Prime Number Help

[Drazzminius]

I'm working on a homework assignment with the following requirement:

Write a Java program that meets the following requirements:

Declare a method to determine whether an integer is a prime number

Use the following method declarations: public static Boolean isPrime (int num)

An integer greater than 1 is a prime number if its only divisor is 1 or itself. For example, isPrime (11) returns true, and isPrime (9) returns false.

Us the isPrime method to find the first thousand prime numbers and display every ten prime numbers in a row, as follows:

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 61 73 79 83 89 97 … …


Important Notes: The input and output must use JOptionPane dialog and display boxes.

Here is what I have so far:

import javax.swing.JOptionPane;

public class PrimeNumber {

	public static boolean isPrime(int num) {
		if (num <  2) return false;
		if (num == 2) return true;
		int n = (int) Math.sqrt(num);
		for (int i = 3; i < n; i += 2) {
			if (num%i == 0) return false;
		}
		return true;
}
	public void main(String[] args) {
		int[] primes = new int[1000];	
		primes[0] = 2;				
		int count = 1;
		int num = 3;
		// Find the first 1000 primes
		while (count<1000) {
			if (isPrime(num)) {
				primes[count] = num; 
				count++;
			}
			num = num + 2;
		}
		String output = (primes[i] + "\t");
		 		
			// Display the result
		JOptionPane.showMessageDialog(null, output, 
			"Assignment 5.1 Output", JOptionPane.INFORMATION_MESSAGE);
		
		System.exit(0);
	}
}

I keep getting this error in JCreator Lite:

C:\Users\Craig\Documents\Downloads\PrimeNumber.java:57: cannot find symbol
symbol : variable i
location: class PrimeNumber
.........String output = (primes[i] + "\t");
........................................^
1 error

Process completed.

What am I doing wrong? Everything else seems to be going okay and the examples I was given in my course seem to line up.

Thanks for any help!

[Drazzminius]

I've made the following updates:

import javax.swing.JOptionPane;

public class PrimeNumber {
	// Main Method
	public static boolean isPrime(int num) {
		
		int[] numbers = new int[0];
		// Reads an input number
		for (int i = 0; i < numbers.length; i++) {
			String numString = JOptionPane.showInputDialog(null,
			"Enter a number: ",
			"Assignment 5.1 Input", JOptionPane.QUESTION_MESSAGE);
			
		// Convert string into integer
		numbers[i] = Integer.parseInt(numString);		
		}
		// Determines primes
		if (num <  2) return false;
		if (num == 2) return true;
		int n = (int) Math.sqrt(num);
		for (int i = 3; i < n; i += 2) {
			if (num&#37;i == 0) return false;
		}
		return true;
	}
	public void main(String[] args) {
		//Declares "primes" as my variable
		int[] primes = new int[1000];	
		primes[0] = 2;				
		int count = 1;
		int num = 3;
		// Finds the first 1000 primes
		while (count<1000) {
			if (isPrime(num)) {
				primes[count] = num; 
				count++;
			}
			num = num + 2;
		}
		// Prepares the output
		String output = "The first 1000 primes are: ";
		for (int i=0; i<primes.length; i++)
		
		output += primes[i] + "\t";
		 		
			// Display the result
		JOptionPane.showMessageDialog(null, output, 
			"Assignment 5.1 Output", JOptionPane.INFORMATION_MESSAGE);
		
		System.exit(0);
	}
}

It shows as building properly, but before it runs I get this error:

--------------------Configuration: <Default>--------------------
java.lang.NoSuchMethodError: main
Exception in thread "main"
Process completed.

???

[Drazzminius]

Here is the latest version. I just can't seem to get the columns to work right.

import javax.swing.JOptionPane;

public class PrimeNumberTest {	
		
		// Check if an integer is a prime
	public static boolean isPrime(int num) {
		if (num <  2) return false;
		if (num == 2) return true;
		int n = (int) Math.sqrt(num);
		for (int i=3; i<n; i+=2) {
			if (num&#37;i == 0) return false;
		}
		return true;
	}
	// Main Method
	public static void main(String[] args) {
		int[] primes = new int[1000];	// Define the array of the first 1000 primes
		primes[0] = 2;					// The smallest prime is 2;
		int count = 1;
		int num = 3;
		// Find the first 1000 primes
		while (count<1000) {
			if (isPrime(num)) {
				primes[count] = num; 
				count++;
			}
			num = num + 2;
		}
		// Opens the Welcome Message
		JOptionPane.showMessageDialog(null, "Click OK to see all prime numbers under 10000", 
		"Assignment 5.1", JOptionPane.INFORMATION_MESSAGE);
		
		// Prepares the output
		String output = "The first 1000 primes are: ";
		for (int i=0; i<primes.length; i++)	{
		//	if (i%10 == 0), primes[i] ("");
			output += primes[i] + "\n";}
		 		
			// Display the result
		JOptionPane.showMessageDialog(null, output, 
			"Assignment 5.1 Output", JOptionPane.INFORMATION_MESSAGE);
		
		System.exit(0);
	}
}

[Nightwalker83]

Have a look in the math forum! There might be a java example for prime numbers in there.