Stack and Queues using Linked Structures

1. Stack and Queues using Linked Structures Kruse and Ryba Ch 4

2. Implementing stacks using arrays • Simple implementation • The size of the stack must be determined when a stack object is declared • Space is wasted if we use less elements • We cannot "push" more elements than the array can hold

3. Dynamic allocation of eachstack element • Allocate memory for each new element dynamically ItemType* itemPtr; ... itemPtr = new ItemType; *itemPtr = newItem;

4. Dynamic allocation of eachstack element (cont.) • How should we preserve the order of the stack elements?

5. Chaining the stack elements together

6. Chaining the stack elements together (cont.) • Each node in the stack should contain two parts: • info: the user's data • next: the address of the next element in the stack

7. Node Type • template<class ItemType> struct NodeType { ItemType info; NodeType* next; };

8. First and last stack elements • We need a data member to store the pointer to the top of the stack • The next element of the last node should contain the value NULL

9. Stack class specification // forward declaration of NodeType (like function prototype) template<class ItemType> struct NodeType; template<class ItemType> class StackType { public: StackType(); ~StackType(); void MakeEmpty(); bool IsEmpty() const; bool IsFull() const; void Push(ItemType); void Pop(ItemType&); private: NodeType<ItemType>* topPtr; };

10. Pushing on a non-empty stack

11. Pushing on a non-empty stack (cont.) • The order of changing the pointers is very important !!

12. Pushing on an empty stack

13. Function Push template <class ItemType> void StackType<ItemType>::Push(ItemType item) { NodeType<ItemType>* location; location = new NodeType<ItemType>; location->info = newItem; location->next = topPtr; topPtr = location; }

14. Popping the top element

15. Popping the top element(cont.) Need to use a temporary pointer

16. Function Pop template <class ItemType> void StackType<ItemType>::Pop(ItemType& item) { NodeType<ItemType>* tempPtr; item = topPtr->info; tempPtr = topPtr; topPtr = topPtr->next; delete tempPtr; }

17. Popping the last element on the stack

18. Other Stack functions template<class ItemType> StackType<ItemType>::StackType() { topPtr = NULL; } template<class ItemType> void StackType<ItemType>::MakeEmpty() { NodeType<ItemType>* tempPtr; while(topPtr != NULL) { tempPtr = topPtr; topPtr = topPtr->next; delete tempPtr; } }

19. Other Stack functions (cont.) template<class ItemType> bool StackType<ItemType>::IsEmpty() const { return(topPtr == NULL); } template<class ItemType> bool StackType<ItemType>::IsFull() const { NodeType<ItemType>* location; location = new NodeType<ItemType>; if(location == NULL) return true; else { delete location; return false; } } template<class ItemType> StackType<ItemType>::~StackType() { MakeEmpty(); }

20. Copy Constructors for stacks • Suppose we want to make a copy of a stack, will the following work? template<class ItemType> void StackType(StackType<ItemType> oldStack, StackType<ItemType>& copy) { StackType<ItemType> tempStack; ItemType item; while(!oldStack.IsEmpty()) { oldStack.Pop(item); tempStack.Push(item); } while(!tempStack.IsEmpty()) { tempStack.Pop(item); copy.Push(item); } }

21. Copy Constructors (cont.) • Shallow Copy: an object is copied to another object without copying any pointed-to data • Deep Copy: makes copies of any pointed-to data When do you need a copy constructor? (1) When parameters are passed by value (2) Return the value of a function   (return thisStack;) (3) Initializing a variable in a declaration   (StackType<int> myStack=yourStack;)

23. Copy constructor for stacks template<class ItemType> Stack Type<ItemType>::StackType(const StackType<ItemType>& anotherStack) { NodeType<ItemType>* ptr1; NodeType<ItemType>* ptr2; if(anotherStack.topPtr == NULL) topPtr = NULL; else { topPtr = new NodeType<ItemType>; topPtr->info = anotherStack.topPtr->info; ptr1 = anotherStack.topPtr->next; ptr2 = topPtr; while(ptr1 !=NULL) { ptr2->next = new NodeType<ItemType>; ptr2 = ptr2->next; ptr2->info = ptr1->info; ptr1 = ptr1->next; } ptr2->next = NULL; } } Alternatively, copy one stack to another using the assignment operator (you need to overload it though!!)

24. Comparing stack implementations

25. Implementing queues using arrays • Simple implementation • The size of the queue must be determined when a stack object is declared • Space is wasted if we use less elements • We cannot "enqueue" more elements than the array can hold

26. Implementing queues using linked lists • Allocate memory for each new element dynamically • Link the queue elements together • Use two pointers, qFront and qRear, to mark the front and rear of the queue

27. Queue class specification // forward declaration of NodeType (like function prototype) template<class ItemType> struct NodeType; template<class ItemType> class QueueType { public: QueueType(); ~QueueType(); void MakeEmpty(); bool IsEmpty() const; bool IsFull() const; void Enqueue(ItemType); void Dequeue(ItemType&); private: NodeType<ItemType>* qFront; NodeType<ItemType>* qRear; };

28. Enqueuing (non-empty queue)

29. Enqueuing (empty queue) • We need to make qFront point to the new node also qFront = NULL New Node qRear = NULL newNode

30. Function Enqueue template <class ItemType> void QueueType<ItemType>::Enqueue(ItemType newItem) { NodeType<ItemType>* newNode; newNode = new NodeType<ItemType>; newNode->info = newItem; newNode->next = NULL; if(qRear == NULL) qFront = newNode; else qRear->next = newNode; qRear = newNode; }

31. Dequeueing (the queue contains more than one element)

32. Dequeueing (the queue contains only one element) • We need to reset qRear to NULL also qFront After dequeue: qFront = NULL qRear = NULL Node qRear

33. Function Dequeue template <class ItemType> void QueueType<ItemType>::Dequeue(ItemType& item) { NodeType<ItemType>* tempPtr; tempPtr = qFront; item = qFront->info; qFront = qFront->next; if(qFront == NULL) qRear = NULL; delete tempPtr; }

34. qRear, qFront revisited • The relative positions of qFront and qRear are important!

35. Other Queue functions template<class ItemType> void QueueType<ItemType>::MakeEmpty() { NodeType<ItemType>* tempPtr; while(qFront != NULL) { tempPtr = qFront; qFront = qFront->next; delete tempPtr; } qRear=NULL; }

36. Other Queue functions (cont.) template<class ItemType> bool QueueType<ItemType>::IsEmpty() const { return(qFront == NULL); } template<class ItemType> bool QueueType<ItemType>::IsFull() const { NodeType<ItemType>* ptr; ptr = new NodeType<ItemType>; if(ptr == NULL) return true; else { delete ptr; return false; } }

37. Other Queue functions (cont.) template<class ItemType> QueueType<ItemType>::~QueueType() { MakeEmpty(); }

38. A circular linked queue design

39. Comparing queue implementations • Memory requirements • Array-based implementation • Assume a queue (size: 100) of strings (80 bytes each) • Assume indices take 2 bytes • Total memory: (80 bytes x 101 slots) + (2 bytes x 2 indexes) = 8084 bytes • Linked-list-based implementation • Assume pointers take 4 bytes • Total memory per node: 80 bytes + 4 bytes = 84 bytes

40. Comparing queue implementations (cont.)

41. Comparing queue implementations (cont.) • Memory requirements • Array-based implementation • Assume a queue (size: 100) of short integers (2 bytes each) • Assume indices take 2 bytes • Total memory: (2 bytes x 101 slots) + (2 bytes x 2 indexes) = 206 bytes • Linked-list-based implementation • Assume pointers take 4 bytes • Total memory per node: 2 bytes + 4 bytes = 6 bytes

42. Comparing queue implementations (cont.)

43. Comparing queue implementations