return prime list equal or below n Please notice that element 0 is kept for 1 although 1 is not prime, this is preserved for usage in special occasions - Java java.lang

Java examples for java.lang:int prime

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Description

return prime list equal or below n Please notice that element 0 is kept for 1 although 1 is not prime, this is preserved for usage in special occasions

Demo Code


//package com.java2s;

public class Main {
    /**/*w w w. j av a  2s  .  c  om*/
     * return prime list equal or below n
     * Please notice that element 0 is kept for 1 although 1 is not prime, this is preserved for usage in special occasions
     * 
     * @param n
     * @return
     */
    public static int[] getAllPrimeBelowN(int n) {
        if (n == 1) {
            return new int[] { 1 };
        }

        int[] temp = new int[n];
        boolean[] flags = new boolean[n + 1];

        temp[0] = 1;
        temp[1] = 2;
        int index = 2;

        int lastPrime = 2;
        while (lastPrime < n) {
            for (int i = 2; i * lastPrime <= n; i++) {
                flags[i * lastPrime] = true;
            }

            boolean found = false;
            for (int i = lastPrime + 1; i <= n; i++) {
                if (!flags[i]) {
                    found = true;
                    lastPrime = i;
                    temp[index++] = lastPrime;
                    break;
                }
            }

            if (!found) {
                break;
            }
        }

        int[] result = new int[index];
        System.arraycopy(temp, 0, result, 0, index);
        return result;

    }
}

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