求1~10000之间的素数个数

首先存储1~100内的素数，然后在据此计算1~10000内素数以及个数

package program;

import java.util.Arrays;

public class suShu100 {

    static boolean[] flag = new boolean[101];
    static int[] arrs = new int[100];
    static int num = 0;

    public static void main(String[] args) {
        // 1 存储100以内的素数
        GenPrime();
        System.out.println(Arrays.toString(arrs));
        System.out.println(num);

        // 2 求10000以内的素数
        int count = 0;
        for (int i = 2; i <= 10000; i++) {
            if (IsPrime(i)) {
                count++;
                // System.out.print(i + " ");
                // if (count % 10 == 0) {
                // System.out.println("");
                // }
            }
        }
        System.out.println(count);

    }

    private static boolean IsPrime(int s) {

        int num2 = 0;
        int k = 0;

        if (s <= 100) {
            if (flag[s]) {
                return true;
            } else {
                return false;
            }
        }
        while (arrs[k] * arrs[k] <= s && k < num) {
            if (s % arrs[k] == 0) {
                return false;
            }
            k++;
        }
        return true;
    }

    private static void GenPrime() {
        int s = 0;
        int j = 0;
        for (int i = 0; i < flag.length; i++) {
            flag[i] = true;
        }
        flag[0] = false;
        flag[1] = false;
        for (int i = 2; i <= 100; i++) {
            if (flag[i]) {
                arrs[s++] = i;

            }

            j = 2 * i;
            while (j <= 100) {
                flag[j] = false;
                j = j + i;
            }
        }
        num = s;
    }

}

结果：

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
25
1229