（2）STL算法之元素计数

2、元素计数算法

STL提供了两个计数功能的算法，分别是count(),以及条件计数的算法count_if();

【count()原型】

template <class InputIterator, class T>

typename iterator_traits<InputIterator>::difference_type

count (InputIterator first, InputIterator last, const T& val);

该函数模板行为等效于以下：

template <class InputIterator,class T>
typename iterator_traits<InputIterator>::difference_type
count(InputIterator first, InputIterator last, const T& val)
{
	typename iterator_traits<InputIterator>::difference_type ret = 0;
	while (first != last)
	{
		if (*first == val)
		{
			++ret;
			++first;
		}
	}
	return ret;
}

【count_if()原型】

template <class InputIterator, class UnaryPredicate>

typename iterator_traits<InputIterator>::difference_type

count_if (InputIterator first, InputIterator last, UnaryPredicate pred);

template <class InputIterator, class UnaryPredicate>
typename iterator_traits<InputIterator>::difference_type
count_if(InputIterator first, InputIterator last, UnaryPredicate pred) //pred 为 true时，才统计元素个数，参数3应该是一个bool类型的函数对象
{
    typename iterator_traits<InputIterator>::difference_type ret = 0;
    while (first != last) {
        if (pred(*first)) ++ret;
        ++first;
    }
    return ret;
}

示例一

#include <iostream>     
#include <algorithm>    
#include <vector>       

int main() {
    // counting elements in array:
    int myints[] = { 10,20,30,30,20,10,10,20 };   
    int mycount = std::count(myints,   myints + 8    , 10);
    std::cout << "10 appears " << mycount << " times.\n";

    // counting elements in container:
    std::vector<int> myvector(myints,    myints + 8    );
    mycount = std::count(myvector.begin(), myvector.end(), 20);
    std::cout << "20 appears " << mycount << " times.\n";

    return 0;
}

示例二

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
using namespace std;

template<class T>
void init_value(T& vect, int first, int last)
{
	if (last > first)
	{
		for (int i = first; i <= last; ++i)
		{
			vect.insert(vect.end(), i);  //尾插
		}
	}
}

void Print(int elem)
{
	cout << elem << " ";
}

bool pred(int elem)
{
	return elem % 2 == 0;   //是否是偶数
}

void main()
{
	vector<int> myvector;
	init_value(myvector, 1, 10);
	for_each(myvector.begin(), myvector.end(), Print);
	cout << endl;

	int ct = count(myvector.begin(), myvector.end(), 4);  //统计等于 4 的数的个数
	int ct_if = count_if(myvector.begin(), myvector.end(), pred);  //统计偶数的个数
	int ct_if1 = count_if(myvector.begin(), myvector.end(), bind2nd(greater<int>(), 2)); //统计大于 2 的个数
	cout << "ct " << ct << endl;   // 1
	cout << "ct_if " << ct_if << endl;  // 5
	cout << "ct_if1  " << ct_if1 << endl;   8
}

这里的bind2nd(greater<int>(), 2)  可以参考 C++11中的std::bind