这段代码似乎同时给我if语句和else语句的结果,为什么？

Question

Console.WriteLine("Hello welcome to the Next-Day calendar");
Console.WriteLine("Please write down the day");
int day = int.Parse(Console.ReadLine());
int day_next = day + 1;

Console.WriteLine("Please write down the month number");
int month = int.Parse(Console.ReadLine());

Console.WriteLine("Please write down the year");
int year = int.Parse(Console.ReadLine());

switch (month) 
{
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 10:
        if (day_next <=31 && day > 0) 
        {
            Console.WriteLine("The date is " + day_next + '/' + month + '/' + year);
        }
        if (day_next == 32) 
        {
            Console.WriteLine("The day is " + 1 + '/' + (month+1) + '/' + year); 
        }
        else
        {
            Console.WriteLine("Error!");
        }
        break;
    case 2:
        if (year % 4 == 0 && day_next <= 29 && day > 0 || year % 4 != 0 && day_next <= 28 && day > 0) 
        {
            Console.WriteLine("The date is " + day_next + '/' + month + '/' + year); 
        }
        if (year % 4 == 0 && day_next == 30 && day > 0 || year % 4 != 0 && day_next == 29 && day > 0) 
        {
            Console.WriteLine("The date is " + 1 + '/' + (month+1) + '/' + year); 
        }
        else
        {
            Console.WriteLine("Error!");
        }
        break;
    case 4:
    case 6:
    case 9:
    case 11:
        if (day_next <= 30 && day > 0) 
        {
            Console.WriteLine("The date is " + day_next + '/' + month + '/' + year);
        }
        if (day_next == 30) 
        {
            Console.WriteLine("The date is " + 1 + '/' + (month + 1) + '/' + year);
        }
        else 
        {
            Console.WriteLine("Error");
        }
        break;
    case 12:
        if (day_next <=31 && day > 0) 
        {
            Console.WriteLine("The date is " + day_next + '/' + month + '/' + year);
        }
        if (day_next == 32) 
        {
            Console.WriteLine("The day is " + 1 + '/' + 1 + '/' + (year + 1)); 
        }
        else
        {
            Console.WriteLine("Error!");
        }
        break;
    default:
        Console.WriteLine("Error!");
        break;
}

如果我输入例如:day = 22,month = 2,year = 2015它将写"日期是2015年2月23日",下一行将写入错误!任何人都能解释一下吗？

Answer 1

你可以else if用来"链"条件::

if (day_next <=31 && day > 0) 
{
    Console.WriteLine("The date is " + day_next + '/' + month + '/' + year);
}
else if (day_next == 32) 
{
    Console.WriteLine("The day is " + 1 + '/' + (month+1) + '/' + year); 
}
else
{
    Console.WriteLine("Error!");
}
break;

发生的事情是第一个if语句是真的,并执行 - 下一个if语句是假的,所以它执行else.使用else if有效链接语句,使它们更像是一个switch.

它相当于:

if (day_next <=31 && day > 0) 
{
    Console.WriteLine("The date is " + day_next + '/' + month + '/' + year);
}
else 
{
    if (day_next == 32) 
    {
        Console.WriteLine("The day is " + 1 + '/' + (month+1) + '/' + year); 
    }
    else
    {
        Console.WriteLine("Error!");
    }
}
break;

我会注意到有更简单的方法可以从月/日/年输入生成日期,但这就是您看到的结果的原因.