Help writing to file

Question

This code is supposed to print all prime numbers between 1 and 100 to a file called PrimeNumbers. However, when I go back and check the file, all it contains is a 1. What is preventing it from printing all of the primes? Thank you.
import java.io.*;
public class Numbers
    public static void main(String[] args) throws IOException
        PrintWriter outputFile = new PrintWriter("PrimeNumbers.txt");
        int num1, num2, num3;
        for(num1=1; num1<= 100; num1++)
            num3 = 0;
            for(num2=1; num2<=num1; num2++)
                if(((num1%num2)==0) & num2!=1 & num2!=num1)
                    num3=1;
            if(num3==0)
                System.out.println(num1 + " is a prime number\n"); 
                outputFile.println(num1);
                outputFile.close();
}Edited by: pfloyd123 on Mar 20, 2009 11:28 PM
Edited by: pfloyd123 on Mar 20, 2009 11:29 PM

Answer 1

pfloyd123 wrote:
This code is supposed to print all prime numbers between 1 and 100 to a file called PrimeNumbers. However, when I go back and check the file, all it contains is a 1. What is preventing it from printing all of the primes? Thank you.
import java.io.*;
public class Numbers
public static void main(String[] args) throws IOException
PrintWriter outputFile = new PrintWriter("PrimeNumbers.txt");
int num1, num2, num3;
for(num1=1; num1<= 100; num1++)
num3 = 0;
for(num2=1; num2<=num1; num2++)
if(((num1%num2)==0) & num2!=1 & num2!=num1)
num3=1;
if(num3==0)
System.out.println(num1 + " is a prime number\n"); 
outputFile.println(num1);
outputFile.close();
}Edited by: pfloyd123 on Mar 20, 2009 11:28 PM
Edited by: pfloyd123 on Mar 20, 2009 11:29 PMI was staring at that code for five minutes trying to workout how it works. So I took a copy, and ran it... and (surpise) it works... and it's not actually too terribly slow... but I made some "performance tweaks", and now it's quit a bit faster.
The console output was the slow part. Wrapping the PrintWriter in a BufferedWriter makes no difference. Hmmm...
package forums;
import java.io.FileWriter;
import java.io.BufferedWriter;
import java.io.PrintWriter;
public class PrimeTester_SieveOfpfloyd123
  private static final int MAX = 1000*1000;
  public static void main(String[] args) throws Exception {
    long start = System.nanoTime();
    PrintWriter out = new PrintWriter("PrimeTester_SieveOfpfloyd123.out");
    //PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("PrimeTester_SieveOfpfloyd123.out")));
    int cnt = 0;
    for ( int a=1; a<=MAX; a++) {
      boolean isPrime = true;
      int n = (int) Math.sqrt(a);
      for ( int b=2; b<n; b++ ) {
        if ( a%b == 0 ) {
          isPrime = false;
          break;
      if ( isPrime) {
        //System.out.print(a);
        //System.out.print(++cnt%20==0 ? '\n' : ' ');
        out.print(a);
        out.print(++cnt%20==0 ? '\n' : ' ');
    long stop = System.nanoTime();
    out.close();
    System.out.println("1.."+MAX+" took "+((stop-start)/1000000.0)+" millis");
}Cheers. Keith.