python - What does the unary operator ~ do in numpy?

Question

I came across a line of code using Python's numpy that looked like this:

~array([0,1,2,3,4,5,4,3,2,1,0,-1,-2])

And it gave the output:

array([-1, -2, -3, -4, -5, -6, -5, -4, -3, -2, -1,  0,  1])

Does the unary operator (~) take an array and apply A -> -(A+1)

If so, whats the point?

It appears to be bitwise negation, just like it is in C. The operator applies to normal Python integers (at least it does for me in 2.5. Maybe I should upgrade...) — Chris Lutz

Blair Conrad Blair Conrad · Accepted Answer · 2010-08-06T21:49:06

Chris Lutz' comment is correct.

~ is the bitwise negation operator

It looks like it turns A to -(A+1) because on many modern computers, negative numbers are represented as the Two's Complement of the corresponding positive integer, where the number is subtracted from 2^(bit length) (that's "two to the power of bit length", not "two exclusive or bit length"...).

In such a system, -1 would be represented as all ones. Of course, so would the sum of a number and its bitwise negative, so we have the situation where

a + ~a = -1        =>
    ~a = -1 - a    =>
    ~a = -(a + 1)

as you noticed.

python - What does the unary operator ~ do in numpy?

4 Answers