wood burning stoves 2.0*
The moose likes Beginning Java and the fly likes GCD calculation using Euclidean algorithm fails Big Moose Saloon
  Search | Java FAQ | Recent Topics | Flagged Topics | Hot Topics | Zero Replies
Register / Login


Win a copy of OCM Java EE 6 Enterprise Architect Exam Guide this week in the OCMJEA forum!
JavaRanch » Java Forums » Java » Beginning Java
Bookmark "GCD calculation using Euclidean algorithm fails" Watch "GCD calculation using Euclidean algorithm fails" New topic
Author

GCD calculation using Euclidean algorithm fails

John Eipe
Ranch Hand

Joined: May 23, 2008
Posts: 215
Hi,

I have here a simple program to find the GCD - Greatest Common Divisor of 2 numbers using Euclidean algorithm. I guess that's the fastest way to compute GCD.

Here is the code:


I believe that the logic is correct. And i don't get any errors. But the output is always 0. (i.e., diff=0)

Please HELP!

www.cs-repository.info
pete stein
Bartender

Joined: Feb 23, 2007
Posts: 1561
This is an important question, but I'm not sure it's a GUI question. I'd like to ask the moderators to move this question to a more appropriate forum.
John Eipe
Ranch Hand

Joined: May 23, 2008
Posts: 215
Thanks Pete. You are right; it's not a GUI question. This got posted there by mistake. How do I raise a request to moderators to move this post?
Maneesh Godbole
Saloon Keeper

Joined: Jul 26, 2007
Posts: 10269
    
    8

Done !


[How to ask questions] [Donate a pint, save a life!] [Onff-turn it on!]
Peter Tellanaki
Greenhorn

Joined: Apr 27, 2009
Posts: 7
hello,

you say
fo(; x-y !=0;){
diff=x-y
....
}
and then you print out diff. so in your for loop when you exit it diff is always zero.

[Edited: This is wrong. Please below see ]
Peter Tellanaki
Greenhorn

Joined: Apr 27, 2009
Posts: 7
sorry ! my wrong answer.
perhaps because of scope ?
you have global diff = 0. and [x and y] diff in for loop perhaps only local ?
Campbell Ritchie
Sheriff

Joined: Oct 13, 2005
Posts: 38509
    
  23
I have only ever used that algorithm in recursive methods.
John Eipe
Ranch Hand

Joined: May 23, 2008
Posts: 215
So you mean I need to change this into a recursive function to make it work.

Isn't there any other way? I wonder why for loop doesn't work.
Peter Tellanaki
Greenhorn

Joined: Apr 27, 2009
Posts: 7
oh my,
i really nedd sleep !
forget all i said. your program will work if both numbers positive. if one positive one negative infinit loop. if both negative 0 always.
the way you use Euclid you must make certain both numbers are >0. but this is ok since +- no effect on gcd. so just sub first
x = Math.max(x, y); y = Math.min(x, y); with fx x = max of absolute values of x and y.
Peter Tellanaki
Greenhorn

Joined: Apr 27, 2009
Posts: 7
so for example

public class GCD {
public static void main(String[] args) throws IOException{
BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
int diff=0;
System.out.println("Enter num1: ");
int x = Integer.parseInt(read.readLine());
System.out.println("Enter num2: ");
int y = Integer.parseInt(read.readLine());
x = Math.max(abs(x), abs(y));
y = Math.min(abs(x), abs(y));
[deleted]

System.out.print("GCD of the nums :"+diff);


}
private static int abs( int arg) {
return arg > 0 ? arg : -arg;
}
}

note: bad style with the static abs, but just to quick fix )

[edit]Delete solution. CR[/edit]
Ulf Dittmer
Marshal

Joined: Mar 22, 2005
Posts: 41624
    
  55
This is where the problem is:

x = Math.max(x, y);
y = Math.min(x, y);


Ping & DNS - my free Android networking tools app
Sridhar Santhanakrishnan
Ranch Hand

Joined: Mar 20, 2007
Posts: 317
@john,

Seems to work fine for me.
Tried it out for 121 and 1001.
Campbell Ritchie
Sheriff

Joined: Oct 13, 2005
Posts: 38509
    
  23
Peter Tellanaki, I am very sorry (please don't be annoyed with me), but I have felt obliged to pull rank and delete part of your submission. If you look on the Beginner's Forum start page, it says
We're all here to learn, so when responding to others, please focus on helping them discover their own solutions, instead of simply providing answers.
Simply providing the answer would prevent John Eipe from learning for himself.
Rob Spoor
Sheriff

Joined: Oct 27, 2005
Posts: 19684
    
  20

Ulf Dittmer wrote:This is where the problem is:

x = Math.max(x, y);
y = Math.min(x, y);

Exactly. You overwrite x with the maximum of x and y. Now if y is larger than x, x and y will end up having the same value.


SCJP 1.4 - SCJP 6 - SCWCD 5 - OCEEJBD 6
How To Ask Questions How To Answer Questions
John Eipe
Ranch Hand

Joined: May 23, 2008
Posts: 215
Sridhar,

How did you get it right? I don't think there are problems with negative numbers. As there are no cases of negative numbers coming in the code. As per the original post - we always take the greater and subtract the lesser one from it.

John Eipe
Ranch Hand

Joined: May 23, 2008
Posts: 215
Great Rob,
that's were the problem lies. I just substituted that part with a swap code:


and it works perfectly fine.

Thank you All.
Rob Spoor
Sheriff

Joined: Oct 27, 2005
Posts: 19684
    
  20

Of course you can change that to the following, eliminating the empty body of the if-statement:
And I think Ulf would like some gratitude as well
Campbell Ritchie
Sheriff

Joined: Oct 13, 2005
Posts: 38509
    
  23
Lots of people get confused about swapping like that.
 
Consider Paul's rocket mass heater.
 
subject: GCD calculation using Euclidean algorithm fails