/* ECP: FILEname=fig4_1.c */
/* 1*/   #include <stdio.h>

/* 2*/   /* Return True Iff N Is Prime */
/* 3*/   int
/* 4*/   IsPrime( unsigned long int N )
/* 5*/   {
/* 6*/       unsigned long int Divisor;

/* 7*/       if( N % 2 )
/* 8*/           for( Divisor = 3; N % Divisor; Divisor += 2 )
/* 9*/               if( Divisor * Divisor > N )
/*10*/                   return 1;

/*11*/       return N == 2 || N == 3;
/*12*/   }


/*13*/   /* Return An Odd Starting Number */
/*14*/   unsigned long int
/*15*/   FirstTrial( void )
/*16*/   {
/*17*/       unsigned long int StartingNum;

/*18*/       printf( "Enter a starting number: " );
/*19*/       if( scanf( "%lu", &StartingNum ) == 1 )
/*20*/           return StartingNum % 2 ? StartingNum : ++StartingNum;

/*21*/       printf( "Bad number entered\n" );
/*22*/       return 0;
/*23*/   }


/*24*/   /* Find Next Prime After Some Starting Point */
/*25*/   main( void )
/*26*/   {
/*27*/       unsigned long int PossiblePrime = FirstTrial( );

/*28*/       for( ; !IsPrime( PossiblePrime ); PossiblePrime += 2 )
/*29*/           ;

/*30*/       printf( "Next largest prime is %lu\n", PossiblePrime );
/*31*/   }