CSI 1340 Introduction to Computer Science II

1
CSI 1340Introduction to Computer Science II

• Chapter 4
• ADTs Stack and Queue

2
Stacks of Plates in Cafeteria
New plates are pushed on top
The next plate is popped from the top
3
What is a Stack?

• A stack is an ordered group of homogeneous items
  (elements), in which the removal and addition of
  stack items can take place only at the top of the
  stack.
• A stack is a LIFO last in, first out structure.
• Entries are taken out of the stack in the reverse
  order of their insertion.

4
Are Stacks Useful?

• Web Surfing
• Lost in hyperlink space
• CNN article on wild goats
• Yahoo search for wild goat cheese recipe
• Ebay bid on goat hair purse
• How did I get here?

5
Stack ADT Operations

• MakeEmpty -- Sets stack to an empty state.
• IsEmpty -- Determines whether the stack is
  currently empty.
• IsFull -- Determines whether the stack is
  currently full.
• Push (ItemType newItem) -- Adds newItem to the
  top of the stack.
• Pop (ItemType item) -- Removes the item at the
  top of the stack and returns it in item.

6
Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top
MAX_ITEMS-1 .
.
. 2
1 items 0
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Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top -1
MAX_ITEMS-1 .
.
. 2
1 items 0
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Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 0
MAX_ITEMS-1 .
.
. 2
1 items 0
V
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Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 1
MAX_ITEMS-1 .
.
. 2
1 C items
0 V
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Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 2
MAX_ITEMS-1 .
.
. 2 S
1 C
items 0 V
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Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 2
MAX_ITEMS-1 .
.
. 2 S
1 C
items 0 V
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Tracing Client Code
S
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 1
MAX_ITEMS-1 .
.
. 2 S
1 C
items 0 V
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Tracing Client Code
S
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 2
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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Tracing Client Code
S
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 2
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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Tracing Client Code
K
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 1
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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Tracing Client Code
K
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 1
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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Tracing Client Code
C
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 0
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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Tracing Client Code
C
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top 0
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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Tracing Client Code
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top -1
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
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End of Trace
V
letter
char letter V StackType charStack charStac
k.Push(letter) charStack.Push(C) charStack.P
ush(S) if ( !charStack.IsEmpty( ))
charStack.Pop(letter) charStack.Push(K) whil
e (!charStack.IsEmpty( ))
charStack.Pop(letter)
Private data top -1
MAX_ITEMS-1 .
.
. 2 K
1 C
items 0 V
21
Just How Generic Are We?

• typedef string GenType
• class StackType
• public
• StackType()
• bool push(const GenType url)
• bool pop(GenType url)
• private
• enum MAXSTACK 100
• int top
• GenType itemMAXSTACK

typedef int GenType class StackType public St
ackType() bool push(const GenType url) bool
pop(GenType url) private enum MAXSTACK
100 int top GenType itemMAXSTACK
22
What is a Class Template?

• A class template allows the compiler to generate
  multiple versions of a class type by using type
  parameters.
• The formal parameter appears in the class
  template definition, and the actual parameter
  appears in the client code. Both are enclosed in
  pointed brackets, lt gt.

23
StackTypeltintgt numStack
ACTUAL PARAMETER
24
StackTypeltfloatgt myStack
ACTUAL PARAMETER
25
Using class templates

• The actual parameter to the template is a data
  type. Any type can be used, either built-in or
  user-defined.
• When using class templates, both the
  specification and implementation should be
  located in the same file (the header file),
  instead of in separate .h and .cpp files.

26
Recall that

• Memory is like a shelf of slots

• Each slot has a number and some content value

27
Addresses in Memory

• When a variable is declared, enough memory to
  hold a value of that type is allocated for it at
  an unused memory location. This is the address
  of the variable.
• int x // 2 bytes
• float number // 4 bytes
• char ch // 1 byte
• 2000 2002
  2006

28
Obtaining Memory Addresses

• We can get the value of a slot. How about the
  slot itself?
• The address of a non-array variable can be
  obtained by using the address-of operator .
• int x
• float number
• char ch
• cout ltlt Address of x is ltlt x ltlt endl
• cout ltlt Address of number is ltlt number ltlt
  endl
• cout ltlt Address of ch is ltlt ch ltlt endl

29
Results

• int x
• float number
• char ch
• cout ltlt Address of x is ltlt x ltlt endl
• cout ltlt Address of number is ltlt number ltlt
  endl
• cout ltlt Address of ch is ltlt ch ltlt endl
• Output
• Address of x is 0012FF7C
• Address of number is 0012FF78
• Address of ch is 0012FF74

30
What is a pointer variable?

• A pointer variable is a variable whose value is
  the address of a location in memory.
• To declare a pointer variable, you must specify
  the type of value that the pointer will point to.
  For example,
• int ptr // ptr will hold the address of an int
• char q // q will hold the address of a char

31
Using a pointer variable
2000
12 x 3000 2000 ptr

• int x
• x 12
• int ptr
• ptr x
• NOTE Because ptr holds the address of x,
• we say that ptr points to x

32
- dereference operator
2000
12 x 3000 2000 ptr

• int x 12
• int ptr x
• coutltltAddr of x ltlt ptr ltlt endl
• coutltltValue of xltlt ptr ltlt endl
• NOTE The value pointed to by ptr is denoted by
  ptr

33
Results

• int x 12
• int ptr x
• coutltltAddr of x ltlt ptr ltlt endl
• coutltltValue of xltlt ptr ltlt endl
• Output
• Addr of x 0012FF7C
• Value of x 12

34
Using the dereference operator
2000 12
5 x 3000 2000 ptr

• int x
• x 12
• int ptr
• ptr x
• ptr 5 // changes the value // at
  address ptr to 5

35
Another Example

• char ch
• ch A
• char q
• q ch
• q Z
• char p
• p q // the right side has value 4000
• // now p and q both point to ch

4000 A
Z ch 5000
6000 4000 4000 q
p
36
The NULL Pointer

• There is a special pointer value called the null
  pointer denoted by NULL to represent a pointer
  variable that is not pointing to anything
• NULL is not a valid memory address
• It is an error to dereference (e.g., x) a
  pointer whose value is NULL. Such an error may
  cause your program to crash, or behave
  erratically. It is the programmers job to
  check for this.
• while (ptr ! NULL)
• . . . // ok to use ptr here

37
Allocation of memory
STATIC ALLOCATION Static
allocation is the allocation of memory space at
compile time.
DYNAMIC ALLOCATION Dynamic
allocation is the allocation of memory space at
run time by using operator new.
38
Using operator new

• If memory is available, new allocates the
  requested object or array and returns a pointer
  to (address of ) the memory allocated.
• Otherwise, the null pointer is returned.
• The dynamically allocated object exists until the
  delete operator destroys it.

39
New Operator

• Creates a new dynamic variable of the specified
  type and returns a pointer to this new dynamic
  variable.
• Example
• double dptr
• dptr new double
• dptr 3.14259

dptr
dptr
3.14259
dptr
40
Dynamically Allocated Data

• char ptr
• ptr new char
• ptr B
• cout ltlt ptr

2000 ptr
41
Dynamically Allocated Data

• char ptr
• ptr new char
• ptr B
• cout ltlt ptr
• NOTE Dynamic data has no variable name

2000 ptr
42
Dynamically Allocated Data

• char ptr
• ptr new char
• ptr B
• cout ltlt ptr
• NOTE Dynamic data has no variable name

43
Dynamically Allocated Data

• char ptr
• ptr new char
• ptr B
• cout ltlt ptr
• delete ptr

2000 ptr NOTE Delete deallocates
the memory pointed to by ptr.
?
44
Using operator delete

• The object or array currently pointed to by the
  pointer is deallocated, and the pointer is
  considered unassigned. The memory is returned to
  the free store.

45
Dynamic Array Allocation
char ptr // ptr is a pointer variable
that // can hold the
address of a char ptr new char 5
// dynamically, during run time, allocates
// memory for 5 characters and places into
// the contents of ptr their beginning
address
6000
6000
ptr
46
Dynamic Array Allocation
char ptr ptr new char 5
strcpy( ptr, Bye ) ptr 1 u // a
pointer can be subscripted cout ltlt ptr 2

6000
u
6000
B y e \0
ptr
47
Declaring a Dynamic Array

• Example
• int arrayptr // (1) Declare a pointer
• arrayptr new int10 // (2) Create an array
  pointed
• // to by the pointer
• arrayptr0 65 // (3) Assign a value to the
  first
• // memory location in the array
• NOTE With dynamic arrays, omit the when
  assigning values or otherwise accessing the array
  elements. After allocation, access is identical
  to statically allocated arrays.

48
Dynamic Array Deallocation
char ptr ptr new char 5
strcpy( ptr, Bye ) ptr 1
u delete ptr // deallocates array
pointed to by ptr // ptr
itself is not deallocated, but
// the value of ptr is considered
unassigned
?
ptr
49
Declaring a Dynamic Record

• Example
• struct RecType
• int iNum
• float fNum
• RecType rptr

50
What is a Queue?

• A queue is an ordered group of homogeneous items
  (elements), in which new elements are added at
  one end (the rear), and elements are removed from
  the other end (the front).
• A queue is a FIFO first in, first out
  structure.
• Like a line at the grocery store.

51
Queue ADT Operations

• MakeEmpty -- Sets queue to an empty state.
• IsEmpty -- Determines whether the queue is
  currently empty.
• IsFull -- Determines whether the queue is
  currently full.
• Enqueue (ItemType newItem) -- Adds newItem to
  the rear of the queue.
• Dequeue (ItemType item) -- Removes the item at
  the front of the queue and returns it in item.

52
Accessing a Dynamic Record

• Example
• rptr new RecType // Creates a record pointed
  to by
• // the pointer
• (rptr). iNum 65 // Assigns a value to the
  first
• // field in the record
• rptr -gt iNum 65 // Alternative syntax

53
Dereferencing Operator w/ Records

• Asterisk and the pointer name are enclosed in
  parentheses.
• Example
• cout ltlt (rptr).iNum

54
Arrow Operator w/Records

• -gt (a hyphen followed by a greater-than symbol)
• For records and classes, it is equivalent to the
  use of the dereferencing operator and
  parentheses.
• Example
• cout ltlt rptr -gt iNum

55
Pointers as Value Parameters

• Example
• void myFunc (int intptr) // Prototype for
  myFunc

56
What happens here?

• int ptr new int
• ptr 3
• ptr new int // changes value of ptr
• ptr 4

3 ptr
3 ptr 4
57
Memory Leak

• A memory leak occurs when dynamic memory (that
  was created using operator new) is left without a
  pointer to it by the program
• int ptr new int
• ptr 8
• int ptr2 new int
• ptr2 -5

8 ptr
-5 ptr2
58
Causing a Memory Leak
8 ptr
-5 ptr2

• int ptr new int
• ptr 8
• int ptr2 new int
• ptr2 -5
• ptr ptr2 // here the 8 becomes
  inaccessible

8 ptr
-5 ptr2
59
A Dangling Pointer

• Occurs when two pointers point to the same
  object and delete is applied to one of them.
• int ptr new int
• ptr 8
• int ptr2 new int
• ptr2 -5
• ptr ptr2

60
Leaving a Dangling Pointer

• int ptr new int
• ptr 8
• int ptr2 new int
• ptr2 -5
• ptr ptr2
• delete ptr2 // ptr is left dangling
• ptr2 NULL

8 ptr NULL
ptr2
61
Classes and Dynamic MemoryAllocation
62

• class Counter
• public
• Counter( )
• Counter(int initCount)
• void incrementCounter( )
• void decrementCounter( )
• void setCounter(int initCount)
• int getCounter( )
• private
• int countptr

countptr
63

• include "counter.h
• CounterCounter( ) // default constructor
• countptr new int
• countptr 0

0
countptr
countptr
64

• CounterCounter(int initCount) // parameterized
  constructor
• countptr new int
• countptr initCount

initCount
countptr
countptr
65

• void CountersetCounter(int initCount)
• countptr initCount

initCount
countptr
countptr
66

• void CounterincrementCounter( )
• countptr countptr 1

0
countptr
1
countptr
67

• void CounterdecrementCounter( )
• countptr countptr - 1

1
countptr
0
countptr
68

• int CountergetCounter( ) const
• return countptr

0
countptr
countptr
69
Destructor Syntax

• Example
• CounterCounter( ) // Implementation for
  destructor
• delete countptr // memory location pointed to
  by
• // countptr is returned to heap
• NOTE the pointer is NOT returned to the heap