Confusing PHP bitwise NOT behavior

Question

In PHP, if I run the following simple program

$number = 9;
var_dump( ~ $number );

my output is

int(-10)

This is confusing to me. I thought ~ was the bitwise NOT operator. So I was expecting something like.

if binary 9 is     00000000000000000000000000001001 
then Bitwise NOT 9 11111111111111111111111111110110

Meaning ~9 should come out as some ludicrously large integer like 4,294,967,286.

What subtly of precedence, type coercion, or something else am I missing here?

It looks like it's being returned as a signed number, in which case -10 is correct. — Chris Forrence
– Chris Forrence, Commented Sep 12, 2013 at 2:11

Alana Storm · Accepted Answer · 2013-09-12 02:44:21Z

4

Your output is defaulting to a signed int - wrap it in decbin to get a binary representation.

Consider:

$number = 9;
var_dump(  bindec(decbin(~ $number)) );

With two's compliment, the MSB of a signed binary number becomes 0-MSB, but every other bit retains its respective positive values.

So for argument's sake (an 8-bit example),

Binary 9: 0000 1001
Inverse:  1111 0110

This results in (-128) + 64 + 32 + 16 + 4 + 2 = -10, so PHP is calculating correctly, its just applying two's compliment to the MSB.

edited Sep 12, 2013 at 2:44

Alana Storm

167k96 gold badges422 silver badges623 bronze badges

answered Sep 12, 2013 at 2:21

calcinai

2,62716 silver badges25 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Alana Storm Over a year ago

+1 for good information, but I'm not sure I follow that — are you saying it's being correctly converted to 11111111111111111111111111110110, and then to 4,294,967,286, but because of integer wrap around it ends up at -10?

Collectives™ on Stack Overflow

Confusing PHP bitwise NOT behavior

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related