CS113 Introduction to C

1
CS113Introduction to C

• Instructor Ioannis A. VetsikasE-mail
  vetsikas_at_cs.cornell.edu
• Lecture 6 September 6

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Create your own types typedef

• Syntax typedef ltsome typegt lttype namegt
• Way to remember it typedef ltsame as a
  declarationgt

define N 3 typedef double scalar typedef
scalar vectorN typedef scalar
matrixNN void add_vectors( vector x, vector
y, vector z ) int i for( i 0 i lt N
i ) xi yi zi
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Structures

• The structure mechanism allows us to aggregate
  elements of different types
• Think of objects (but not quite!)
• Your struct definition generally goes outside all
  functions

struct card_struct int pips char
suit typedef struct card_struct card
struct card_struct int pips char
suit c1, c2 typedef struct card_struct
card
void some_function() struct card_struct
a card b / a, b have same types / b.pips
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4
Structures (continued)

• Syntax
• struct ltname/taggt
• component-declarations
• ltvariable name(s)gt
• Access elements/fields with dot operator
• b.pips or a.suit
• Structure Names/Tags
• If a structure type does not have a name, then
  only a finite number of structures can be created
• Structure tags form their own namespace!
• Structure fields also form their own namespace

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Structures (continued)

• Once a structure is declared you can use it as
  though it were a default data type (e.g. int)
• struct point int x,y
• struct rect struct point pt1, pt2
• Can return from and also pass structures to
  functions (call-by-value)
• int ptinrect (struct point p, struct point r)
• return p.xgtr.pt1.x p.xltr.pt2.x
• p.ygtr.pt1.y p.yltr.pt2.y

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Structures (continued)

• Another example of a function
• struct point midpoint (struct point a, struct
  point b)
• struct m (a.xb.x)/2, (a.yb.y)/2
• return m
• Usually one uses typedef to name the struct in
  some way and thus does not have to put struct
  lttaggt all the time
• What does ab do, when we have defined
• struct point ab ?
• Can also define pointers to structures in the
  same way as for simple data types
• struct point pt, point_ptr
• We can use the , operators in the same way as
  before
• point_ptr or pt

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Structures accessing via pointer

• If p is a pointer to a structure and x is an
  element of the structure then to access this
  element one puts (p).x or more commonly p-gtx

struct card_struct int pips char
suit typedef struct card_struct card void
set_card( card c ) c-gtpips 3
c-gtsuit A void main() card a
set_card( a )
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Pointers to Structures (continued)

• Call by value expensive (and slow) to pass
  structures between functions
• Use pointer instead (this simulates call by
  reference)
• Operators -gt and . are left-to-right associative
  and have maximum precedence along with () and
• e.g. pp-gtx increments field x, not pointer pp

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Structures initialize

• You may initialize a variable corresponding to a
  structure that was defined by initializing all
  its elements as follows
• struct name var init_element_1, ,
  init_element_n

include ltstdio.hgt struct address_struct char
street char city_and_state long
zip_code typedef struct address_struct
address void main() address a "1449
Crosby Drive", "Fort Washington, PA", 19034
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Unions

• Similar to structures, but they can only hold one
  of the elements at a time
• So they use the same spot in memory to save any
  of the possible elements.
• Memory for union is max of memory needed for each
  element

union int_or_float int i float f union
int_or_float a / need to keep track of, on
own, which type is held / / a.i always an int,
a.f always a float /
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sizeof Operator

• sizeof expression/object or sizeof lttype namegt
  returns the size of the type of the expression or
  the type in bytes
• the expression is not evaluated
• E.g. sizeof a, sizeof a13 (a is array) or
  sizeof int
• Typing sizeof a/sizeof a0 returns?
• Cannot be applied to functions

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Read from KR

• Covered 6.1-6.4 today

Other issues

• Homework 2 due on Friday
• You MUST print the output exactly in the same way
  as in the examples which are given
• Do not forget to include your name and student
  ID when you submit your programs by email give
  me printout