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pythonbitwise-operatorsbit

Bitwise operator ~ isn't simply "flipping bits" in python. Why is that?

I'd expect bin(~0b111000) to return the value 0b000111 because to my understanding the NOT operation would return the opposite bit as output.

I keep reading that "~x: Returns the complement of x - the number you get by switching each 1 for a 0 and each 0 for a 1" so I don't exactly know where my logic breaks down.

Why does it show -(x + 1) instead of just literally flipping all bits?

Solution

  • It is flipping all the bits!

    You seem to think of 0b111000 as a 6bit value. That is not the case. All integers in python3 have (at least conceptually) infinitely many bits. So imagine 0b111000 to be shorthand for 0b[...]00000111000.

    Now, flipping all the bits results in 0b[...]11111000111. Notice how in this case the [...] stands for infinitely many ones, so mathematically this gets interesting. And we simply cannot print infinitely many ones, so there is no way to directly print this number.

    However, since this is 2s complement, this simply means: The number which, if we add 0b111001 to it, becomes 0. And that is why you see -0b111001.