~1 and ~0 giving strange results in python 3

Question

&, |, ^, and ~ are all bitwise operators in python. &, ^, and | are all working fine for me - when i take, say, 1|0, I get 1. But ~ is giving me strange results. ~1 gives me -2, and ~0 gives me -1. Is this because I'm using integers or something? I'm running python 3.

I'm hoping to get 1 from ~0, and 0 from ~1 (the integers). Is this possible?

@nos `-2` and `-1` respectively. Interestingly `~True` and `~False` give the same results — Cory Kramer, Jan 22 '15 at 19:12
it is all in the documentation https://wiki.python.org/moin/BitwiseOperators — Padraic Cunningham, Jan 22 '15 at 19:15
possible duplicate of [bit-wise operation unary ~ (invert)](http://stackoverflow.com/questions/7278779/bit-wise-operation-unary-invert) — Jasper, Jan 22 '15 at 19:20

score 5 · Accepted Answer · answered Jan 22 '15 at 19:14

5

From here

~x

Returns the complement of x - the number you get by switching each 1 for a 0 and each 0 for a 1. This is the same as -x - 1.

Following the last part of that statement:

-1 - 1 does indeed equal -2

and

-0 - 1 does indeed equal -1

answered Jan 22 '15 at 19:14

Cory Kramer

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Jasper · Answer 2 · 2015-01-22T19:19:27.213

That's because of the two's complement implementation of integers.

If you switch all bits from 0000 0000 (assuming 8 bit integers here, but it's still valid for larger ones), you get 1111 1111. In two's complement interpretation, that's -1, because to represent -1, you take 1, invert all bits and add one:

   0000 0001 (= 1)
-> 1111 1110 (inverted)
-> 1111 1111 (added one, now this is '-1')

The same works for your second example.

~1 and ~0 giving strange results in python 3

2 Answers2