Help! Factoring application

[blueduiker]

Hi everyone, I enrolled in APCS this year and am new to java. So far I have been able to do all my projects with ease however this one seems difficult. Here is what my teacher wants the code to look like:
----------------------------------------------------------------------------
Sample Output:

Please enter an integer: 150

Factored:

150 = 2 x 3 x 5 x 5
----------------------------------------------------------------------------
Could someone please help me? Here are some rules: we have not learned/not supposed to use arrays (idk if that helps), we know switch statements, for/do/while loops, if-statements, scanner class, math class, and some other stuff. Oh and this has to be done in java. Please respond as soon as possible. Thanks!

[blueduiker]

also it has to work for all numbers not just 150

[mrmatt1111]

What have you done so far? What approaches have you tried?

[bubbless]

Something like this?

int number = 150;
int factor = 0;

for(int i = 2; i <= number; i++) {
  if((number % i) == 0) {
    factor = i;
    break;
  }
}

if(factor != 0) {
  System.out.println("Factored: "+number+" = "+factor+" x "+number/factor);
} else {
  System.out.println("Not possible.");
}

You can put it in a loop until you have the final result.

[blueduiker]

i tried doing an extensive number of if statements but that hasn't worked because there are an infinite number of prime numbers, so I deleted the code

[blueduiker]

yeah i think that might work
thanks!

[blueduiker]

hmm i need to find a way to eliminate all none prime numbers
my code looks like:

import java.util.Scanner;
import java.text.DecimalFormat;
public class factor{
public static void main (String[] args) {

//This is my declaration section
int user_input, factor;
Scanner scan = new Scanner(System.in);

//This section receives the users input
System.out.print ("Please enter an integer to be factored:");
user_input = scan.nextInt();

System.out.println ("Factored:");
System.out.print (user_input + " = ");

for (int i = 2; i < user_input; i++) {
if ((user_input % i) == 0) {
factor = i;
System.out.print (factor + " x ");
}
}
}
}



The output for 24 with this code is:
Please enter an integer to be factored: [24]
Factored:
24 = 2 x 3 x 4 x 6 x 8 x 12 x



Also how do i get rid of that extra multiplication sign

[blueduiker]

Also 25 returns as 5 x
how do i have it return as 5 x 5

[bubbless]

Code tags please.

[blueduiker]

what are code tags?
sorry

[bubbless]

Put [ CODE ] and [ /CODE ] around your code. (without the spaces)

[blueduiker]

code looks like this so far:

import java.util.Scanner;
import java.text.DecimalFormat;
public class factor{
public static void main (String[] args) {

//This is my declaration section
int user_input, factor;
Scanner scan = new Scanner(System.in);

//This section receives the users input
System.out.print ("Please enter an integer to be factored:");
user_input = scan.nextInt();

System.out.println ("Factored:");
System.out.print (user_input + " = ");

for (int i = 2; i < user_input; i++) {
if ((user_input % i) == 0) {
factor = i;
System.out.print (factor + " x ");
}
}
}
}

[bubbless]

Try this as a method.

public method getNextFactor(int number) {
  int factor = 0;
  for (int i = 2; i < number; i++) {
    if ((number % i) == 0) {
      factor = i;
      break;
    }
  }
  return factor;
}

Use this method inside a loop to find all the factors.

[rdtindsm]

OP seems to have the right idea on displaying his ouput. Will leave it to him to figure that out.

What I have noticed is that he is incrementing his factor after he verifies that it is a factor. This will not be correct.

Consider a number like 40 (2 x 2 x 2 x5);
You will find 2, increment to 3 (not a factor), then increment to 4 which is a factor. But 4 is 2 x 2. You need to loop with the value 2 until you find that it is no longer a factor. Don't worry about the number being prime. You will find them as you progress in the algorithm.

http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

The sieve of eratosthenes merely finds the primes, but the reference will help you understand what you are doing.

Factor(X)
n=2
while (not end condition)
   if (n is a factor of X) {
   add n to factor list
   X = X / n
   } // if
   while(n is a factor of X {
      add n to factor list
      X = X / n
   }  //while
   if((  X  equals 1) ){ 
        end condition = true
   }  //if
   increment n
} //while
end Factor

edit: moved increment statement after if test, removed "and (n divides X) evenly.
"X" MUST be 1 on the last iteration, or there is another prime factor

edit:

[blueduiker]

thanks for all the hel

[blueduiker]

sorry i meant "help" not "hel"

[Eranga]

Hi blueduiker, if you have free time please read our FAQ page. You can find lots of useful things from there.