Bit Manipulation Hacker's Delight in Python

Question

I’m trying to get into the book "Hacker’s Delight." The first formula in chapter 2 is x & (x -1) and is supposed to "turn off the rightmost 1-bit in a word producing 0 if none
(e.g. 01011000 -> 0101000)." I have no idea why someone would want to do this.

I translated this into python as
bin(0b1011000 and (0b1011000 - 1)) and got
'0b1010111'. Is this correct?

I tried leaving out the "b" designating leading zeros and got this wild result '0b11110110110100110111'.

Am I close to correct?

Asked By: Ryan

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Source

Answer 1

Try these a few examples and see if you can tell the correctness yourself:

>>>x = 15
>>>bin(x)
'0b1111'

>>>x & (x -1)
14
>>>bin(14)
'0b1110'

# now try this x = 10
>>>x = 10
>>>bin(x)
'0b1010'
>>>x & (x - 1)
8
>>>bin(8)
'0b1000`     # <---- see the rightmost bit is gone *rightmost 1*

Answered By: Daniel Hao

Bit Manipulation Hacker's Delight in Python

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