SCJP Brainteaser (9)

[Shaan Shar]

What would be the value of X and Y

public class Sample {
    public static void main(String[] args) {
        int x = 1984;
        int y = 2001;
        x ^= y ^= x ^= y;
    }
}

Please put your thoughts here and yes important thing you shouldn't use JAVA Compiler for this exercise.....

It would be great if you use only your brain not JVM.

[Shaan Shar]

Any predictions ??? or no predictions..?
[ November 10, 2006: Message edited by: Sharma Ji ]

[Joe San]

Hi Sharma Ji,

I'm weak with what ^ and >> << operators does...I can say onething is that the code compiles and it is interpreted as below,

x = x ^(y^=y^(x=x^y))

How it runs is like the bold part is evaluated first, the result is evaluated with the italicized part which is then evaluated with x and the result assigned to x.

Am I correct???

[Shaan Shar]

Originally posted by Jothi Shankar Kumar Sankararaj:
Hi Sharma Ji,

I'm weak with what ^ and >> << operators does...I can say onething is that the code compiles and it is interpreted as below,

x = x ^(y^=y^(x=x^y))

How it runs is like the bold part is evaluated first, the result is evaluated with the italicized part which is then evaluated with x and the result assigned to x.

Am I correct???

So what would be your final answers ??

what would be the final values of X and Y

[Joe San]

Hi Sharma Ji,

I have taken several mock exams and when I come across questions that ask about the use of ^ operators, I just skip them and guess them at the and after attempting all other questions. They are Integer bit wise operators and I'm afraid of them simply because I dont know what 1984 and 2001 will look like in bits. If anyone in this post tells me an easy way to sneak the bit pattern out, that would be of great help to me.

The point is the way the compiler interprets it is as shown above in my post. Is that right or wrong. I dont want to compile it and give the answer but rather I want to do it using my brain

So anyone please explain this.

[Shaan Shar]

Originally posted by Sharma Ji:
Any predictions ??? or no predictions..?

Bump ???

No solutions till now....

[anvesh charuvaka]

The value of x is copied into y therefore y=1984
The value of x becomes 0


The ^ operator known as XOR has the following truth table

0 0 0
0 1 1
1 0 1
1 1 0

i.e if both the bits are same then the result is false. if they differ then the result is true.


Solution

After this operation
x = x^y;
in the bit positions where x and y differ the resultant bit is set to 1 and where they are same the resultant bit is set to 0.

y=y^x;
now x has the bit pattern as described above.
for the positions where x and y had same bit patterns the bit value in y at that position is XORed with 0 which returns the same bit because

0 ^ 0 = 0
1 ^ 0 = 1

for the bit positions where x and y had different bits the bit value in y at that positions is XORed with 1 which returns the inversion of that bit because

0 ^ 1 = 1
1 ^ 1 = 0

hence the result in y is x.


now the value of x is simpler to solve
the final expression
x=x^y;

can be re written as

x=x^(y^x) // here the y value is the previous y value
and x value has not changed after y=y^x
=> x=x^x^y // previous value of y
now any number XORed with itself is 0

[ November 14, 2006: Message edited by: anvesh charuvaka ]

[ November 14, 2006: Message edited by: anvesh charuvaka ]
[ November 14, 2006: Message edited by: anvesh charuvaka ]

[Luiz Claudio Fran�a dos Santos]

It's a bug http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4672380

[Luiz Claudio Fran�a dos Santos]

try this one

 int x = 2;
       x = x * (x = 1);
       System.out.println(x);

What is the output?
[ November 14, 2006: Message edited by: Luiz Claudio Fran�a dos Santos ]

[Naveen Zed]

There lies a big calculation
The code behind is I think

  x=1984^(y=2001^(x=x^y)) 

in above code x=x^y gives (1984)11111000000 ^ (2001)11111010001 ==010001

so agin XOR with 2001(11111000000)^17(010001) gives 1984(11111000000)

finally y is 1984
again x=1984^1984 gives 0

So the final answar is x=0 and y=1984

[Naveen Zed]

sorry there is a tyo in above answar

in above code x=x^y gives (1984)11111000000 ^ (2001)11111010001 ==010001

so agin XOR with 2001(11111010001)^17(010001) gives 1984(11111000000)

finally y is 1984
again x=1984^1984 gives 0

So the final answar is x=0 and y=1984