Implement Sieve of Eratosthenes and prime number checker

Author: gourabofficial

Basic/Prime_Number/SieveOfEratosthenes.java

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+
+import java.util.Scanner;
+
+public class SieveOfEratosthenes {
+
+  public static void main(String[] args) {
+    Scanner sc = new Scanner(System.in);
+    System.out.print("Findes prime upto: ");
+    int num = sc.nextInt();
+
+    boolean[] isPrime = new boolean[num + 1];
+
+    for (int i = 2; i <= num; i++) {
+      isPrime[i] = true;
+
+    }
+
+    isPrime[0] = false;
+    isPrime[1] = false;
+
+    // Start from 2 and mark all multiples as not prime
+    for (int i = 2; i * i <= num; i++) {
+      if (isPrime[i]) {
+        for (int j = i * i; j <= num; j += i) {
+          isPrime[j] = false;
+        }
+      }
+    }
+
+    System.out.println("Prime numbers up to " + num + ":");
+    
+    for (int i = 2; i < num; i++) {
+      if (isPrime[i]) {
+        System.out.println(i + " ");
+      }
+    }
+
+  }
+
+}

Basic/Prime_Number/prime_Checker.java

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+
+import java.util.Scanner;
+
+public class prime_Checker {
+  public static void main(String[] args) {
+    Scanner sc = new Scanner(System.in);
+    System.out.print("Enter the number: ");
+    int num = sc.nextInt();
+    // System.out.println(Math.sqrt(4));
+
+    if (isPrime(num)) {
+      System.out.println(num+ " is Prime Number");
+    } else {
+      System.out.println(num + " is not Prime ");
+    }
+  }
+
+  static boolean isPrime(int num)
+  {
+    if (num <= 1)
+      return false;
+    
+    if (num == 2 )
+      return true;
+
+    for (int i = 2; i <= Math.sqrt(num); i++) {
+      if (num % i == 0) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+}

Basic/Prime_Number/print_n_number.java

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+
+import java.util.Scanner;
+
+public class print_n_number {
+  public static void main(String[] args) {
+    Scanner sc = new Scanner(System.in);
+    System.out.print("Enter the N number to check for prime numbers: ");
+    int num = sc.nextInt();
+
+    System.out.println("Prime numbers up to " + num + " : ");
+    for (int i = 2; i <= num; i++) {
+      if (isPrimeCheck(i)) {
+        System.out.println(i + " ");
+      }
+    }
+    System.out.println();
+  }
+  
+  static boolean isPrimeCheck(int num) {
+    if (num <= 1)
+      return false;
+    if (num == 2)
+      return true;
+
+    for (int i = 2; i <= Math.sqrt(num); i++) {
+      if (num % 2 == 0) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+}