binary operation to get operands from result value

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x & y =R where '&' is a bitwise AND operator
then how do I get a value x using values R and y?

Posted 2-Aug-11 20:21pm

Shreyajit

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Sergey Alexandrovich Kryukov 3-Aug-11 3:20am

Is it interview question? Very elementary, but makes sense to check up basic understanding of things. You really did not know the answer, did you? Because you don't need any knowledge, just a pure logic.
--SA

Shreyajit 3-Aug-11 5:08am

No I do have the basic knowledge. What I want that is there any solution for this kind of problems so that I can get my operand values back from the result value with the help of bitwise operators

CPallini 3-Aug-11 3:24am

I suppose you should be persuaded, at least on statistical grounds, that bitwise and is a lossy operation. :-D

5 solutions

Solution 4

Generally speaking you cannot.
Suppose, for instance, y=0, then R=0 regardless of the value of x.
On the other hand, if all bits of y are set then you have full info about x, since x=R.
In other words:

if a bit of y is set (equal to 1) then corrensponding bit of x is equal to the corresponding bit of R.
if a bit of y is reset (equal to 0) then R provides no useful info and the corrensponding bit of x may be either 1 or 0.

Posted 2-Aug-11 21:20pm

CPallini

Updated 2-Aug-11 21:21pm

v2

Comments

Sergey Alexandrovich Kryukov 3-Aug-11 3:31am

Clear explanation, my 5.
--SA

CPallini 3-Aug-11 3:39am

Thanks, you already had my five too. :-)

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-08-02T21:13:00

It is not possible. For example, consider the following case:

x=0110 //will be unknown
&
Y=0101
=====
R=0100

You can restore all bits except bits #1 and #3, where both the bit of known operand and a bit of result are both 0. In this case, the bit of the first (unknown) operand can be either 0 or 1, it would not affect the result bit. In this example, x can be 1110, 0110, 1100 or 0100.

—SA

OriginalGriff · Accepted Answer · 2011-08-02T21:15:00

Solution 2

You can't - AND "throws away" information.
If you have the binary values 101 and 110, and you AND them together, you get the binary result 100. from that result you can say that both X and Y are members of the set { 100, 101, 110, 111 } - if there are only three bits involved - but you cannot say which value was assigned to which variable.

Posted 2-Aug-11 21:15pm

OriginalGriff

Comments

Sergey Alexandrovich Kryukov 3-Aug-11 3:17am

Too late: I was the first with the same answer :-). Same time, in fact, my 5.
--SA

YvesDaoust · Accepted Answer · 2011-08-02T21:26:00

Solution 5

The XOR operation is reversible.

If R = x ^ y, then x = y ^ R.

Posted 2-Aug-11 21:26pm

YvesDaoust

Comments

Sergey Alexandrovich Kryukov 3-Aug-11 3:30am

Perfectly true, so what? It does not answer the question. Addition and subtraction are also reversible...
(I did not vote.)
--SA

YvesDaoust 5-Aug-11 3:34am

Just answering the question that's behind the question.

Agus Syahputra · Accepted Answer · 2011-08-02T21:16:00

With '&' operator, you cannot get the x value.

Suppose x=5 and y=4, then r=4

Behind the scenes it would done like so :

x= 0101
y= 0100
   ----&
r= 0100

And if we have x=5 and y=6, then we also get r=4

Behind the scenes it would done like so :

x= 0101
y= 0110
   ----&
r= 0100

You see, 5 & 4 and 5 & 6 have the same result.

So, how would you get the x value ? The answer is never. Because '&' operator does the bitwise operation, not arithmetic operation.

binary operation to get operands from result value

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