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| <TITLE>count(), count_if()</TITLE> |
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| <H2>count(), count_if()</H2> |
| <P><B>Library:</B> <A HREF="2-9.html">Algorithms</A></P> |
| |
| <PRE><HR><B><I>Function</I></B><HR></PRE> |
| |
| <UL> |
| <LI><A HREF="#sec1">Local Index</A></LI> |
| <LI><A HREF="#sec2">Summary</A></LI> |
| <LI><A HREF="#sec3">Synopsis</A></LI> |
| <LI><A HREF="#sec4">Description</A></LI> |
| <LI><A HREF="#sec5">Complexity</A></LI> |
| <LI><A HREF="#sec6">Example</A></LI> |
| <LI><A HREF="#sec7">Warnings</A></LI> |
| <LI><A HREF="#sec8">Standards Conformance</A></LI> |
| </UL> |
| <A NAME="sec1"><H3>Local Index</H3></A> |
| No Entries |
| <A NAME="sec2"><H3>Summary</H3></A> |
| <P>Algorithm that counts the number of elements in a range that satisfy a given condition</P> |
| <A NAME="sec3"><H3>Synopsis</H3></A> |
| |
| <PRE>#include <algorithm> |
| |
| namespace std { |
| template<class InputIterator, class T> |
| typename iterator_traits<InputIterator>::difference_type |
| count(InputIterator start, InputIterator finish, |
| const T& value); |
| |
| template <class InputIterator, class T, class Size> |
| void count(InputIterator start, InputIterator finish, |
| const T& value, Size& n); |
| |
| template<class InputIterator, class Predicate> |
| typename iterator_traits<InputIterator>::difference_type |
| count_if(InputIterator start, InputIterator finish, |
| Predicate pred); |
| |
| template <class InputIterator, class Predicate, class Size> |
| void count_if(InputIterator start, InputIterator finish, |
| Predicate pred, Size& n); |
| } |
| </PRE> |
| <A NAME="sec4"><H3>Description</H3></A> |
| <BLOCKQUOTE><HR><B> |
| NOTE -- The second versions of the count() and count_if() functions are not part of the C++ Standard, but are included here as extensions for compatibility purposes. See <A HREF="B.html">Appendix B</A> for a complete list of extensions of this implementation. |
| </B><HR></BLOCKQUOTE> |
| <P>The <SAMP>count()</SAMP> algorithm compares <SAMP>value</SAMP> to elements in the sequence defined by iterators <SAMP>start</SAMP> and <SAMP>finish</SAMP>. The first version of <SAMP>count()</SAMP> returns the number of matches. The second version, which is provided for backwards compatibility or as an alternative to the first in case the template <B><I><A HREF="iterator-traits.html">iterator_traits</A></I></B> isn't provided, increments a counting value <SAMP>n</SAMP> each time it finds a match. In other words, <SAMP>count()</SAMP> returns (or adds to <SAMP>n</SAMP>) the number of iterators <SAMP>i</SAMP> in the range <SAMP>[start, finish)</SAMP> for which the following condition holds:</P> |
| <P><SAMP>*i == value</SAMP></P> |
| <P>Type <SAMP>T</SAMP> must be <SAMP>EqualityComparable</SAMP>.</P> |
| <A NAME="sec5"><H3>Complexity</H3></A> |
| <P>The <SAMP><A HREF="count.html">count_if()</A></SAMP> algorithm lets you specify a predicate, and returns the number of times an element in the sequence satisfies the predicate (or increments <SAMP>n</SAMP> that number of times). That is, <SAMP>count_if()</SAMP> returns (or adds to <SAMP>n</SAMP>) the number of iterators <SAMP>i</SAMP> in the range <SAMP>[start, finish)</SAMP> for which the following condition holds: </P> |
| <P><SAMP>pred(*i) == true.</SAMP></P> |
| <P>Both <SAMP>count()</SAMP> and <SAMP><A HREF="count.html">count_if()</A></SAMP> perform exactly <SAMP>finish - start</SAMP> applications of the corresponding predicate.</P> |
| <A NAME="sec6"><H3>Example</H3></A> |
| |
| <UL><PRE>// |
| // count.cpp |
| // |
| |
| #include <algorithm> // for count, count_if |
| #include <functional> // bind2nd, less |
| #include <iostream> // for cout, endl |
| #include <vector> // for vector |
| |
| |
| |
| int main () |
| { |
| // Typedef for convenience. |
| typedef std::vector<short, std::allocator<short> > vector; |
| |
| const vector::value_type a [] = { |
| 1, 2, 3, 4, 5, 5, 7, 8, 9, 10 |
| }; |
| vector::size_type i = 0, j = 0, k = 0; |
| |
| // Set up a vector. |
| const vector v (a + 0, a + 10); |
| |
| std::count (v.begin (), v.end (), 5, i); // Count fives |
| std::count (v.begin (), v.end (), 6, j); // Count sixes |
| |
| // Count all elements less than 8. |
| std::count_if (v.begin(), v.end(), |
| std::bind2nd(std::less<vector::value_type>(), 8), k); |
| |
| std::cout << i << " " << j << " " << k << std::endl; |
| |
| return 0; |
| } |
| |
| |
| Program Output: |
| </PRE></UL> |
| <UL><PRE>2 0 7 |
| </PRE></UL> |
| <A NAME="sec7"><H3>Warnings</H3></A> |
| <P>If your compiler does not support partial specialization, the first version of both <SAMP>count()</SAMP> and <SAMP><A HREF="count.html">count_if()</A></SAMP>, the version that returns the count, is not available.</P> |
| <A NAME="sec8"><H3>Standards Conformance</H3></A> |
| <P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.1.6</I></P> |
| |
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