Meaning of bitwise and(&) of a positive and negative number?

Question

Can anyone help what n&-n means?? And what is the significance of it.

Answer 1

It's just a bitwise-and of the number. Negative numbers are represented as two's complement.

So for instance, bitwise and of 7&(-7) is x00000111 & x11111001 = x00000001 = 1

Answer 2

Because x & -x = {0, 1, 2, 1, 4, 1, 2, 1, 8, 1, 2, 1, 4, 1, 2, 1, 16, 1, 2, 1, 4, 1, 2, 1, 8, 1, 2, 1, 4, 1, 2, 1, 32} for x from 0 to 32. It is used to jumpy in the for sequences for some applications. The applications can be to store accumulated records.

for(;x < N;x += x&-x) {
    // do something here
    ++tr[x];
}

The loop traverses very fast because it looks for the next power of two to jump.

Answer 3

On pretty much every system that most people actually care about, it will give you the highest power of 2 that n is evenly divisible by.

Answer 4

I would add a self-explanatory example to the Mark Randsom's wonderful exposition.

010010000 | +144 ~
----------|-------
101101111 | -145 +
        1 |
----------|-------
101110000 | -144 

101110000 | -144 & 
010010000 | +144
----------|-------
000010000 |   16`

Answer 5

As @aestrivex has mentioned, it is a way of writing 1.Even i encountered this

for (int y = x; y > 0; y -= y & -y)

and it just means y=y-1 because
7&(-7) is x00000111 & x11111001 = x00000001 = 1

Answer 6

I believe it is a trick to figure out if n is a power of 2. (n == (n & -n)) IFF n is a power of 2 (1,2,4,8).