Sieve of Eratosthenes in java

Sieve of Eratosthenes is the ancient algorithm to find prime numbers up to a given number.

Algorithm

1. Generate integers from 2 to n (Given number).

2. Counting from 2 mark every 2nd integer. (multiples of 2)

3. Now, starting from 3 mark every third integer. (multiples of 3)

4. Finally, marking from 5 mark every 5th integer.(multiples of 5)

Program

import java.util.Scanner;

public class SievePrimeFactors  {
   public static void main(String args[]) {
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter a number");
      int num = sc.nextInt();
      boolean[] bool = new boolean[num];
     
      for (int i = 0; i< bool.length; i++) {
         bool[i] = true;
      }
      for (int i = 2; i< Math.sqrt(num); i++) {
         if(bool[i] == true) {
            for(int j = (i*i); j<num; j = j+i) {
               bool[j] = false;
            }
         }
      }
      System.out.println("List of prime numbers upto given number are : ");
      for (int i = 2; i< bool.length; i++) {
         if(bool[i]==true) {
            System.out.println(i);
         }
      }
   }
}

Output

Enter a number
50
List of prime numbers up to given number are :
2
3
5
7
11
13
17
19
23
29
31
37
41
43
47

George John

Updated on: 25-Jun-2020

3K+ Views

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