C++实现——由年月日推算是星期几

#include <iostream>
#include <string>
#include <cmath>

using namespace std;
/*
语法：result = weekday(int N, int M, int d)
参数：
N, M, d：年月日，例如：2003, 11, 4
返回值：0：星期天，1星期一……
注意：
需要cmath
适用于1582年10月15日之后, 因为罗马教皇格里高利十三世在这一天启用新历法.
*/
//给定年月日，输出当天是星期几
int weekday(int N, int M, int d)
{
int m, n, c, y, w;
    m = (M - 2) % 12;
if (M >= 3) n = N; else n = N - 1;
    c = n / 100;
    y = n % 100;
    w = (int)(d + floor(13 * m / 5) + y + floor(y / 4) + floor(c / 4) - 2 * c) % 7;
while (w<0) w += 7;
return w;
}

//测试函数
int main(){

int year, month, day;
string week[] = {"日","一","二","三","四","五","六"};
while (cin >> year >> month >> day){

cout << year << "年" << month << "月" << day << "日是星期" << week[weekday(year, month, day)] << endl;
    }
}