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Write a program that read an integer and display its smallest prime factors in ascending order. For example, if the input is 60, the output should be 2,3,5...
I came out with below code, but I get 2 2 3 5.How to have only one 2 ?

What I have tried:

package chapter4;

import java.util.Scanner;

public class Exercise4_16 {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		System.out.print("Enter an integer: ");
		Exercise4_16 exe = new Exercise4_16();
		int value = input.nextInt();
		exe.primeFactors(value);
	}

	public void primeFactors(int value) {
		while (value % 2 == 0) {
			System.out.print(2 + " ");
			value = value / 2;
		}

		for (int i = 3; i <= Math.sqrt(value); i = i + 2) {
			while (value % i == 0) {
				System.out.print(i + " ");
				value /= i;
			}
		}

		if (value >= 2) {
			System.out.print(value);
		}
	}
}
Posted
Updated 22-Dec-17 22:18pm

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
C#
private int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on your line:
C#
exe.primeFactors(value);

and run your app. Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
 
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Quote:
For example, if the input is 60, the output should be 2,3,5...
I came out with below code, but I get 2 2 3 5.How to have only one 2 ?

Probably because 60=2*2*3*5
One can guess that 27 will give 3*3*3 doe same reason.

You need to change your code to print a factor once, no matter how many times it is in the integer. You need to separate the print of factor and the reduction on integer.
 
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Comments
wseng 25-Dec-17 2:47am    
How can I print a factor once?

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