CHAPTER 7 
=========

bitwise operators
-----------------

 o act on integral expressions
 o precedence: ~ ; <<,>> ; & ; ^ ; |  

example
-------

int a = 33333, b = -77777;

Expression   Representation                         Value 


a            00000000 00000000 10000010 00110101    33333 
b            11111111 11111110 11010000 00101111    -77777
a & b        00000000 00000000 10000000 00100101    32805 
a ^ b        11111111 11111110 01010010 00011010    -110054 
a | b        11111111 11111110 11010010 00111111    -77249 
~(a | b)     00000000 00000001 00101101 11000000    77248 
~a & ~b      00000000 00000001 00101101 11000000    77248 

===================================================================

left and right shift
--------------------
 
 o integral promotion of each operand
 o the type of the result is the promoted type of the left-hand side
   operand

example
-------

char     c = 'Z';
int      a = 1 << 31; /* shift 1 to the high bit */
unsigned b = 1 << 31;

Expression   Representation                         Action
 
c		00000000 00000000 00000000 01011010    unshifted
c << 4	00000000 00000000 00000101 10100000    left-shifted 4 

a		10000000 00000000 00000000 00000000    unshifted 
a >> 3	11110000 00000000 00000000 00000000    right-shifted 3 

b		10000000 00000000 00000000 00000000    unshifted 
b >> 3	00010000 00000000 00000000 00000000    right-shifted 3 

Example:
int a=1, b=2;
(a+b << 12*a >> b) is equivalent to (((a+b)<<(12*a))>>b)
and yields 00001100 00000000

====================================================================

Operator precedence and associativity, a final look
---------------------------------------------------

	 Operators                           Associativity
-------------------------------------------------------------
() [] . -> ++(postfix) --(postfix)           left to right
-------------------------------------------------------------
+(unary) -(unary) ++(prefix) --(prefix) !    right to left
sizeof(type) &(address) *(dereference) ~
-------------------------------------------------------------
*  /  %                                      left to right
-------------------------------------------------------------
+  -                                         left to right
-------------------------------------------------------------
<< >>                                        left to right
-------------------------------------------------------------
<  <=  >  >=                                 left to right
-------------------------------------------------------------
== !=                                        left to right
-------------------------------------------------------------
&                                            left to right
-------------------------------------------------------------
^                                            left to right
-------------------------------------------------------------
|                                            left to right
-------------------------------------------------------------
&&                                           left to right
-------------------------------------------------------------
||                                           left to right
-------------------------------------------------------------
?:                                           right to left
-------------------------------------------------------------
=  +=  -=  *=  /=  &= >>= etc                right to left
-------------------------------------------------------------
,(comma operator)                            left to right
-------------------------------------------------------------

====================================

(int-to-binary.c)

/* Bit print an int expression */

#include <limits.h>
#include <stdio.h>

void bit_print(int a)
{
   int   i;
   int   n = sizeof(int) * CHAR_BIT;   /* in limits.h */
   int   mask = 1 << (n - 1);          /* mask = 100...0 */

   for (i = 1; i <= n; ++i) {
      putchar(((a & mask) == 0) ? '0' : '1');
      a <<= 1;
      if (i % CHAR_BIT == 0 && i < n)
         putchar(' ');
   }
}

int main(void)
{
   int   k;

   for ( ; ; ) {
      printf("Input an integer:  ");
      scanf("%d", &k);
      printf("\n%7d = ", k);
      bit_print(k);
      printf("\n\n");
   }
   return 0;
}

======================================================================

/* Pack 4 characters into an int */

#include <limits.h>

int pack(char a, char b, char c, char d)
{
   int   p = a;     /* p will be packed with a, b, c, d */

   p = (p << CHAR_BIT) | b;
   p = (p << CHAR_BIT) | c;
   p = (p << CHAR_BIT) | d;
   return p;
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/* Unpack a byte from an int */

#include <limits.h>

char unpack(int p, int k)               /* k = 0, 1, 2, or 3 */
{
   int        n = k * CHAR_BIT;         /* n = 0, 8, 16, or 24 */
   unsigned   mask = ((1<<CHAR_BIT)-1); /* low-order byte */

   mask <<= n;
   return ((p & mask) >> n);
}