CODEWITHSUNDEEP
language-agnosticbit-manipulationbitwise-operators
Kapil
Kapil

Reputation: 9959

Opposite of Bitwise OR operation

I compute

c = a 'OR' b // bitwise OR operation here

Now given only values of c and b how can I compute the original value of a?

Upvotes: 8

Views: 10250

Answers (6)

Burkhard
Burkhard

Reputation: 14738

Since a OR 1 is always 1 and a OR 0 is always a you can only find the value of a if b is 0.

Edit: AND and OR are lossy operations (cannot always be reversed). Whereas XOR and NOT are lossless/reversible.

Upvotes: 4

Cambium
Cambium

Reputation: 19392

This is impossible.

A simple case to demonstrate my point (assuming a, b, and c are all 1-bit):

If 'b' is 1, 'c' will always be 1, you cannot determine the value of 'a'.

Upvotes: 25

Guffa
Guffa

Reputation: 700212

That's not possible, the or operation is not reversible. There are many different values of a that give the same value for c.

You can get one possible value of a by doing an and operation with the complement of b.

a = c & ~b

Upvotes: 4

pierroz
pierroz

Reputation: 7870

from a mathematic point of view it is just not possible to deduce A from C and B. if, for the nth bit you have the value 1 in C and in B you can't know if the nth bit in A equals 0 or 1

Upvotes: 1

joeslice
joeslice

Reputation: 3454

You cannot reliably go back. For example, a = 0010 and b = 0011. a OR b = 0011. The same result is true if a was different (0001 or 0011 for example).

Upvotes: 5

Philippe Leybaert
Philippe Leybaert

Reputation: 171744

That's not possible. There is no way to determine what 'a' will be

Upvotes: 1

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