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# reversed and & or operators

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This is for people who know about the and, or, xor, etc. operators. Is there a way to reverse the effect of and & or? For example: 204 & 170 = 136 11001100 (204) & 10101010 (170) = 10001000 (136) Now can we get the number back to 204 using just 170 and 136? ----------------------------------------------- Here comes the Thnikkaman!

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Unfortunately not, let's look at the & operator:

a b AND
1 1 1
1 0 0
0 1 0
0 0 0

So let's think we only know b & the result and let's try to guess what a was:

b AND Guess of a
1 1 must be 1
0 0 0 or 1
1 0 must be 0
0 0 0 or 1

So we know for certain only the bits of a where the corresponding bit of b was 1. It works quite the same for OR too, but reversed.

[edited by - clb on June 6, 2004 5:30:23 AM]

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Xor can be reversed:

A xor B = C <==> C xor B = A

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