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:
[cpp] view plain copy print ? // 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) ; //返回字符串中从0开始的，长度为1的字符串 } 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) ; //第三个参数叫做谓词 // print the count of elements that start with letter 'S' cout << "Number of elements that start with letter \"S\" = " << result << endl ; } count例子: [cpp] view plain copy print ? // 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 ; }
