【STL】标准库中count与count_if函数说明与例子

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  ;}