這段代碼似乎同時給了我if語句和else語句的結果，爲什麼？

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; 
} 

如果我例如輸入：天= 22個月= 2年= 2015 它會寫「的日期是23/2/2015」，而下一行會寫錯誤！有人可以解釋這一點嗎？

Answer 1

您可以使用else if爲「鏈」的條件::

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; 

發生了什麼事是第一次if說法是正確的，並且執行 - 接下來if說法是假的，所以它執行else。使用else if有效地鏈接這些語句，使它們更像switch。

它等同於：

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; 

我要指出，有多簡單的方法來從月/日/年輸入的日期，不過這是您所看到的結果的原因。