Bitwise Operators - Uses

• Efficient storage.
  • Status bits. Eg, Mouse event key mask.
  • Eg, representing Connect-Four board.
  • Packing or unpacking values into a single int/long.
    
    A common use of the bitwise operators (shifts with ands to extract values and ors to add values) is to work with multiple values that have been encoded in one int. Bit-fields are another way to do this. For example, let's say you have the following integer variables: age (range 0-127), gender (range 0-1), height (range 0-128). These can be packed and unpacked into/from one short (two-byte integer) like this (or many similar variations).
    

    //define the variables 

    
    

    int   age, gender, height;
    short packed_info;
    . . .
    // packing
    packed_info = (((age << 1) | gender) << 7) | height;
    . . .
    // unpacking
    height = packed_info & 0x7f;
    gender = (packed_info >>> 7) & 1;
    age    = (packed_info >>> 8);
    

    
    
  • Setting flag bits
    Some library functions take an int that contains bits, each of which represents a true/false (boolean) value. This saves a lot of space and can be fast to process.
  
  
• Efficient computation
  
  
  
  • On some (esp old) machines, shifting is faster than multiplying or dividing by powers of two.
    

    y = x << 3;        // Assigns 8*x to y.
    y = (x << 2) + x;  // Assigns 5*x to y.

    
    
  • Flipping between on and off with xor
    
  
  
  Sometimes xor is used to flip between 1 and 0.
  
  
  

  x = x ^ 1;      // Or the more cryptic x ^= 1;

  
  
  In a loop that will change x alternately between 0 and 1.
  
  
  
• Problems
  
  
  
  • How could you use bit operations to test whether a number is odd?
    
  • How would you multiply by 256?
    
  • How would you swap 2 integers, without using temporary variable?