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7<title>The MPL Reference Manual: count_if</title>
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11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./count.html" class="navigation-link">Prev</a>&nbsp;<a href="./lower-bound.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./count.html" class="navigation-link">Back</a>&nbsp;<a href="./lower-bound.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./querying-algorithms.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./algorithms.html" class="navigation-link">Algorithms</a> / <a href="./querying-algorithms.html" class="navigation-link">Querying Algorithms</a> / <a href="./count-if.html" class="navigation-link">count_if</a></td>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="count-if">
15<h1><a class="toc-backref" href="./querying-algorithms.html#id407" name="count-if">count_if</a></h1>
16<div class="section" id="querying-count-synopsis">
17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
18<pre class="literal-block">
19template&lt;
20      typename Sequence
21    , typename Pred
22    &gt;
23struct <a href="./count-if.html" class="identifier">count_if</a>
24{
25    typedef <em>unspecified</em> type;
26};
27</pre>
28</div>
29<div class="section" id="querying-count-description">
30<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
31<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>
32</div>
33<div class="section" id="querying-count-header">
34<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
35<pre class="literal-block">
36#include &lt;<a href="../../../../boost/mpl/count_if.hpp" class="header">boost/mpl/count_if.hpp</a>&gt;
37</pre>
38</div>
39<div class="section" id="querying-count-parameters">
40<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
41<table border="1" class="table">
42<colgroup>
43<col width="19%" />
44<col width="38%" />
45<col width="43%" />
46</colgroup>
47<thead valign="bottom">
48<tr><th>Parameter</th>
49<th>Requirement</th>
50<th>Description</th>
51</tr>
52</thead>
53<tbody valign="top">
54<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
55<td><a class="reference" href="./forward-sequence.html">Forward Sequence</a></td>
56<td>A sequence to be examined.</td>
57</tr>
58<tr><td><tt class="literal"><span class="pre">Pred</span></tt></td>
59<td>Unary <a class="reference" href="./lambda-expression.html">Lambda Expression</a></td>
60<td>A count condition.</td>
61</tr>
62</tbody>
63</table>
64</div>
65<div class="section" id="querying-count-expression-semantics">
66<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
67<p>For any <a class="reference" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt> and unary <a class="reference" href="./lambda-expression.html">Lambda Expression</a> <tt class="literal"><span class="pre">pred</span></tt>:</p>
68<pre class="literal-block">
69typedef <a href="./count-if.html" class="identifier">count_if</a>&lt;s,pred&gt;::type n;
70</pre>
71<table class="field-list" frame="void" rules="none">
72<col class="field-name" />
73<col class="field-body" />
74<tbody valign="top">
75<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><a class="reference" href="./integral-constant.html">Integral Constant</a>.</td>
76</tr>
77<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
78<pre class="last literal-block">
79typedef <a href="./lambda.html" class="identifier">lambda</a>&lt;pred&gt;::type p;
80typedef <a href="./fold.html" class="identifier">fold</a>&lt; 
81      s
82    , <a href="./long.html" class="identifier">long_</a>&lt;0&gt;
83    , <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,_2&gt;, <a href="./next.html" class="identifier">next</a>&lt;_1&gt;, _1 &gt;
84    &gt;::type n;
85</pre>
86</td>
87</tr>
88</tbody>
89</table>
90</div>
91<div class="section" id="querying-count-complexity">
92<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
93<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>
94</div>
95<div class="section" id="querying-count-example">
96<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
97<pre class="literal-block">
98typedef <a href="./vector.html" class="identifier">vector</a>&lt;int,char,long,short,char,long,double,long&gt; types;
99   
100<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 );
101<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 );
102<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 );
103</pre>
104</div>
105<div class="section" id="querying-count-see-also">
106<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
107<p><a class="reference" href="./querying-algorithms.html">Querying Algorithms</a>, <a class="refentry reference" href="./count.html"><tt class="refentry literal"><span class="pre">count</span></tt></a>, <a class="refentry reference" href="./find.html"><tt class="refentry literal"><span class="pre">find</span></tt></a>, <a class="refentry reference" href="./find-if.html"><tt class="refentry literal"><span class="pre">find_if</span></tt></a>, <a class="refentry reference" href="./contains.html"><tt class="refentry literal"><span class="pre">contains</span></tt></a></p>
108<!-- modtime: November 10, 2004 04:43:18 +0000 -->
109<!-- Algorithms/Querying Algorithms//lower_bound |60 -->
110</div>
111</div>
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