CSC195, Class 14: Fun with Bitwise Operations

Overview:
* Summary of bitwise operations
* Using bitwise operations
* Lab 

Notes:
* No homework (yet) so that you have time to finish the labs.
* Read K&R 6.1, 6.7-6.9
* CS195/201 passed the division.  It's on to the curriculum 
  committee.
* Snacks.  Sam is easy to manipulate.  
* Any questions on yesterday's work?

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C provides a host of operations on integers that reflect
the underlying bit representation of those integers

* &
* |
* ~
* ^
* <<
* >>

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Why would a programming language provide these operations?
* Reminds you that the language is low-level.
* You might want to do "funky low-level manipulations"
* Efficiently interact directly with .... other hardware.

* We could apply Gries-style logic to everything!  Yay!

* "Smart" programmers can choose appropriate operations.
  + Divide by 4: Shift right two bits.
  + Multiply by 8: Shift left three bits.

* Efficient storage of sets of Boolean values (and other
  small values)
* E.g. permissions:
b b b b b b b b b
r w x r w x r w x
u u u g g g o o o

We can use the bitwise operations to combine settings:

Writeable by other: 2
Readable by group is 32
Writable by other and readable by group: |

Good C programmers, trying to be efficient, often define
a set of constants that can be logically combined.

#define WRITEABLE 02
#define READABLE  04
#define EXECUTABLE 01

myprog = WRITABLE | EXECUTABLE

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REFLECTIONS

* Integer values that start with 0 are octal
  + Octal 07: Binary 111
  + Octal 06: Binary 110
  + Octal 070: Binary 111000
* Integer values that start with 0x are hexidecimal
* Example of potentially good testing
* How to write "printbits"