C++ STL（4）：计算元素个数

#include <iostream>
#include <algorithm>
#include <functional>

//计算元素个数
int main()
{
	/************************************************************************/
	//count
	/************************************************************************/
	//计算范围[first,last)内与value相等的个数
	/*
		template<class InputIterator, class T> inline
		size_t count(
			InputIterator First,
			InputIterator Last,
			const T& Value
		);
	*/

	int a[] = {2, 0, 4, 6, 0 ,3, 1, -7};
	const int N = sizeof a / sizeof a[0];
	std::cout << "Numbers of equal to zeros is: " 
		<< std::count(a, a + N, 0) << std::endl;    //Numbers of equal to zeros is: 2

	/************************************************************************/
	//count_if
	/************************************************************************/
	//count_if在范围[first,last)内满足条件的元素个数
	/*
		template<class InputIterator, class Predicate>
		typename iterator_traits<InputIterator>::difference_type count_if(
			InputIterator _First, 
			InputIterator _Last,
			Predicate _Pred
		);
	*/
	std::cout << "Numbers of equal to zeros is: " << 
	std::count_if(a, a + N, std::bind1st(std::equal_to<int>(),0)) 
		<< std::endl;    //Numbers of equal to zeros is: 2

	std::cout << "Numbers of greater equal 0 is: " << 
	std::count_if(a, a + N, std::bind1st(std::less_equal<int>(),0)) 
		<< std::endl;    //Numbers of greater equal 0 is: 7

	return 0;
}

测试结果：

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