C语言求素数(质数)Eratosthenes经典算法

//使用Eratosthenes方法找出指定范围内的所有质数

#include

#define SIZE 500 //该方法能够求出2*SIZE 之内的质数#define TRUE 1

#define FALSE 0

int

main()

{

char sieve[ SIZE ]; /* the sieve */

char *sp; /* pointer to access the sieve */

int number; /* number we’re computing */

/*

** Set the entire sieve to TRUE.

*/

for(sp = sieve; sp<&sieve[ SIZE ]; )

*sp++ = TRUE;

/*** Process each number from 3 to as many as the sieve holds. (Note: the

** loop is terminated from inside.)

*/

for( number = 3; ; number += 2 ){

/*

** Set the pointer to the proper element in the sieve, and stop

** the loop if we’ve gone too far.

*/

sp = &sieve[ 0 ] + ( number-3 ) / 2;

if(sp>= &sieve[ SIZE ] )

break;

/*

** Now advance the pointer by multiples of the number and set

** each subsequent entry FALSE.

*/

while(sp += number, sp<&sieve[ SIZE ] )

*sp = FALSE;

}

/*

** Go through the entire sieve now and print the numbers corresponding

** to the locations that remain TRUE.

*/

printf( "2\t" );

for( number = 3, sp = &sieve[ 0 ];

sp<&sieve[ SIZE ];

number += 2, sp++ ){

if( *sp )

printf( "%d\t", number );

}

system("pause");

return EXIT_SUCCESS; }