CHAPTER 7 
=========

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

 o act on integral epxressions
 o system dependent
 o we assume here 8-bit bytes, 4-byte words, ascii character code

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 

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

/* Bit print an int expression */

#include <limits.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(' ');
   }
}

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

/* Test bit print */

include <stdio.h>

void   bit_print(int a);

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 = 255;           /* low-order byte */

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

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

/* Create employee data in a short int. */

short create_employee_data(int id_no,int job_type,char gender)
{
   short   employee = 0;     /* start with all bits off */

   employee |= (gender == 'm' || gender == 'M') ? 0 : 1;
   employee |= job_type << 1;
   employee |= id_no << 7;
   return employee;
}

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

enumeration types
-----------------

enum - keyword

type declaration:
enum day {sun, mon, tue, wed, thu, fri, sat};

sun, mon, ... are int constants: sun==0, mon==1, ...

'enum day' is a type, declaring variables:
enum day d1, d2;

d1 = fri;

if (d1 == d2) ...

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

enumerators can be intialized
-----------------------------

enum suit {clubs = 1, diamonds, hearts, spades} a, b, c;

enum fruit {apple = 7, pear, orange = 3, lemon}  frt;

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

OK if tag name is missing, 
enum {fir, pine} tree;
but cannot define more variables of the type:

defining a shorter type name:
enum day {sun, mon, tue, wed, thu, fri, sat};
typdef enum day day;