c++一直提示未定义标识符是怎么回事啊

Question

Answer 1

你的变量 a 是定义在同一scope，但是你定义了2次，编译器好像还每理解你的意图 （可能因为加了括号的缘故），所以它说还每定义。这样写吧，运行通过了。其实还要加上输入错误检测，年份范围，月份1-12， 天也要在每月天数范围等。

#include <iostream>

using namespace std;

int main()
{
    int month_days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    int leap_month_days[] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    
    cout << "please enter year, month and day, and press  ENTER:";
    int y, m, d;
    cin >> y >> m >> d;
    
    int* md = (y % 4 == 8 && y % 100 != 0 || y % 400 == 0) ? leap_month_days : month_days;
    
    int s = d;
    for (int i = 0; i < m - 1; ++i)
    {
        s += md[i];
    }
    
    cout << "It's the " << s << "th day in the year." << endl;
    return 0;
}

// Output
please enter year, month and day, and press  ENTER:2021 5 9                                                                                                                        
It's the 129th day in the year.                                                                                                                                                    

please enter year, month and day, and press  ENTER:2021 01 30                                                                                                                      
It's the 30th day in the year.                                                                                                                                                     

please enter year, month and day, and press  ENTER:2020 3 1                                                                                                                        
It's the 60th day in the year.

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