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<div class="section" id="count-if">
<h1><a class="toc-backref" href="./querying-algorithms.html#id1479">count_if</a></h1>
<div class="section" id="id567">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    , typename Pred
    &gt;
struct <a href="./count-if.html" class="identifier">count_if</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
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<div class="section" id="id568">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns the number of elements in <tt class="literal"><span class="pre">Sequence</span></tt> that satisfy the predicate <tt class="literal"><span class="pre">Pred</span></tt>.</p>
</div>
<div class="section" id="id569">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/count_if.hpp" class="header">boost/mpl/count_if.hpp</a>&gt;
</pre>
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<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
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<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
<td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td>
<td>A sequence to be examined.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">Pred</span></tt></td>
<td>Unary <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a></td>
<td>A count condition.</td>
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<div class="section" id="id571">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt> and unary <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a> <tt class="literal"><span class="pre">pred</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./count-if.html" class="identifier">count_if</a>&lt;s,pred&gt;::type n;
</pre>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first"><a class="reference internal" href="./integral-constant.html">Integral Constant</a>.</p>
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<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
<pre class="last literal-block">
typedef <a href="./lambda.html" class="identifier">lambda</a>&lt;pred&gt;::type p;
typedef <a href="./fold.html" class="identifier">fold</a>&lt;
      s
    , <a href="./long.html" class="identifier">long_</a>&lt;0&gt;
    , <a href="./if.html" class="identifier">if_</a>&lt; <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">1</span></tt>&lt;p,<a href="./placeholders.html" class="identifier">_2</a>&gt;, <a href="./next.html" class="identifier">next</a>&lt;<a href="./placeholders.html" class="identifier">_1</a>&gt;, <a href="./placeholders.html" class="identifier">_1</a> &gt;
    &gt;::type n;
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<div class="section" id="id572">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Linear. Exactly <tt class="literal"><span class="pre"><a href="./size.html" class="identifier">size</a>&lt;s&gt;::value</span></tt> applications of <tt class="literal"><span class="pre">pred</span></tt>.</p>
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<div class="section" id="id573">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./vector.html" class="identifier">vector</a>&lt;int,char,long,short,char,long,double,long&gt; types;

<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./count-if.html" class="identifier">count_if</a>&lt; types, is_float&lt;_&gt; &gt;::value), ==, 1 );
<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./count-if.html" class="identifier">count_if</a>&lt; types, is_same&lt;_,char&gt; &gt;::value), ==, 2 );
<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./count-if.html" class="identifier">count_if</a>&lt; types, is_same&lt;_,void&gt; &gt;::value), ==, 0 );
</pre>
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<div class="section" id="id574">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./querying-algorithms.html">Querying Algorithms</a>, <a class="reference internal" href="./count.html">count</a>, <a class="reference internal" href="./find.html">find</a>, <a class="reference internal" href="./find-if.html">find_if</a>, <a class="reference internal" href="./contains.html">contains</a></p>
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