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2. 学习C＃
3. if...else statement

if...else statement

if语句后面可以跟一个可选的else语句，该语句在布尔表达式为false时执行。

语法 (Syntax)

C＃中if...else语句的语法是 -

if(boolean_expression) {
   /* statement(s) will execute if the boolean expression is true */
} else {
   /* statement(s) will execute if the boolean expression is false */
}

如果布尔表达式的计算结果为true ，则执行if block代码if block ，否则执行代码else block 。

流程图 (Flow Diagram)

例子 (Example)

using System;
namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         /* local variable definition */
         int a = 100;
         /* check the boolean condition */
         if (a < 20) {
            /* if condition is true then print the following */
            Console.WriteLine("a is less than 20");
         } else {
            /* if condition is false then print the following */
            Console.WriteLine("a is not less than 20");
         }
         Console.WriteLine("value of a is : {0}", a);
         Console.ReadLine();
      }
   }
}

编译并执行上述代码时，会产生以下结果 -

a is not less than 20;
value of a is : 100

if...else if...else (The if...else if...else Statement)

if语句后面可以跟一个else if...else语句，这对于使用单个if ... else if语句测试各种条件非常有用。

当使用if，else if，else语句时，要记住几点。

• 一个if可以有零个或一个其他的，它必须在任何其他if之后。

• 如果是的话，if可以有零到多个，并且它们必须在else之前。

• 一旦else成功，其余的其他if或者其他都将被测试。

语法 (Syntax)

C＃中if...else if...else语句的语法是 -

if(boolean_expression 1) {
   /* Executes when the boolean expression 1 is true */
} 
else if( boolean_expression 2) {
   /* Executes when the boolean expression 2 is true */
} 
else if( boolean_expression 3) {
   /* Executes when the boolean expression 3 is true */
} else {
   /* executes when the none of the above condition is true */
}

例子 (Example)

using System;
namespace DecisionMaking {
   class Program {
      static void Main(string[] args) {
         /* local variable definition */
         int a = 100;
         /* check the boolean condition */
         if (a == 10) {
            /* if condition is true then print the following */
            Console.WriteLine("Value of a is 10");
         } 
         else if (a == 20) {
            /* if else if condition is true */
            Console.WriteLine("Value of a is 20");
         } 
         else if (a == 30) {
            /* if else if condition is true  */
            Console.WriteLine("Value of a is 30");
         } else {
            /* if none of the conditions is true */
            Console.WriteLine("None of the values is matching");
         }
         Console.WriteLine("Exact value of a is: {0}", a);
         Console.ReadLine();
      }
   }
}

编译并执行上述代码时，会产生以下结果 -

None of the values is matching
Exact value of a is: 100