GCD calculation using Euclidean algorithm fails

[John Eipe]

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:

 
    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(x, y);
        y = Math.min(x, y);
        
 
        
        for(;(x-y)!=0;){
            diff = x-y;
            x = Math.max(diff,y);
            y = Math.min(diff,y);
        }
        
        System.out.print("GCD of the nums :"+diff);
        
        
    }
}   

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!

[pete stein]

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]

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]

Done !

[Peter Tellanaki]

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]

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]

I have only ever used that algorithm in recursive methods.

public static int gcd(int i, int j)
{
   int k = Math.max(i, j);
   int l = Math.min(i, j);
   int result = . . .
   if . . .
      result = gcd( . . . );
   else . . .
   return result;
}

[John Eipe]

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]

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]

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]

This is where the problem is:

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

[Sridhar Santhanakrishnan]

@john,

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

[Campbell Ritchie]

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]

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.

[John Eipe]

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]

Great Rob,
that's were the problem lies. I just substituted that part with a swap code:

int temp;
if(x==Math.max(x, y)){
            
        }
        else{
            temp=x;
            x=y;
            y=temp;
        }    

and it works perfectly fine.

Thank you All.

[Rob Spoor]

Of course you can change that to the following, eliminating the empty body of the if-statement:

if (x != Math.max(x, y))
{
    int temp = x; // declaration moved to inside the if-statement; it's not needed outside
    x = y;
    y = temp;
}    

And I think Ulf would like some gratitude as well

[Campbell Ritchie]

Lots of people get confused about swapping like that.