From 0e0fcb5d2a5bd3721ba047c197b2fd60acf48f2c Mon Sep 17 00:00:00 2001
From: Raghav Bajaj <73180455+Raghav-Bajaj@users.noreply.github.com>
Date: Tue, 31 Oct 2023 20:07:36 +0530
Subject: [PATCH] Update PrimeNumberGenerator.java

optimizing primeNumberGenerator with sieve of Eratosthenes
---
 PrimeNumberGenerator.java | 54 +++++++++++----------------------------
 1 file changed, 15 insertions(+), 39 deletions(-)

diff --git a/PrimeNumberGenerator.java b/PrimeNumberGenerator.java
index 98c37dd8a16..b125ee3c95a 100644
--- a/PrimeNumberGenerator.java
+++ b/PrimeNumberGenerator.java
@@ -1,62 +1,38 @@
-import java.util.ArrayList;
-import java.util.List;
 import java.util.Scanner;
 
 public class PrimeNumberGenerator {
     public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
 
-        System.out.print("Enter the start of the range: ");
-        int startRange = sc.nextInt();
-
         System.out.print("Enter the end of the range: ");
         int endRange = sc.nextInt();
 
-        if (startRange > endRange) {
-            System.out.println("Invalid range. The start of the range should be less than or equal to the end.");
-        } else {
-            List<Integer> primeNumbers = generatePrimesInRange(startRange, endRange);
+        boolean[] isPrime = sieveOfEratosthenes(endRange);
 
-            System.out.println("Prime numbers between " + startRange + " and " + endRange + ":");
-            for (int prime : primeNumbers) {
-                System.out.print(prime + " ");
+        System.out.println("Prime numbers up to " + endRange + ":");
+        for (int i = 2; i <= endRange; i++) {
+            if (isPrime[i]) {
+                System.out.print(i + " ");
             }
         }
 
         sc.close();
     }
 
-    // Function to generate prime numbers within a specified range
-    public static List<Integer> generatePrimesInRange(int start, int end) {
-        List<Integer> primeNumbers = new ArrayList<>();
-
-        for (int num = start; num <= end; num++) {
-            if (isPrime(num)) {
-                primeNumbers.add(num);
-            }
-        }
-
-        return primeNumbers;
-    }
-
-    // Function to check if a number is prime
-    public static boolean isPrime(int num) {
-        if (num <= 1) {
-            return false;
-        }
-        if (num <= 3) {
-            return true;
-        }
-        if (num % 2 == 0 || num % 3 == 0) {
-            return false;
+    public static boolean[] sieveOfEratosthenes(int n) {
+        boolean[] isPrime = new boolean[n + 1];
+        for (int i = 2; i <= n; i++) {
+            isPrime[i] = true;
         }
 
-        for (int i = 5; i * i <= num; i += 6) {
-            if (num % i == 0 || num % (i + 2) == 0) {
-                return false;
+        for (int p = 2; p * p <= n; p++) {
+            if (isPrime[p]) {
+                for (int i = p * p; i <= n; i += p) {
+                    isPrime[i] = false;
+                }
             }
         }
 
-        return true;
+        return isPrime;
     }
 }