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count_if [C++ Reference]
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<h2><a name="count_if" id="count_if">count_if</a></h2>
<div class="level2">
<p>
Syntax:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#include <algorithm></span>
typename iterator_traits<span class="sy0"><</span>input_iterator<span class="sy0">>::</span><span class="me2">difference_type</span>
count_if<span class="br0">(</span> input_iterator start<span class="sy0">,</span> input_iterator end<span class="sy0">,</span> UnaryPred p <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
The <code>count_if</code> function returns the number of elements between <code>start</code> and <code>end</code>
for which the predicate <code>p</code> returns true.
</p>
<p>
For example, the following code uses <code>count_if</code> with a predicate that returns
true for the integer 3 to count the number of items in an array that are equal
to 3:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">int</span> nums<span class="br0">[</span><span class="br0">]</span> <span class="sy0">=</span> <span class="br0">{</span> 0<span class="sy0">,</span> 1<span class="sy0">,</span> 2<span class="sy0">,</span> 3<span class="sy0">,</span> 4<span class="sy0">,</span> 5<span class="sy0">,</span> 9<span class="sy0">,</span> 3<span class="sy0">,</span> 13 <span class="br0">}</span><span class="sy0">;</span>
<span class="kw4">int</span> start <span class="sy0">=</span> <span class="nu0">0</span><span class="sy0">;</span>
<span class="kw4">int</span> end <span class="sy0">=</span> <span class="nu0">9</span><span class="sy0">;</span>
<span class="kw4">int</span> target_value <span class="sy0">=</span> <span class="nu0">3</span><span class="sy0">;</span>
<span class="kw4">int</span> num_items <span class="sy0">=</span> count_if<span class="br0">(</span> nums<span class="sy0">+</span>start<span class="sy0">,</span>
nums<span class="sy0">+</span>end<span class="sy0">,</span>
bind2nd<span class="br0">(</span>equal_to<span class="sy0"><</span>int<span class="sy0">></span><span class="br0">(</span><span class="br0">)</span><span class="sy0">,</span> target_value<span class="br0">)</span> <span class="br0">)</span><span class="sy0">;</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"nums[] contains "</span> <span class="sy0"><<</span> num_items <span class="sy0"><<</span> <span class="st0">" items matching "</span> <span class="sy0"><<</span> target_value <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
<p>
When run, the above code displays the following output:
</p>
<pre class="code">
nums[] contains 2 items matching 3</pre>
<p>
Alternatively, the same result can be achieved with the <code>boost::bind</code>:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">int</span> num_items <span class="sy0">=</span> count_if<span class="br0">(</span> nums<span class="sy0">+</span>start<span class="sy0">,</span>
nums<span class="sy0">+</span>end<span class="sy0">,</span>
boost<span class="sy0">::</span><span class="me2">bind</span><span class="br0">(</span>equal_to<span class="sy0"><</span>int<span class="sy0">></span><span class="br0">(</span><span class="br0">)</span><span class="sy0">,</span> target_value<span class="sy0">,</span> _1<span class="br0">)</span> <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
<code>count_if</code> can also be used on containers such as <a href="../../stl/vector/start.html" class="wikilink1" title="stl:vector:start">vectors</a>:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="kw4">int</span> num_items <span class="sy0">=</span> count_if<span class="br0">(</span> nums.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">,</span>
nums.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">,</span>
boost<span class="sy0">::</span><span class="me2">bind</span><span class="br0">(</span>equal_to<span class="sy0"><</span>int<span class="sy0">></span><span class="br0">(</span><span class="br0">)</span><span class="sy0">,</span> target_value<span class="sy0">,</span> _1<span class="br0">)</span> <span class="br0">)</span><span class="sy0">;</span></pre>
<p>
Related Topics: <a href="../../stl/algorithm/count.html" class="wikilink1" title="stl:algorithm:count">count</a>
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