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pythonbitwise-operatorsnor

Python NOR Returning Odd Values

I am trying to get a NOR of 2 values

a = 0b1010
b = 0b0101

print(~ (a | b))

The current output is -16, but if I do this by hand

   1010
OR 0101
--------
   1111

NOT 1111
--------
    0000

So, this should give a value of 0, not -16. Why does it do this? How can I fix this?

Solution

  • These operations are being done with 32-bit integers (or 64-bit integers in a 64-bit version of Python).

        0000 0000 0000 0000 0000 0000 0000 1010
 OR 0000 0000 0000 0000 0000 0000 0000 0101
 ------------------------------------------
    0000 0000 0000 0000 0000 0000 0000 1111

NOT 0000 0000 0000 0000 0000 0000 0000 1111
-------------------------------------------
    1111 1111 1111 1111 1111 1111 1111 0000

    Which, taken as a signed integer, is the two's complement representation of -16, because you have to add 16 to reach zero (and a carry).

    To fix it, explicitly xor with 0b1111 instead of using ~.

    print((a | b) ^ 0b1111)