meaning of (number) & (-number)

Question

What is the meaning of (number) & (-number)? I have searched it but was unable to find the meaning

I want to use i & (-i) in for l

Answer 1

Assuming that negative values are using two's complement. Then -number can be calculated as (~number)+1, flip the bits and add 1.

For example if number = 92. Then this is what it would look like in binary:

number               0000 0000 0000 0000 0000 0000 0101 1100
~number              1111 1111 1111 1111 1111 1111 1010 0011
(~number) + 1        1111 1111 1111 1111 1111 1111 1010 0100
-number              1111 1111 1111 1111 1111 1111 1010 0100
(number) & (-number) 0000 0000 0000 0000 0000 0000 0000 0100

You can see from the example above that (number) & (-number) gives you the least bit.

You can see the code run online on IDE One: http://ideone.com/WzpxSD

Here is some C code:

#include 
#include 
#include 
using namespace std;

void printIntBits(int num);
void printExpression(char *text, int value);

int main() {
  int number = 92;

  printExpression("number", number);
  printExpression("~number", ~number);
  printExpression("(~number) + 1", (~number) + 1);
  printExpression("-number", -number);
  printExpression("(number) & (-number)", (number) & (-number));

  return 0;
}

void printExpression(char *text, int value) {
  printf("%-20s", text);
  printIntBits(value);
  printf("\n");
}

void printIntBits(int num) {
    for(int i = 0; i < 8; i++) {
        int mask = (0xF0000000 >> (i * 4));
        int portion = (num & mask) >> ((7 - i) * 4);
      cout << " " << std::bitset<4>(portion);
    }
}

Also here is a version in C# .NET: https://dotnetfiddle.net/ai7Eq6