0

I find '&' in python means 'and' operation based on bit expression. Recently, I find a very smart code and one line is like 'i & -i' where i is a integer. How to understand the result of 'i & -i'. In addition, how python deal with negative integer '-i' for bit manipulation?

OneCricketeer
  • 179,855
  • 19
  • 132
  • 245
Complex
  • 1
  • 1
    Negative numbers are in [two's complement](https://en.wikipedia.org/wiki/Two's_complement) with an infinite number of leading ones (just like positive numbers have an infinite number of leading zeros). – Kevin Feb 12 '17 at 19:30
  • 1
    Yes, bit manipulation is very important because that's how your CPU does maths! [Very interesting read](https://graphics.stanford.edu/~seander/bithacks.html), as well as [this](https://www.hackerearth.com/practice/notes/bit-manipulation/). – ForceBru Feb 12 '17 at 19:31
  • 2
    Essentially a duplicate of http://stackoverflow.com/questions/41969429/why-the-bit-operation-i-i-equals-to-rightmost-bit, aside from the slight caveat that Python integers aren't fixed-width. – user2357112 Feb 12 '17 at 19:33
  • @user2357112 the proof I gave there also applies to left-infinite bit strings so it proves it for Python-type integers as well – harold Feb 12 '17 at 21:14
  • @user2357112 did you realize that you are genius? – Complex Feb 13 '17 at 22:46

1 Answers1

1

"i & -i" - this is clear all bits "1", but last significant one. For example:

i = 10(dec) = 00001010(bin)

i & -i will be 00000010(bin) = 2(dec)

By math terms, "i & -i" returns maximal 2^N, which is divider of "i".

More examples:

i(dec)   i(bin)     i&-i
 1            1        1
 5          101        1
 8         1000     1000
12         1100     0100
olegarch
  • 3,670
  • 1
  • 20
  • 19