Soeriksen Sieve method

Principle:

Number of records for the first 2~n. 2 as the smallest prime number. So more than 2 are not prime, from the recording medium crossed off, after scanning again. Use 3 as the minimum prime number. 3 times times the number of strokes off, so down, to find out all prime numbers.

As you can see in the table:

2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
2	3	-	5	-	7	-	9	-	11	-	13	-	15	-	17	-	19	-
2	3	-	5	-	7	-	-	-	11	-	13	-	-	-	17	-	19	-

Code:

Infer whether it is a prime number:

BOOL Is_prime (int n) {for    (int i=2;i*i<=n;i++) {        if (n%i==0)            return false;    }    return n!=1;}

Ed Sieve Method:

const int MAX  = 1000;int prime[max];bool is_prime[max];int sieve (int n) {    int p=0;    for (int i=0;i<=n;i++)        is_prime[i] = true;    Is_prime[0] = is_prime[1] = false;    for (int i=2;i<=n;i++) {        if (Is_prime[i]) {            prime[p++] = i;            for (int j=2*i;j<=n;j+=i)  is_prime[j] = false;        }    }    return p;}

Soeriksen Sieve method