Why is (x & 0) different from (x & ~1)?

Question

Why is x & 0 different from x & ~1?

Shouldn't ~1 and 0 be the same since 0 is the complement of ~1?

x = 21
print(f"x = {bin(x)}")
print(f"x & 0 = {bin(x & 0)}")
print(f"x & ~1 = {bin(x & ~1)}")

results in

x = 0b10101
x & 0 = 0b0
x & ~1 = 0b10100

@EugeneSh. not quite true in Python, since integers are arbitrary length you'd need an infinite number of leading `1`s. — Mark Ransom, Jun 25 '21 at 17:05
@EugeneSh. That's 1s complement; Python doesn't use that. `~x` is equal to `-(x+1)` [for an integer `x`](https://github.com/python/cpython/blob/32bd68c839adb7b42af12366ab0892303115d1d1/Objects/longobject.c#L4300-L4303). — Mustafa Aydın, Jun 25 '21 at 17:11
@MarkRansom Actually I have missed the fact the question is for Python, so I guess my comment is inaccurate. I will remove it to prevent confusion. — Eugene Sh., Jun 25 '21 at 17:13

score 1 · Accepted Answer · answered Jun 25 '21 at 17:13

1

You are getting confused between ~and not here,

x & (not 1) will be equal to 0

as not 1 is 0 but ~1 is -0b10 which is -2 for a 2-bit representation.

You can read more on ~ operator in this link

answered Jun 25 '21 at 17:13

Shreyansh

But in Python, bitwise NOT is used using this (~) symbol – Aldrin Saurov Sarker Jun 25 '21 at 17:16
https://www.geeksforgeeks.org/python-bitwise-operators/ – Aldrin Saurov Sarker Jun 25 '21 at 17:16
@AldrinSaurovSarker the bitwise note is ~ only, but bitwise not of 1 is not 0, not 1 is 0. as 1 is represented as 0b01 hence ~(0b01) is -0b10. – Shreyansh Jun 25 '21 at 17:19

score 0 · Answer 2 · answered Jun 25 '21 at 17:14

0

~ and & are bitwise operations, not logical operations. The logical expression "x and False" indeed yields False, as does "not True":

print x and False
print x and not True

answered Jun 25 '21 at 17:14

Erpelstolz

2 Answers2