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Array-based Implementation

Array-based Implementation. An array Q of maximum size N Need to keep track the front and rear of the queue: f : index of the front object r : index immediately past the rear element Note: Q [ r ] is empty (does not store any object). Array-based Implementation. Front element: Q [ f ]

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Array-based Implementation

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  1. Array-based Implementation • An array Q of maximum size N • Need to keep track the front and rear of the queue: f: index of the front object r: index immediately past the rear element • Note: Q[r] is empty (does not store any object)

  2. Array-based Implementation • Front element: Q[f] • Rear element: Q[r – 1] • Queue is empty: f = r • Queue size: r – f

  3. Algorithm dequeue(): if (isEmpty()) throw QueueEmptyException; temp = Q[f]; f = f + 1; return temp; Algorithm enqueue(object): if (r == N) throw QueueFullException; Q[r] = object; r = r + 1; Dequeue() and Enqueue()

  4. Analogy: A snake chases its tail Front element: Q[f] Rear element: Q[r – 1] Incrementing f, r f = (f + 1) mod N r = (r + 1) mod N mod: Java operator “%” Circular Array Implementation

  5. Queue size = (N – f + r) mod N → verify this Queue is empty: f = r When r reaches and overlaps with f, the queue is full: r = f To distinguish between empty and full states, we impose a constraint: Q can hold at most N – 1 objects (one cell is wasted). So r never overlaps with f, except when the queue is empty. Circular Array Implementation

  6. Algorithm enqueue(object): if (size() == N – 1) throw QueueFullException; Q[r] = object; r = (r + 1) mod N; Algorithm dequeue(): if (isEmpty()) throw QueueEmptyException; temp = Q[f]; f = (f + 1) mod N; return temp; Pseudo-code

  7. Algorithm front(): if (isEmpty()) throw QueueEmptyException; return Q[f]; Algorithm isEmpty(): return (f = r); Algorithm size(): return ((N – f + r) mod N); Homework: Remove the constraint “Q can hold at most N – 1 objects”. That is, Q can store up to N objects. Implement the Queue ADT using a circular array. Note: there is no corresponding built-in Java class for queue ADT Pseudo-code

  8. Double-Ended Queue ADT Deque (pronounced “deck”) Allows insertion and deletion at both the front and the rear of the queue Deque ADT: operations addFirst(e): insert e at the beginning of the deque addLast(e): insert e at the end of the deque removeFirst(): remove and return the first element removeLast(): remove and return the last element getFirst(): return the first element getLast(): return the last element isEmpty(): return true if deque is empty; false otherwise size(): return the number of objects in the deque 8

  9. Implementation Choices Array (homework) Singly linked list: removing at the tail costs θ(n) Doubly linked list 9

  10. removeLast() and addLast() 10

  11. Implementing Stacks and Queues with Deques 11

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