What is a queue? First-in first-out data structure (FIFO) Queues - PDF document

What is a queue? • First-in first-out data structure (FIFO) Queues • New objects are placed at rear • Removal restricted to front • Examples? Queue ADT Operations Example • enqueue (o): Insert o at rear of queue • enqueue(5) • enqueue(9) – Input: Object; Output: None • enqueue(3) • enqueue(7) • dequeue (): Remove object at front; error if empty • dequeue() • size() – Input: None; Output: Object removed • enqueue(7) • enqueue(3) • size (): Return number of objects in queue – Input: None; Output: Integer • dequeue() • enqueue(5) • isEmpty (): Return a boolean indicating queue empty • front() • dequeue() – Input: None; Output: Boolean • dequeue() • first (): Return object at front without removing; error if empty • dequeue() – Input: None; Output: Object • isEmpty() Queue Interface Underlying Representation int size(); • Array versus Linked List – Pros and cons? bool isEmpty(); • Running time? Object front() throws QueueEmptyException; – size – isEmpty void enqueue(Object obj); – enqueue – dequeue Object dequeue() throws QueueEmptyException; – front 1

Array Implementation Array Implementation 0 1 2 3 4 5 6 … n-1 0 1 2 3 4 5 6 … n-1 enqueue(5) enqueue(5) 5 3 5 3 enqueue(3) enqueue(3) 0 1 2 3 4 5 6 … n-1 0 1 2 3 4 5 6 … n-1 3 dequeue() ? dequeue() ? 0 1 2 3 4 5 6 … n-1 Circular Array Circular Array f r f r 0 1 2 3 4 5 6 … n-1 0 1 2 3 4 5 6 … n-1 • f – stores index of cell which stores first element • How do you add a new element? of queue – insert at array[r] • r – stores index of next available cell in queue – increment r • How do you remove an element? • Initially, f = r = 0 – return array[f] • How do you add a new element? – increment f • How do you remove an element? • What happens when r >= n-1? Circular Array Circular Array r f f r f dequeue 0 1 2 3 4 5 6 … n-1 0 1 2 0 1 2 f r r f • Need to be able to wrap around r =(2+1)%3= 0 enqueue enqueue • Modulo – % 0 1 2 0 1 2 – increment f using (f+1)%n f r enqueue – increment r using (r+1)%n 0 1 2 2

Algorithms Deque • size • Double-ended queue – return (N-f+r) mod N • isEmpty – insertFirst – return (f == r) • front – insertLast – if isEmpty then throw QueueEmptyException – removeFirst – return array[f] • dequeue – removeLast – if isEmpty then throw QueueEmptyException – temp = array[f] – first – f = (f+1)%N – return temp – last • enqueue – if size == N-1 then throw QueueFullException SIZE MUST BE < N-1 – size – array[r] = object – r = (r+1)%N – isEmpty Example Doubly Linked List • insertFirst(3) Object1 Object2 Object3 prev prev prev • insertFirst(5) header trailer next next next • first() • removeFirst() • Algorithms • insertLast(7) – insertFirst • last() • removeFirst() – removeLast • removeLast() 3