I hope you:

1
CS110 Lecture 23 Object Oriented Programming
Misc. Lost Details
Jack Tumblinjet_at_cs.northwestern.edu

• I hope you
• Have completed PA-4, due today
• Used Blackboard to turn in PA-4
• Have started on Project 5, DUE WED. DEC 3
• Know all about structures pointers Chap.
  12 except 12-8

2
Question from TAs

• Infinite loops what happens if you do this?
  eventually?
• int main()
• int i
• for(i 0 i gt -1 i)
• // do something useful...
• return 0

3
Question from TAs
Variable i keeps growing the set of integers
isn't infinite only holds numbers from -231 to
231. when i reaches 231, then i will produce
i -231, AND THEN THE LOOP STOPS!

• Infinite loops what happens if you do this?
  eventually?
• int main()
• int i
• for(i 0 i gt -1 i)
• // do something useful...
• return 0

4
NEW! Blackboard turn-in

• For project 4 and 5, use 'Digital Drop Box'
  Blackboard Tools button, Digital
  Drop Box, Add File, then Send File
• Be sure to SEND, don't just ADD FILE We don't
  receive your file until you hit 'send'!
• No need to turn in paper printouts

5
Collections of Objects (C)(May help on
Project 5)

• A Game uses a bunch of somethings
• Several different classes (robots, trees,
  clouds, castles, dogs, cats,...)
• Each with different actions abilities
  blink(), armsUp(), armsDown(), sleep(), walk(),
  run(),
• May want to create/destroy objects as game
  progresses
• How can we do that? first ask
• How can we store them?
• Hint think about a changing collection of
  objects as a changing collection of struct
  variables.

6
Collections of Objects(May help on Project 5)

• Idea 0 Declare sets of individually named struct
  variables for every possible object your program
  will ever need
• fishT oneFish, twoFish, redFish, blueFish
• carT carGo, carPet, carToon, carTell
• lokT lokNess, iwazLok, shewasLok, swanLok
• All have complete member variables that
  describe absolutely everything that can change
  'name', 'isAlive', 'age', 'position',
  'isHungry', isDry', 'energy'...
• Idea 1 Fixed Arrays of struct variables an
  array of fishT objects, an array of carT, an
  array of lokT. Tedious, inflexible, fixed cant
  easily create, destroy, swap, rearrange...
• Idea 2 Dynamically-Alloc'd Arrays of struct
  variables make a pointer-to-fishT, a
  pointer-to-carT, a pointer-to-lokT, then reserve
  memory for the array...

7
Collections of Objects(useful, but not
included in Project 5)

• Idea 3 Make a new struct to point to any ONE
  thing(optional not needed for Project 5)
• typedef struct holderT int kindOf / if
  kindOf0, we are EMPTY /
• fishT pF / if kindOf1, point to a
  fishT / carT pC / if kindOf2,
  point to a carT / lokT pL / if
  kindOf3, point to a lokT /
• holderT
• The 'kindOf' member variable tells you which
  pointer (pF, pC, pL, or none) to use. All the
  other pointers are kept NULL.

8
Collections of Objects
typedef struct holderT int kindOf / if
kind0, we are EMPTY! / fishT pF
/ if kind1, points to a fishT / carT
pC / if kind2, points to a carT /
lokT pL / if kind3, points to a lokT
/ holderT

• Idea 3 (cont'd)
• Create an object? set the value for kindOf,
  then malloc()an object for the selected pointer
• holderT myn
• ...myn.kindOf 2 / We will hold a carT
  object /myn.pC (carT )malloc(1
  sizeof(carT))myn.pF NULL myn.pL NULL
• Destroy an object? use kindof to find its
  pointer call free() ...
• Access an object? Lets try it.... assume a 'carT'
  object. Write code to print its floating-point
  member 'x_vel' ...

9
Data Nesting

• Many forms of data are naturally nested USE
  THIS!
• Universe?SolarSystem?planet?continent?country?stat
  e/province?town?precinct?street?building?room?chai
  r
• Matrix?row?element
• Movie?frame?scanline?pixel?color
• Least restrictive, clearest data nesting tool
  Structures
• Nested Structures members are other structure
  types
• allows separate struct name for each nesting
  level cityT( street,road,hiway
  members)?streetT(house,building members),?
  houseT(yard,room,hall members),?roomT(door,window,
  furnit. members)? furnitureT(desk,chair,bed
  members),?deskT(drawer,leg,top,side...)?
• Wonderful Structures can use arrays as members
  (fixed or dynamic allocd)

10
Collections of Objects (C)(May help on
Project 5)

• We humans tend to organize our world as
  collections of objects
• We give names to classes of objects (or kinds
  of objects) birds, clouds, trees, people,
  triangles, books, animals, buildings, chairs,
  tables, etc.
• Each kind of object has many variable parts
  name, address, salary, dept, office, SS,
  supervisor, etc.
• An object is just one named, countable,
  discrete object Bob Cratchit, emp1,
  newest_hire, etc.
• Each class of object has many associated actions
  hire, fire, demote, promote, retire, re-assign,
  work overtime, travel, vacation, training, move
  to new city, ...

11
Collections of Objects (C)(May help on
Project 5)

• Our best strategy in C Organize the entire
  program as a collection of objects
• A class of object data structure type we
  define and name
• its member variables define each part of the
  object,
• One (data-structure-type) variable one
  countable object
• Each class of object has its own set of related
  functions wagonInit(), wagonDraw(),
  wagonMove(), etc.

12
Collections of Objects (C)

• The strategy known as "Object-Oriented
  Programming" (C, Java, Modula,
  Smalltalk,...) Organize the entire program as a
  collection of objects
• A class of object data structure type we
  define and name
• its member variables define each part of the
  object,
• One (data-structure-type) variable one
  countable object
• Each class of object has its own set of member
  functions wagon.Init(), wagon.Draw(),
  wagon.Move(), etc.
• In C, 'class' ( a struct its 'member'
  functions )

13
Collections of Objects (C)

• Key idea of object oriented programming
• Object data structure all its related
  functions
• Use nesting of classes to keep things simple
• cityT ? streetT ? houseT ? roomT ? chairT...
• Writing cityT functions? Please don't make me
  worry about the number of chairs in each room!.

14
Misc. Lost Details

• Congratulations!
• You now know all the essentials of C
• You're ready for C
• You're ready to learn a few key algorithms
• There are a few leftover details to clean up
  first

15
"Hey! I can even point to myself!"
a data structure

• Data structure members can be (almost) any type,
  including
• all basic types int, char, float, double
• all derived types enum, arrays, pointers,
  structures
• even a pointer to our own kind of struct! if
  you're careful but not
  array of our own kind of struct (do you know
  why?)
• typedef struct employeeT/employee record
  /
• char name / string variable /int
  IDnum / employee ID /float salary / in
  dollars /char ssn11 / xxx-xx-xxxx /stru
  ct employeeT pBoss / immediate
  supervisor /struct employeeT pPeon / team
  member /
• employeeT

16
Self-pointing is tricky...

• Data structure members can be (almost) any type,
  including
• even a pointer to our own kind of struct!
• SURPRISE! THIS SYNTAX DOESNT WORK!
• typedef struct employeeT /employee record
  /
• char name / string constant /int
  IDnum / employee ID /float salary / in
  dollars /char ssn11 / xxx-xx-xxxx /empl
  oyeeT pBoss / immediate supervisor /employeeT
  pPeon / team member /
• employeeT

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Self-pointing is tricky...

• Instead, you must do this
• typedef struct employeeT /employee record
  /
• char name / string constant /int
  IDnum / employee ID /float salary / in
  dollars /char ssn11 / xxx-xx-xxxx /str
  uct employeeT pBoss / immediate
  supervisor /struct employeeT pPeon / team
  member /
• employeeT
• ?HUNH? WHAT HAPPENED HERE?
• actual datatype is struct employeeT, but
  typedef renames datatype to employeeT
• These lines are compiled before typedef can do
  the renaming,so they must use the actual
  datatype name struct employee

18
scanf() more of the story
(CHAPTER 11-6 IN BOOK look at 11-5 too!)

• int scanf(format string, ptr1, ptr2, ptr3, )
• Format String
• Can set max of chars for each data conversion
• Can set separators (besides whitespace)
• cnt scanf(3i, 80s,my_int, msg)
• Return value of successful conversions (or 1
  if nothing at all matches)
• Destinations are ALL pointers/array names.

19
scanf() more of the story
(CHAPTER 11-6 IN BOOK look at 11-5 too!)

• int scanf(format string, ptr1, ptr2, ptr3, )
• Format String
• Can set max of chars for each data conversion
• Can set separators (besides whitespace)
• cnt scanf(3i, 80s,my_int, msg)
• Return value of successful conversions (or 1
  if nothing at all matches)
• Destinations are ALL pointers/array names.

20
scanf() more of the story
(CHAPTER 11-6 IN BOOK look at 11-5 too!)

• int scanf(format string, ptr1, ptr2, ptr3, )
• Format String
• Can set max of chars for each data conversion
• Can set separators (besides whitespace)
• cnt scanf(3i, 80s,my_int, msg)
• Return value of successful conversions (or 1
  if nothing at all matches)
• Destinations are ALL pointers/array names.

21
scanf() more of the story
(CHAPTER 11-6 IN BOOK look at 11-5 too!)

• int scanf(format string, ptr1, ptr2, ptr3, )
• Format String
• Can set max of chars for each data conversion
• Can set separators (besides whitespace)
• cnt scanf(3i, 80s,my_int, msg)
• Return value of successful conversions (or 1
  if nothing at all matches)
• Destinations are ALL pointers/array names.

22
scanf() more of the story
(CHAPTER 11-6 IN BOOK look at 11-5 too!!)

• int scanf(format string, ptr1, ptr2, ptr3, )
• Format String
• Can set max of chars for each data conversion
• Can set separators (besides whitespace)
• To read an entire line (up to 80 chars) of text
  as a string, replace 80s with 80\n (note
  no s)
• cnt scanf(3i, 80\n,my_int, msg)

read a string, but dont stop until you read 80
chars or find a \n
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Format Strings Can be Variable!

• So far, weve used only string constantsscanf(
  d,val)printf(Value is d\n,val)
• But we can use string variables too(recall
  'string' variable? use a pointer or array name)
• char inMsg10 dchar outMsg20
  Value is d\n... scanf(inMsg,val)
  printf(outMsg,val)

24
(Recall) Literals

• Numerical literals in C are obviousint i
  float base_e i3 base_e
  2.718281828OR, more compactlyint i 3
  float base_e 2.718281828These literals set
  the values for i, base_e, and are not
  L-valuesthey do not have an address. pV 3
  pF 2.71828 3 pF

ERROR!!
25
String Literals?

• String Literals in C are a little strange
  they ARE L-values, they DO have address char
  pMsg...
• pMsg Are we having fun yet?
• OR, more compactlychar pMsg Are we having
  fun yet?
• Pointer pMsg gets the address of a string
  literal. Literal's chars are in 'locked'
  memory--const.ERROR if you try to change its
  contents!
• strcpy(Real Soon now!,pSrc)

String literal
ERROR!!
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String Constants

• Reminder Arrays arent movable!
• We can move a pointer pMsg to a string literals
  addresschar pMsg
• pMsg Are we having fun yet? // OK!
• And we can initialize an array msg by copying a
  string literalchar msg20 Are we having
  fun yet? // OK!
• But we cant move an array name msg to that
  locationchar msg20
• msg Not this way, no! // ERROR!

address
all chars
27
Build a String sprintf()

• int sprintf( pStr, format_string, arg0, arg1,
  )
• Think string printf it constructs a formatted,
  printable string of chars in memory, starting at
  location pStr.
• Arguments pStr char. pointer to showing where
  to write string
• format_string (same as used in printf)
• arg0, arg1, list of values used in format
  string
• Return value number of non-null chars written
  to pStrWARNING!! Reserve a block of memory at
  pStr first!

28
More about string.h

• Uses Char Pointer arguments and return types
  (pass-by-reference)
• YOU must ensure enough memory was reserved for
  all operations of all string.h functions!
• All have form str(n)xxx (dest, src, n), and
  include length strlen, copy
  strcpy, compare strcmp, concatenate strcat,
  find char strchr, find string strstr.
• Other, more obscure string.h functions strerror(
  ), strpbrk(), strtok(), strcspn(), strspn(),

MAJOR source of subtle bugs!
29
Jargon Buffer

• Buffer temporary storage for a data sequence
• A pointer is not a buffer until it points to a
  reserved block of memory
• Reserved by dynamic allocation (malloc,free) or
• Reserved by a declared array (still in scope, or
  static)
• CAUTION never over-run a buffermake sure all
  string lengths lt buffer length
• HELPFUL All string.h functions have an n
  version strncpy, strncmp, strncat, strnchr
  operates on only the first n characters

30
Subtlety of strncpy, strncat

• char strncpy(char dest, char src, int n)
• Always copies n chars to dest. Of these, it
• Copies src chars until null \0 char found,
  then
• Copies additional null chars after that.
• CAREFUL! If src longer than n, NO null char!!
• char strncat(char dest, char src, int n)
• Appends no more than n chars from src onto dest
• CAREFUL! If src longer than n, NO null char!!

31
Other useful string.h functions

• int strncmp(char str1, char str2, int n)
• Compares first n characters of strings 1 2
• Returns the DIFFERENCES 0 perfect match!
• char strchr(char str, char ch)
• Searches string str for the 1st char ch it
  finds,
• Returns pointer to the char it found, or NULL
• char strstr(char str, char ch)
• Searches string str1 for the 1st string str2
  it finds,
• Returns pointer to the string it found, or NULL.

32
Finish up

• Worthwhile topics
• Search, Sort Chapter 8-5
• Chap 14 Linked lists
• Intro to C
• Skipped so far
• Chapter 13-1,2 Binary files,
• Chapters 13-4 fgets/getssscanf,
  fputs/putssprintf
• Chap 15 Bitwise operators, bit-shift
• Appendix G pre-proc directives include
• Appendix I command-line arguments
• Appendix J Function pointers