nested 循环

Objective-C编程语言允许在另一个循环中使用一个循环。 以下部分显示了几个例子来说明这个概念。

语法 (Syntax)

C语言中nested for loop语句的语法如下 -

for ( init; condition; increment ) {
   for ( init; condition; increment ) {
      statement(s);
   }
   statement(s);
}

Objective-C编程语言中nested while loop语句的语法如下 -

while(condition) {
   while(condition) {
      statement(s);
   }
   statement(s);
}

Objective-C编程语言中nested do...while loop语句的语法如下 -

do {
   statement(s);
   do {
      statement(s);
   } while( condition );
} while( condition );

关于循环嵌套的最后一点是你可以将任何类型的循环放在任何其他类型的循环中。 例如，for循环可以在while循环内，反之亦然。

例子 (Example)

以下程序使用嵌套for循环来查找从2到100的素数 -

#import <Foundation/Foundation.h>
int main () {
   /* local variable definition */
   int i, j;
   for(i=2; i<100; i++) {
      for(j=2; j <= (i/j); j++)
        if(!(i%j)) break;        // if factor found, not prime
      if(j > (i/j)) NSLog(@"%d is prime\n", i);
   }
   return 0;
}

编译并执行上述代码时，会产生以下结果 -

2013-09-07 22:40:01.004 demo[1027] 2 is prime
2013-09-07 22:40:01.005 demo[1027] 3 is prime
2013-09-07 22:40:01.005 demo[1027] 5 is prime
2013-09-07 22:40:01.005 demo[1027] 7 is prime
2013-09-07 22:40:01.005 demo[1027] 11 is prime
2013-09-07 22:40:01.005 demo[1027] 13 is prime
2013-09-07 22:40:01.005 demo[1027] 17 is prime
2013-09-07 22:40:01.005 demo[1027] 19 is prime
2013-09-07 22:40:01.005 demo[1027] 23 is prime
2013-09-07 22:40:01.005 demo[1027] 29 is prime
2013-09-07 22:40:01.005 demo[1027] 31 is prime
2013-09-07 22:40:01.005 demo[1027] 37 is prime
2013-09-07 22:40:01.005 demo[1027] 41 is prime
2013-09-07 22:40:01.005 demo[1027] 43 is prime
2013-09-07 22:40:01.005 demo[1027] 47 is prime
2013-09-07 22:40:01.005 demo[1027] 53 is prime
2013-09-07 22:40:01.005 demo[1027] 59 is prime
2013-09-07 22:40:01.005 demo[1027] 61 is prime
2013-09-07 22:40:01.005 demo[1027] 67 is prime
2013-09-07 22:40:01.005 demo[1027] 71 is prime
2013-09-07 22:40:01.005 demo[1027] 73 is prime
2013-09-07 22:40:01.005 demo[1027] 79 is prime
2013-09-07 22:40:01.005 demo[1027] 83 is prime
2013-09-07 22:40:01.005 demo[1027] 89 is prime
2013-09-07 22:40:01.005 demo[1027] 97 is prime