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<Head>
<Title>count_if</Title>
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<H1>count_if</H1>
<Table CellPadding=0 CellSpacing=0 width=100%>
<TR>
<TD Align=left><Img src = "algorithms.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
<TD Align=right><Img src = "function.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
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<TR>
<TD Align=left VAlign=top><b>Category</b>: algorithms</TD>
<TD Align=right VAlign=top><b>Component type</b>: function</TD>
</TR>
</Table>
<h3>Prototype</h3>
<tt>Count_if</tt> is an overloaded name: there are two <tt>count_if</tt> functions.
<pre>
template <class InputIterator, class Predicate>
iterator_traits<InputIterator>::difference_type
count_if(InputIterator first, InputIterator last, Predicate pred);
template <class <A href="InputIterator.html">InputIterator</A>, class <A href="Predicate.html">Predicate</A>, class Size>
void count_if(InputIterator first, InputIterator last,
Predicate pred,
Size& n);
</pre>
<h3>Description</h3>
<tt>Count_if</tt> finds the number of elements in <tt>[first, last)</tt> that satisfy
the predicate <tt>pred</tt>. More precisely, the first version of <tt>count_if</tt>
returns the number of iterators <tt>i</tt> in <tt>[first, last)</tt> such that
<tt>pred(*i)</tt> is <tt>true</tt>. The second version of <tt>count</tt> adds to <tt>n</tt>
the number of iterators <tt>i</tt> in <tt>[first, last)</tt> such that
<tt>pred(*i)</tt> is <tt>true</tt>.
<P>
The second version of <tt>count_if</tt> was the one defined in the original
STL, and the first version is the one defined in the
draft C++ standard; the definition was changed because the
older interface was clumsy and error-prone. The older interface
required the use of a temporary variable, which had to be
initialized to 0 before the call to <tt>count_if</tt>.
<P>
Both interfaces are currently supported <A href="#1">[1]</A>, for
reasons of backward compatibility, but eventually the older
version will be removed.
<h3>Definition</h3>
Defined in the standard header <A href="algorithm">algorithm</A>, and in the nonstandard
backward-compatibility header <A href="algo.h">algo.h</A>.
<h3>Requirements on types</h3>
For the first version, which takes three arguments:
<UL>
<LI>
<tt>InputIterator</tt> is a model of <A href="InputIterator.html">Input Iterator</A>.
<LI>
<tt>Predicate</tt> is a model of <A href="Predicate.html">Predicate</A>.
<LI>
<tt>InputIterator</tt>'s value type is convertible to <tt>Predicate</tt>'s argument
type.
</UL>
For the second version, which takes four arguments:
<UL>
<LI>
<tt>InputIterator</tt> is a model of <A href="InputIterator.html">Input Iterator</A>.
<LI>
<tt>Predicate</tt> is a model of <A href="Predicate.html">Predicate</A>.
<LI>
<tt>Size</tt> is an integral type that can hold values of
<tt>InputIterator</tt>'s distance type.
<LI>
<tt>InputIterator</tt>'s value type is convertible to <tt>Predicate</tt>'s argument
type.
</UL>
<h3>Preconditions</h3>
For the first version:
<UL>
<LI>
<tt>[first, last)</tt> is a valid range.
</UL>
For the second version:
<UL>
<LI>
<tt>[first, last)</tt> is a valid range.
<LI>
<tt>n</tt> plus the number of elements that satisfy <tt>pred</tt> does not exceed
the maximum value of type <tt>Size</tt>.
</UL>
<h3>Complexity</h3>
Linear. Exactly <tt>last - first</tt> applications of <tt>pred</tt>.
<h3>Example</h3>
<pre>
int main() {
int A[] = { 2, 0, 4, 6, 0, 3, 1, -7 };
const int N = sizeof(A) / sizeof(int);
cout << "Number of even elements: "
<< count_if(A, A + N,
<A href="unary_compose.html">compose1</A>(<A href="binder2nd.html">bind2nd</A>(<A href="equal_to.html">equal_to</A><int>(), 0),
<A href="binder2nd.html">bind2nd</A>(<A href="modulus.html">modulus</A><int>(), 2)))
<< endl;
}
</pre>
<h3>Notes</h3>
<P><A name="1">[1]</A>
The new <tt>count</tt> interface uses the <tt><A href="iterator_traits.html">iterator_traits</A></tt> class, which
relies on a C++ feature known as <i>partial specialization</i>. Many of
today's compilers don't implement the complete standard; in
particular, many compilers do not support partial specialization. If
your compiler does not support partial specialization, then you will
not be able to use the newer version of <tt>count</tt>, or any other STL
components that involve <tt><A href="iterator_traits.html">iterator_traits</A></tt>.
<h3>See also</h3>
<tt><A href="count.html">count</A></tt>, <tt><A href="find.html">find</A></tt>, <tt><A href="find_if.html">find_if</A></tt>
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