1. WIKI教程
2. 学习D.
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)

D编程语言中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 。

D编程语言将任何non-zero和non-null值假定为true ，如果它zero或null ，则将其假定为false值。

流程图 (Flow Diagram)

例子 (Example)

import std.stdio;
int main () { 
   /* local variable definition */ 
   int a = 100; 
   /* check the boolean condition */ 
   if( a < 20 ) { 
      /* if condition is true then print the following */ 
      writefln("a is less than 20" ); 
   } else { 
      /* if condition is false then print the following */
      writefln("a is not less than 20" ); 
   } 
   writefln("value of a is : %d", a); 
   return 0; 
}

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。

• 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if if或else都不会被测试。

语法 (Syntax)

D编程语言中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)

import std.stdio;
int main () { 
   /* local variable definition */ 
   int a = 100; 
   /* check the boolean condition */ 
   if( a == 10 ) { 
      /* if condition is true then print the following */ 
      writefln("Value of a is 10" ); 
   } else if( a == 20 ) { 
      /* if else if condition is true */ 
      writefln("Value of a is 20" ); 
   } else if( a == 30 ) { 
      /* if else if condition is true  */ 
      writefln("Value of a is 30" ); 
   } else { 
      /* if none of the conditions is true */ 
      writefln("None of the values is matching" ); 
   } 
   writefln("Exact value of a is: %d", a ); 
   return 0; 
}

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