroutput"><span class="identifier">for_each</span></code>,
65          what was passed is just a functor, no more, no less.
66        </p>
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70<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
71        Distributed under the Boost Software License, Version 1.0. (See accompanying
72        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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