Question

import java.util.Scanner;

public class SieveOfEratosthenes {
   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);
         }
      }
   }
}

Can you comment the code along with adding improvements to it. I also need it to print out it's execution and run time. The language is in java. Make sure it prints out the run time

after the execution and printing of the prime numbers. This program is the Sieve of Eratosthenes that calculates the prime numbers leading up to a number.http://www.algolist.net/Algorithms/Number_theoretic/Sieve_of_Eratosthenes

Answer 1

The above code efficiently prints the prime numbers.

I have added inline comments based on Sieve of Eratosthenes to find prime number and a statement to print the run time after printing the prime numbers.

>> Code :

import java.util.Scanner;

public class SieveOfEratosthenes {
public static void main(String args[]) {
long startTime = System.currentTimeMillis(); // initialise startTime with the current time in millis.
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number");// accept the input value until which the prime numbers need to be printed.
int num = sc.nextInt();
boolean[] bool = new boolean[num]; // create a boolean array from 0 to num.
  
for (int i = 0; i< bool.length; i++) {
bool[i] = true; // initialise all the values to true
}
for (int i = 2; i< Math.sqrt(num); i++) { // starting from index 2 the first prime number .Count upto num^2.
if(bool[i] == true) {
for(int j = (i*i); j<num; j = j+i) {
bool[j] = false; // initialise to false if the given number is not prime.
}
}
}
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); // lists all the numbers which are initialised to true in boolean array.
}
}
long endTime = System.currentTimeMillis();// initialise endTime to current time in millis.
long totalTime = endTime - startTime; // subtract the startTime and endTime to get the total run time in miilis
System.out.println(totalTime + " is the total run time );
}
}