sieve2.c

该程序用于在一个给定的数组中寻找素数

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
int main(int argc, char *argv[])
{
        // We will compute all primes less than the value specified on the
        // command line, or, if no argument, all primes less than 100.
        int max = 100;                           // Assign a default value
				max =atoi(argv[1]);
        // Create an array that specifies whether each number is prime or not.
        int maxhalf=(max+1)/2;
        char * isprime = ( char*)malloc(maxhalf+1); //only save odd
        int ct=0;
		int i;
        // Assume that all numbers are primes, until proven otherwise.
        for( i = 0; i <= maxhalf; i++)
        {
					isprime[i] = 1;
        //	ct++;
        }

        // However, we know that 0 and 1 are not primes.  Make a note of it.
        isprime[0] = 0; //1
				isprime[1] = 1;  //3
        // To compute all primes less than max, we need to rule out
        // multiples of all integers less than the square root of max.
        int n = (int) ceil(sqrt(max));  // See java.lang.Math class

        // Now, for each integer i from 0 to n:
        //   If i is a prime, then none of its multiples are primes,
        //   so indicate this in the array.  If i is not a prime, then
        //   its multiples have already been ruled out by one of the
        //   prime factors of i, so we can skip this case.

			 for( i = 3; i <= n; i+=2) {
            if (isprime[(i-1)/2])        //除法指令不算慢    // If i is a prime,
                for(int j = 3*i; j <= max; j = j +i +i) // loop through multiples,
                {
                   isprime[(j-1)/2] = 0;               // they are not prime.
                //    ct++;
                }
        }


        // Now go look for the largest prime:
        int largest=2;
        if(max <=2)
        {
        	largest=2;
        }
        else if((max%2==1)&&(isprime[(max-1)/2]))
        {
        	largest=max;
        }
        else
        {
        	for(largest = max-1-(max %2); !isprime[(largest-1)/2]; largest-=2)
        	{
        	//	ct++;  // empty loop body
        	}
      	}

        // Output the result
			printf("The largest prime less than or equal to %d is %d, cycle time:%d\n" ,max, largest,ct);
			return 1;
}