Implementing a Queue as a Linked Structure CS 308 – Data Structures

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

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

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

Enqueuing (non-empty queue)

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

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

Dequeueing (the queue contains more than one element)

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

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

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

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

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

Other Queue functions (cont.) template QueueType() QueueType ::~QueueType() { MakeEmpty(); }

A circular linked queue design

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

Comparing queue implementations (cont.)

Comparing queue implementations 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 (cont.)

Comparing queue implementations (cont.)

Comparing queue implementations Big-O Comparison of Queue Operations OperationArray Implementation Linked Implementation Class constructorO(1) MakeEmptyO(1)O(N) IsFullO(1) IsEmptyO(1) EnqueueO(1) DequeueO(1) DestructorO(1)O(N)

Exercises 5, 6