The Ugly Truth

1
The Ugly Truth

• Computer programming languages, tools make
  tinkering and elaboration too easy!
• RESULT Simple programs grow endlessly, become
  unreliable, ugly, confusing, impossible to
  understand and improve.EXAMPLE MS Office2000
  1,182 files, 104 folders, 193.65 MB, MS Word
  2.0 40 files, 8 folders, 0.78 MB!
• The history of CS is saw-toothed we keep
  throwing away the big ugly mess we made and
  starting all over again!

2
Modular Design

• ...Is our best hope for reliable programs.
  IdeaEven HUGE programs are kept simple

• Many separate modules
• Elaborate, but step-by-step
• Self-contained,independent,
• Orderly, nested interfaces,
• Details hidden, protected,
• One task?one function
• Separate, tested functions in separate, tested
  libraries,
• Fix the function bodies dont change the
  interfaces!

• One huge mess,
• Complicated, Tangled,
• Interdependent,
• Disorganized, with
• Scattered details everywhere,
• Redundant,
• Impossible to test,fixing part A breaks part B
• A massive heap
• of quick, partial fixes

3
Modular Design Core Platitudes

• No problem is inherently complicated and
  tangledit is just poorly understood.
• Complicated and Tangled too many details
  exposed!
• Short , Simple, and Separate modules are Best.
  Elaborate and Simple and Separate are good/OK.
  Complicated and Tangled ones are VERY VERY BAD,
  even if the program is short!
• Hide the Detailskeep interfaces simple
  general.
• Solve only one problem at a time! Dont confuse
  yourself

4
Modular Design

• (Chapter 10.1 has good pig-latin example)
• Break problem into just a few primary steps.
• Critically examine/refine each step
• Re-usable parts?
• Any surprises, any forgotten steps?
• Any simpler way? Is your refinement confusing?
• Need to back up, re-organize the larger problem?
• Repeat apply above to each step you just made.

5
Modular Design

• typedef struct int score / current score
  /gameInfoT / Structure stub / int
  main() int run gameInfoT game run
  initGame(game) if(TRUErun)
  getInput(game) computeOutput(game)
  displayOutput(game) endGame(it)

• Write function stubs and data structure
  stubs.
• FIRST make it run,
• THEN make it do something.

6
Modules for Big Programs

• Manage a 500 function program, 10 authors?
• Answer One module per author
• A module is a pair of files
• A .h file (the interface) that holds
• Function prototypes
• include statements
• define statements
• A .c file (the implementation) that holds
• Function bodies

7
Modules and Data Structures

• Good idea
• Organize programs to give each data structure its
  own private set of functions.
• Put the structure its fcns in one module
• Example birdT structure will have functions
  for different behaviorsbirdInit(), BirdHop(),
  BirdFly(), BirdFeed(), BirdPreen(), BirdChirp(),
  BirdSing(), BirdSleep()
• Module bird.h, bird.c has birdT struct, functions

8
Nested Modules

• One module can rely on a sub-module
• module1.h may start with include
  module1A.h include module1B.h
• module1a.h may start with include
  module1A1.h include module1A2.h and so
  on
• One structure can have a smaller sub-structure
  as a member type

9
Big Caution though--

• Though NESTING of modules is allowed, be careful
  that your modules dont have any circular
  dependencies
• CAUTION! circular includes will not work!

/ my1.h file / include my2.h
/ my2.h file / include my1.h
10
A Good Check
Draw a graph of dependencies to be sure No
upward pointing arrows!

• .

Myprog.c include objects.h int main()
Objects.h include bird.hinclude
tree.hinclude car.h
Tree.h include oak.hinclude
birch.h include maple.h
Bird.h include owls.hinclude
sparrow.h include hawk.h
car.h include chevy.h include ford.h
11
static Three Meanings?

• Static local variable
• Memory stays reserved, always undisturbed, even
  when its variable name is out-of-scope
• memory usage similar to malloc()
• Static function
• Function name is valid (scope) only within the
  module
• Cant be called/accessed by functions not in
  module
• Static global variable (GLOBALBAD
  IDEA!!)
• Name is valid (scope) only within the module
• Cant be called/accessed by functions not in
  module