while(condition)
{
   statement(s);
}
#include <stdio.h>
 
int main ()
{
   /* 局部变量定义 */
   int a = 10;

   /* while 循环执行 */
   while( a < 20 )
   {
      printf("a 的值: %d\n", a);
      a++;
   }
 
   return 0;
}
for ( init; condition; increment )
{
   statement(s);
}
#include <stdio.h>
 
int main ()
{
   /* for 循环执行 */
   for( int a = 10; a < 20; a = a + 1 )
   {
      printf("a 的值: %d\n", a);
   }
 
   return 0;
}
do
{
   statement(s);

}while( condition );
#include <stdio.h>
 
int main ()
{
   /* 局部变量定义 */
   int a = 10;

   /* do 循环执行 */
   do
   {
       printf("a 的值: %d\n", a);
       a = a + 1;
   }while( a < 20 );
 
   return 0;
}
for (initialization; condition; increment/decrement)
{
    statement(s);
    for (initialization; condition; increment/decrement)
    {
        statement(s);
        ... ... ...
    }
    ... ... ...
}
while (condition1)
{
    statement(s);
    while (condition2)
    {
        statement(s);
        ... ... ...
    }
    ... ... ...
}
do
{
    statement(s);
    do
    {
        statement(s);
        ... ... ...
    }while (condition2);
    ... ... ...
}while (condition1);
#include <stdio.h>
 
int main ()
{
   /* 局部变量定义 */
   int i, j;
   
   for(i=2; i<100; i++) {
      for(j=2; j <= (i/j); j++)
        if(!(i%j)) break; // 如果找到,则不是质数
      if(j > (i/j)) printf("%d 是质数\n", i);
   }
 
   return 0;
}
#include <stdio.h>
int main()
{
    int i=1,j;
    while (i <= 5)
    {
        j=1;
        while (j <= i )
        {
            printf("%d ",j);
            j++;
        }
        printf("\n");
        i++;
    }
    return 0;
}
#include <stdio.h>
int main()
{
    int i=1,j;
    do
    {
        j=1;
        do
        {
            printf("*");
            j++;
        }while(j <= i);
        i++;
        printf("\n");
    }while(i <= 5);
    return 0;
}
#include <stdio.h>
 
int main ()
{
   /* 局部变量定义 */
   int a = 10;

   /* while 循环执行 */
   while( a < 20 )
   {
      printf("a 的值: %d\n", a);
      a++;
      if( a > 15)
      {
         /* 使用 break 语句终止循环 */
          break;
      }
   }
 
   return 0;
}
#include <stdio.h>
 
int main ()
{
   /* 局部变量定义 */
   int a = 10;

   /* do 循环执行 */
   do
   {
      if( a == 15)
      {
         /* 跳过迭代 */
         a = a + 1;
         continue;
      }
      printf("a 的值: %d\n", a);
      a++;
     
   }while( a < 20 );
 
   return 0;
}
goto label;
..
.
label: statement;
#include <stdio.h>
 
int main ()
{
   /* 局部变量定义 */
   int a = 10;

   /* do 循环执行 */
   LOOP:do
   {
      if( a == 15)
      {
         /* 跳过迭代 */
         a = a + 1;
         goto LOOP;
      }
      printf("a 的值: %d\n", a);
      a++;
     
   }while( a < 20 );
 
   return 0;
}
#include <stdio.h>
 
int main ()
{
   for( ; ; )
   {
      printf("该循环会永远执行下去!\n");
   }
   return 0;
}