C++的std::multiplies

一、介绍

std::multiplies是乘法的二元函数对象。常被用于std::transform或者std::accumulate等的运算算子。

template <class T> struct multiplies {
  T operator() (const T& x, const T& y) const {return x*y;}
  typedef T first_argument_type;
  typedef T second_argument_type;
  typedef T result_type;
};

二、实例

2.1两个数组的元素分别对应相乘

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
 
int main()
{
    std::vector<int> v1 = {10, 20, 30, 40, 50};
    std::vector<int> v2 = { 1, 2, 3, 4, 5 };
 
    std::vector<int> result(5);
    std::transform(v1.begin(), v1.end(), v2.begin(), result.begin(), std::multiplies<int>());
 
    for (int i : result) {
        std::cout << i << "\t";
    }
    std::cout << std::endl;
 
    return 0;
}

 

2.2 角度与弧度的变换

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
 
int main(int argc, const char * argv[])
{
	std::vector<double> a = { 0, 30, 45, 60, 90, 180 };
	std::vector<double> r(a.size());
	const double pi = std::acos(-1); // since C++20 use std::numbers::pi
 
	std::transform(a.begin(), a.end(), r.begin(),
		std::bind1st(std::multiplies<double>(), pi / 180.));
	//  an equivalent lambda is: [pi](double a){ return a*pi / 180.; });
 
	for (std::size_t n = 0; n < a.size(); ++n)
		std::cout << a[n] << "° = " << std::fixed << r[n]
		<< " rad\n" << std::defaultfloat;
 
        return 0 ;
}

2.3 计算阶乘

#include <iostream>     // std::cout
#include <functional>   // std::multiplies
#include <numeric>      // std::partial_sum
 
int main(int argc, const char * argv[])
{
	int numbers[9];
	int factorials[9];
	for (int i = 0; i < 9; i++) numbers[i] = i + 1;
	std::partial_sum(numbers, numbers + 9, factorials, std::multiplies<int>());
	for (int i = 0; i < 9; i++)
		std::cout << numbers[i] << "! is " << factorials[i] << '\n';
        return 0 ;
}

参考：

http://www.cplusplus.com/reference/functional/multiplies/