鍍金池/ 教程/ C/ Objective-C if...else 語句
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Objective-C if...else 語句

if 語句后面可以通過一個可選的else語句,布爾表達式為假時執(zhí)行。

語法:

在Objective-C編程語言的if... else語句的語法是:

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

如果布爾表達式的值為true,那么將要執(zhí)行的if block代碼塊,else block 代碼塊將被執(zhí)行。

Objective-C語言的編程語言假設(shè)為真,任何非零和非空值,如果它是零或者為null,那么它被假定為false。

流程圖:

Objective-C if...else statement

例如:

#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
" );
   }
   else
   {
       /* if condition is false then print the following */
       NSLog(@"a is not less than 20
" );
   }
   NSLog(@"value of a is : %d
", a);
 
   return 0;
}

上面的代碼編譯和執(zhí)行時,它會產(chǎn)生以下結(jié)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 語句

if語句后面可以跟一個可選的 else if...else 語句,這是非常有用的,如果使用單... else if語句來測試各種條件。 

當使用 if , else if , else 語句時,有幾點要牢記:

  • 一個if可以有零個或一個else,它必須跟在else if之后。

  • 一個if 可以有零或許多else if,他們必須出現(xiàn)在else之前。

  • else if 一旦成功,否則,剩余的else if'將不會被測試執(zhí)行。

語法:

if...else if...else 語句語法在Objective-C編程語言是:

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 */
}

例如:

#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
" );
   }
   else if( a == 20 )
   {
       /* if else if condition is true */
       NSLog(@"Value of a is 20
" );
   }
   else if( a == 30 )
   {
       /* if else if condition is true  */
       NSLog(@"Value of a is 30
" );
   }
   else
   {
       /* if none of the conditions is true */
       NSLog(@"None of the values is matching
" );
   }
   NSLog(@"Exact value of a is: %d
", a );
 
   return 0;
}

上面的代碼編譯和執(zhí)行時,它會產(chǎn)生以下結(jié)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