count_if函数原型如下：
template<class _InIt, class _Pr> inline
typename iterator_traits<_InIt>::difference_type
count_if(_InIt _First, _InIt _Last, _Pr _Pred);
前两个参数是iterator(迭代器)，表示查找
半闭合区间
的前后两个位置，第三个参数为一个用户定义的predicate function object，而
predicate意思就是说是一个返回值是bool型的仿函数(function object，也称functor)。
count函数原型如下：
template<class InputIterator, class T> inline
size_t count(
InputIterator First,
InputIterator Last,
const T& Value
)
前两个参数如上，只是第三个参数是一个T类型的常量，相比之下，count_if具有更大的灵活性。
count_if例子:
// countif.cpp
// compile with: /EHsc
//
// Functions:
//
// count_if - Count items in a range that satisfy a predicate.
//
// begin - Returns an iterator that points to the first element in
// a sequence.
//
// end - Returns an iterator that points one past the end of a
// sequence.
// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)
#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <vector>
using namespace std;
// Return true if string str starts with letter 'S'
int MatchFirstChar( const string& str)
{
string s("S") ;
return s == str.substr(0,1) ;
}
int main()
{
const int VECTOR_SIZE = 8 ;
// Define a template class vector of strings
typedef vector<string > StringVector ;
//Define an iterator for template class vector of strings
typedef StringVector::iterator StringVectorIt ;
StringVector NamesVect(VECTOR_SIZE) ; //vector containing names
StringVectorIt start, end, it ;
ptrdiff_t result = 0 ; // stores count of elements
// that match value.
// Initialize vector NamesVect
NamesVect[0] = "She" ;
NamesVect[1] = "Sells" ;
NamesVect[2] = "Sea" ;
NamesVect[3] = "Shells" ;
NamesVect[4] = "by" ;
NamesVect[5] = "the" ;
NamesVect[6] = "Sea" ;
NamesVect[7] = "Shore" ;
start = NamesVect.begin() ; // location of first
// element of NamesVect
end = NamesVect.end() ; // one past the location
// last element of NamesVect
// print content of NamesVect
cout << "NamesVect { " ;
for(it = start; it != end; it++)
cout << *it << " " ;
cout << " }\n" << endl ;
// Count the number of elements in the range [first, last +1)
// that start with letter 'S'
result = count_if(start, end, MatchFirstChar) ;
// print the count of elements that start with letter 'S'
cout << "Number of elements that start with letter \"S\" = "
<< result << endl ;
}
count例子:
// count.cpp
// compile with: /EHsc
//
// Functions:
//
// count - Count items in a range that match a value.
// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)
#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <vector>
using namespace std;
int main()
{
const int VECTOR_SIZE = 8 ;
// Define a template class vector of strings
typedef vector<string > StringVector ;
//Define an iterator for template class vector of strings
typedef StringVector::iterator StringVectorIt ;
StringVector NamesVect(VECTOR_SIZE) ; //vector containing names
string value("Sea") ; // stores the value used
// to count matching elements
StringVectorIt start, end, it ;
ptrdiff_t result = 0 ; // stores count of elements
// that match value.
// Initialize vector NamesVect
NamesVect[0] = "She" ;
NamesVect[1] = "Sells" ;
NamesVect[2] = "Sea" ;
NamesVect[3] = "Shells" ;
NamesVect[4] = "by" ;
NamesVect[5] = "the" ;
NamesVect[6] = "Sea" ;
NamesVect[7] = "Shore" ;
start = NamesVect.begin() ; // location of first
// element of NamesVect
end = NamesVect.end() ; // one past the location
// last element of NamesVect
// print content of NamesVect
cout << "NamesVect { " ;
for(it = start; it != end; it++)
cout << *it << " " ;
cout << " }\n" << endl ;
// Count the number of elements in the range [first, last +1)
// that match value.
result = count(start, end, value) ;
// print the count of elements that match value
cout << "Number of elements that match \"Sea\" = "
<< result << endl ;
}