tslnc04 tslnc04 - 7 months ago 12
Python Question

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?

Answer

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)