1. WIKI教程
2. 学习Objective_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)

Objective-C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 ，那么将执行if block代码if block ，否则将执行代码else block 。

Objective-C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 <Foundation/Foundation.h>
int main () {
   /* local variable definition */
   int a = 100;
   /* check the boolean condition */
   if( a < 20 ) {
      /* if condition is true then print the following */
      NSLog(@"a is less than 20\n" );
   } else {
      /* if condition is false then print the following */
      NSLog(@"a is not less than 20\n" );
   }
   NSLog(@"value of a is : %d\n", a);
   return 0;
}

编译并执行上述代码时，会产生以下结果 -

2013-09-07 22:04:10.199 demo[3537] a is not less than 20
2013-09-07 22:04:10.200 demo[3537] 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)

Objective-C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 <Foundation/Foundation.h>
int main () {
   /* local variable definition */
   int a = 100;
   /* check the boolean condition */
   if( a == 10 ) {
      /* if condition is true then print the following */
      NSLog(@"Value of a is 10\n" );
   } else if( a == 20 ) {
      /* if else if condition is true */
      NSLog(@"Value of a is 20\n" );
   } else if( a == 30 ) {
      /* if else if condition is true  */
      NSLog(@"Value of a is 30\n" );
   } else {
      /* if none of the conditions is true */
      NSLog(@"None of the values is matching\n" );
   }
   NSLog(@"Exact value of a is: %d\n", a );
   return 0;
}

编译并执行上述代码时，会产生以下结果 -

2013-09-07 22:05:34.168 demo[8465] None of the values is matching
2013-09-07 22:05:34.168 demo[8465] Exact value of a is: 100