#define    N   100                                          
   #define    A   2                                            
                                                               
   main()                                                      
   {                                                           
       int  a;                                                 
                                                               
       a  =  A;                                                
       while( a < N )                                          
       {                                                       
            printf("%d\n", a);                                 
            a *= a;                                            
       }                                                       
   }                                                           
                                                            
Output                                                      
                                                               
   2                                                           
   4                                                           
   16