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It is provided "as is" without express or implied warranty. -- --><Head><Title>count_if</Title><!-- Generated by htmldoc --></HEAD><BODY TEXT="#000000" LINK="#006600" ALINK="#003300" VLINK="#7C7F87" BGCOLOR="#FFFFFF"><A HREF="/"><IMG SRC="/images/common/sgilogo_small.gif" ALT="SGI Logo" WIDTH="80" HEIGHT="72" BORDER="0"></A><P><!--end header--><BR Clear><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></TR><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_typecount_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 satisfythe 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 originalSTL, and the first version is the one defined in the draft C++ standard; the definition was changed because theolder interface was clumsy and error-prone. The older interfacerequired the use of a temporary variable, which had to beinitialized to 0 before the call to <tt>count_if</tt>.<P>Both interfaces are currently supported <A href="#1">[1]</A>, forreasons of backward compatibility, but eventually the olderversion will be removed.<h3>Definition</h3>Defined in the standard header <A href="algorithm">algorithm</A>, and in the nonstandardbackward-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, whichrelies on a C++ feature known as <i>partial specialization</i>. Many oftoday's compilers don't implement the complete standard; inparticular, many compilers do not support partial specialization. Ifyour compiler does not support partial specialization, then you willnot be able to use the newer version of <tt>count</tt>, or any other STLcomponents 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><!-- start footer --><!-- Footer Begins --><STYLE TYPE="text/css"><!--TD.footer, TD.footer A{ font-family: Arial, helvetica, sans-serif; font-size: 8pt;}A.home {font-family: Arial, helvetica, sans-serif;}--></STYLE><P><A CLASS="home" HREF="index.html">STL Home</A><P><TABLE WIDTH="600" CELLPADDING="0" CELLPADDING="0" BORDER="0"> <TR> <TD ALIGN="RIGHT" CLASS="footer"><A HREF="/company_info/terms.html" TARGET="_top">terms of use</A> | <A HREF="/company_info/privacy.html" TARGET="_top">privacy policy</A></TD> <TD ALIGN="CENTER" CLASS="footer"> | </TD> <TD ALIGN="LEFT" CLASS="footer"><A HREF="/cgi-bin/feedback/" TARGET="_top">contact us</A></TD> </TR><TR> <TD ALIGN="RIGHT" CLASS="footer">Copyright © 1993-2003 Silicon Graphics, Inc. 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