Thread: Prime number generator program missing return statement

Hello 03EVOAWD.

You are getting this error because of this method:

public static boolean isPrime(int num) {   }

You need to return a result of type boolean.

For example:

  import javax.swing.*; import java.util.Scanner;   public class PrimeNumber{   public static boolean isPrime(int num){   Scanner input = new Scanner(System.in);   int Odd = input.nextInt(); int Count = 0; int Prime = 1; int Num1;   String checker= JOptionPane.showInputDialog("Input your Values:"); Integer Number1 = Integer.parseInt(checker);   do{   Num1 = (Prime + 2) / 2;     boolean isPrime = true; Count++;   if(Num1 != 1){ isPrime = false; break;} }   while(Count <= 1000);   String message2= String.format("All Prime Numbers: %d\n", Odd); JOptionPane.showMessageDialog(null, message2); [B]return false;[/B]   }//End method   }//End Class PrimeNumber

I'm not sure if this will work as expected but it will stop that error.

Ummmm try adding:

public static void main(String[] args){   }

You will need to call your boolean method from within this main method.

Ooh... don't compute every prime seprately If you know what range you need prime numbers up to, use the Sieve of Eratosthenes, or if you're feeling bold the Sieve of Atkin.

What are you expecting to happen?

Originally Posted by 03EVOAWD

find the first thousand prime numbers and display every ten prime numbers in a row, ----eventually

Right now I just want it to ask me to input a value

System.out.println("Enter a value: "); Scanner input = new Scanner(System.in); int Odd = input.nextInt();

Try this. Only problem is it shows each prime number one by one..

import javax.swing.JOptionPane;   public class PrimeNumber { public static void main (String[] args) {   final int Maximum_Primes = 1000;//Number of primes to display   final int Primes_Per_Line = 10; //Display 10 per line   int count = 0;//Count number of prime numbers   int number = 2;   System.out.println("The first 1000 prime numbers are \n");   while (count < Maximum_Primes) { boolean isPrime = true;   int temp = number/2; // had to declare new variable since you cannot do math in a for loop   for (int divisor = 2;divisor<=temp;divisor++) //you cannot declare divisor 3 times here and it   //cannot be split on multiple lines { if (number % divisor == 0){ isPrime = false; break; } } if (isPrime) { count++; if (count % Primes_Per_Line != 0)//need to make sure that it will print when its not // equal to 0 {   System.out.print(number + " "); [B]JOptionPane.showMessageDialog(null, number + " ");[/B] } else System.out.println(number + "."); //JOptionPane.showMessageDialog(null, number + " "); } number++; } } }

I've used an ArrayList here to store the results and then display them with the JOptionPane.

The JOptionPane is one massive box at the moment. You'll need to figure out how to add a new line after every 10 results or something.

package guiclient;   import javax.swing.JOptionPane; import java.util.ArrayList;   public class PrimeNumber { public static void main (String[] args) {   [B]ArrayList<String> al = new ArrayList<String>();[/B]   final int Maximum_Primes = 1000;//Number of primes to display   final int Primes_Per_Line = 10; //Display 10 per line   int count = 0;//Count number of prime numbers   int number = 2;   System.out.println("The first 1000 prime numbers are \n");   while (count < Maximum_Primes) { boolean isPrime = true;   int temp = number/2; // had to declare new variable since you cannot do math in a for loop   for (int divisor = 2;divisor<=temp;divisor++) //you cannot declare divisor 3 times here and it   //cannot be split on multiple lines { if (number % divisor == 0){ isPrime = false; break; } } if (isPrime) { count++; if (count % Primes_Per_Line != 0)//need to make sure that it will print when its not // equal to 0 {   System.out.print(number + " "); [B] al.add(number + " ");[/B]   } else System.out.println(number + "."); [B] al.add(number + " ");[/B] } number++; }   [B]JOptionPane.showMessageDialog(null, al);[/B]   } }

Your lucky i'm bored today

Try this:

import javax.swing.JOptionPane; import java.util.ArrayList;   public class PrimeNumber { public static void main (String[] args) {   ArrayList<String> al = new ArrayList<String>(); [B]int alCount = 0;[/B]   final int Maximum_Primes = 100;//Number of primes to display   final int Primes_Per_Line = 10; //Display 10 per line   int count = 0;//Count number of prime numbers   int number = 2;   System.out.println("The first 1000 prime numbers are \n");   while (count < Maximum_Primes) { boolean isPrime = true;   int temp = number/2; // had to declare new variable since you cannot do math in a for loop   for (int divisor = 2;divisor<=temp;divisor++) //you cannot declare divisor 3 times here and it   //cannot be split on multiple lines { if (number % divisor == 0){ isPrime = false; break; } } if (isPrime) { count++; if (count % Primes_Per_Line != 0)//need to make sure that it will print when its not // equal to 0 {   System.out.print(number + " ");   [B] alCount++;   if(alCount == 10){ al.add(number + "\n"); alCount = 0; }else{ al.add(number + " "); }[/B]     } else System.out.println(number + ".");   } number++; }   JOptionPane.showMessageDialog(null, al);   } }