count_if用法

template <class InputIterator, class Predicate> typename iterator_traits<InputIterator>::difference_type count_if ( ForwardIterator first, ForwardIterator last, Predicate pred );

<algorithm>

Return number of elements in range satisfying condition

Returns the number of elements in the range [first,last) for which condition pred is true.

The behavior of this function template is equivalent to:

template <class InputIterator, class Predicate> ptrdiff_t count_if ( InputIterator first, InputIterator last, Predicate pred ) { ptrdiff_t ret=0; while (first != last) if (pred(*first++)) ++ret; return ret; }

Parameters

first, lastForward iterators to the initial and final positions of the sequence of elements. The range used is [first,last), which contains all the elements between first and last, including the element pointed
by first but not the element pointed by last.predUnary predicate taking an element in the range as argument, and returning a value indicating the falsehood (with false, or a zero value) or truth (true, or non-zero) of some condition applied to it. This can
either be a pointer to a function or an object whose class overloads operator().

Return value

The number of elements in the range [first,last) for which cond applied to it returns true.

The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type, generally defined as the same type as ptrdiff_t.

#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;;
class Student{
public:
int No;
string strName;
int grade;
Student(int No,string strName,int grade):No(No),strName(strName),grade(grade){};
/*bool operator ==(int grade){
return this->grade==grade;
}*/

};
class MatchExpress{
int grade;
public:
MatchExpress(int grade):grade(grade){};
bool operator()(Student& s){
return s.grade>grade;
}
};
int main(){
vector<Student>v;
Student s1(1000,"张三",80);
Student s2(1001,"李四",85);
Student s3(1002,"王五",80);
Student s4(1003,"赵六",80);
v.push_back(s1);
v.push_back(s2);
v.push_back(s3);
v.push_back(s4);
int nCount;
nCount=count_if(v.begin(),v.end(),MatchExpress(80));
cout<<nCount<<endl;
}