Java code for factorial problems

[guyfrompluto]

Here is a starter program that prints prime factors of a supplied numbers as a string.It's quite buggy as the input 8 yields 1.2.4 when what i wanted is 1.2.2.2 and I don't understand why the return in line 32 (commented) is not executed.

//program to compute prime factors of a number



import javax.swing.JOptionPane;
import java.util.Arrays;

public class test {

    
    public static void main(String[] args) {
        int num;
        for (int counter=0;counter<10;counter++){
        String input=JOptionPane.showInputDialog("Enter a number.");
        try{
            num=Integer.parseInt(input);
        }
        catch(NumberFormatException nfe){
            input=JOptionPane.showInputDialog("Invalid input. Enter an integer.");
            num=Integer.parseInt(input);
        }
        System.out.println(calculateFactors("1.",num));
        }


    }
    
    public static String calculateFactors(String list,int num){ /*list is passed to preserve the string during recursion*/
        for (int i=2;i<=num;i++){
            if(isPrime(num)){
                list=list+Integer.toString(num)+"."; //why isn't this executed???
                return list;
            }
            if(isPrime(i)){
                if(num%i==0){
                    list=list+Integer.toString(i)+".";
                    num=num/i;
                    calculateFactors(list,num);
                }
            }
        }
        return list+Integer.toString(num);
        
    }



    static boolean isPrime(int num){
            for(int i=2;i<num;i++){
                if(num%i==0){
                    return false;
                }
            }
            return true;
        }

}

I am new to programming and recursion is confusing which is probably the reason for my problems. Any help and suggestions would be much appreciated.

[jon.kiparsky]

Two points, while I find my way to your actual problem

- when you're marching up the line of numbers in your isPrime(), you can actually stop when you get to the halfway point, since you know that there is no integer factor of n greater than n/2.

- Could you maybe keep track of the primes that you've found, so you don't have to look them all up each time?
(this is me being subtle...)

[guyfrompluto]

I think I was using too much recursion in the program. Also, I ignored the while loop and sticked with the for loop which didnt let me change the counter variable during the program making recursion inevitable. Here is an improved version:

//program to compute prime factors of a number



import javax.swing.JOptionPane;

public class test {

    
    public static void main(String[] args) {
        int num;
        for (int counter=0;counter<10;counter++){
        String input=JOptionPane.showInputDialog("Enter a number.");
        try{
            num=Integer.parseInt(input);
        }
        catch(NumberFormatException nfe){
            input=JOptionPane.showInputDialog("Invalid input. Enter an integer.");
            num=Integer.parseInt(input);
        }
        System.out.println(calculateFactors("1.",num));
        }


    }
    
    public static String calculateFactors(String list,int num){
    	int i=2;
        while (i<num){
            
            if(isPrime(i)){
                if(num%i==0){
                    list=list+Integer.toString(i)+".";
                    num=num/i;
                    continue;
                }
                else{
                	i++;
                }
            }
            else{
            	i++;
            }
        }
            
                    if(isPrime(num)){
                        list=list+Integer.toString(num)+"."; //ok executed now.
                        return list;
                    }
                    
                
            
                    return "error in program";
        
    }



    static boolean isPrime(int num){
            for(int i=2;i<num/2;i++){
                if(num%i==0){
                    return false;
                }
            }
            return true;
        }

}