Data Structures and Algorithms for Engineers

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1 Data Structures and Algorithms for Engineers David Vernon Carnegie Mellon University Africa Data Structures and Algorithms for Engineers 1 Carnegie Mellon University Africa

2 Lecture 11 Queue ADT Implementation using List ADT (array and linked-list) Comparison of order of complexity Dedicated ADT Circular queues Queue applications Data Structures and Algorithms for Engineers 2 Carnegie Mellon University Africa

3 Queues A queue is another special type of list insertions are made at one end, called the tail of the queue deletions take place at the other end, called the head thus, the last one added is always the last one available for deletion also referred to as FiFO list (First In First Out) Data Structures and Algorithms for Engineers 3 Carnegie Mellon University Africa

4 Queues Head Tail Data Structures and Algorithms for Engineers 4 Carnegie Mellon University Africa

5 Queue Operations Declare: Q : The function value of Declare(Q) is an empty queue Data Structures and Algorithms for Engineers 5 Carnegie Mellon University Africa

6 Queue Operations Empty: Q : The function Empty causes the queue to be emptied and it returns position End(Q) Data Structures and Algorithms for Engineers 6 Carnegie Mellon University Africa

7 Queue Operations IsEmpty: Q B : The function value IsEmpty(Q) is true if Q is empty; otherwise it is false Data Structures and Algorithms for Engineers 7 Carnegie Mellon University Africa

8 Queue Operations Head: Q E : The function value Head(Q) is the first element in the list; if the queue is empty, the value is undefined Data Structures and Algorithms for Engineers 8 Carnegie Mellon University Africa

9 Queue Operations Enqueue: E Q Q : Enqueue(e, Q) Add an element e the the tail of the queue Head Tail Head Tail Data Structures and Algorithms for Engineers 9 Carnegie Mellon University Africa

10 Queue Operations Dequeue: Q E : Dequeue(Q) Remove the element from the head of the queue: i.e. return the first element and delete it from the queue Head Tail Head Tail Data Structures and Algorithms for Engineers 10 Carnegie Mellon University Africa

11 Queue Operations All these operations can be directly implemented using the LIST ADT operations on a queue Q Again, it may be more efficient to use a dedicated implementation And, again, it depends what you want: code efficiency or software re-use (i.e. utilization efficiency) Data Structures and Algorithms for Engineers 11 Carnegie Mellon University Africa

12 Queue Operations Declare(Q) Empty(Q) Head(Q) Retrieve(First(Q), Q) Enqueue(e, Q) Insert(e, End(Q), Q) Dequeque(Q) Retrieve(First(Q), Q) Delete(First(Q), Q) Data Structures and Algorithms for Engineers 12 Carnegie Mellon University Africa

13 Queue Errors Queue overflow errors occur when you attempt to enqueue() an element in a queue that is full Queue underflow errors occur when you attempt to dequeue() an element from an empty queue Your ADT implementation should provide guards that catch these errors Data Structures and Algorithms for Engineers 13 Carnegie Mellon University Africa

14 Queue Implementation The List ADT can be implemented As an array As a linked-list So, therefore, so can the Queue ADT What are the advantages and disadvantages of the these two options? When would you pick one implementation over the other? Data Structures and Algorithms for Engineers 14 Carnegie Mellon University Africa

15 Queue Operations Array Linked-List Declare(Q) O(1) O(1) Empty(Q) O(1) O(n) Head(Q) O(1) O(1) Retrieve(First(Q), Q) Enqueue(e, Q) O(1) O(n) why? Insert(e, End(Q), Q) Dequeque(Q) O(n) why? O(1) Retrieve(First(Q), Q) Delete(First(Q), Q) Data Structures and Algorithms for Engineers 15 Carnegie Mellon University Africa

16 Queue Implementation Reusing the List ADT involves some compromises Alternative is to create a new Queue ADT With an implementation that avoids these compromises Data Structures and Algorithms for Engineers 16 Carnegie Mellon University Africa

17 Queue Implementation typedef struct { int front; int rear; ITEM_TYPE items[max_items]; } QUEUE_TYPE; Data Structures and Algorithms for Engineers 17 Carnegie Mellon University Africa

18 Queue Implementation void empty(queue_type *queue) { queue->front = 0; queue->rear = 0; return(end(queue)); } Data Structures and Algorithms for Engineers 18 Carnegie Mellon University Africa

19 Queue Implementation bool is_empty(queue_type *queue) { if (queue->front == queue->rear) return(true); else return(false) } Data Structures and Algorithms for Engineers 19 Carnegie Mellon University Africa

20 Queue Implementation void enqueue(element_type e, QUEUE_TYPE *queue) { items[rear] = e; rear = (rear +1) % MAX_ITEMS; } Data Structures and Algorithms for Engineers 20 Carnegie Mellon University Africa

21 Queue Implementation int is_full(queue_type *queue) { if (( rear + 1 ) % MAX_ITEMS == front ) return(true); else return(false); } Data Structures and Algorithms for Engineers 21 Carnegie Mellon University Africa

22 Queue Implementation void enqueue(item_type e, QUEUE_TYPE *queue) { if (!is_full(queue)) { items[rear] = e; rear = (rear +1) % MAX_ITEMS; } else { error( Queue overflow: queue is already full ); } } Data Structures and Algorithms for Engineers 22 Carnegie Mellon University Africa

23 Queue Implementation void dequeue(item_type *e, QUEUE_TYPE *queue) { if (!is_empty(queue)) { *e = items[front]; front = (front+1) % MAX_ITEMS; } else { error( Queue underflow: queue is empty ); } } Data Structures and Algorithms for Engineers 23 Carnegie Mellon University Africa

24 Queue Implementation Can you see a particular problem with the linked-list implementation? How would you fix it? Data Structures and Algorithms for Engineers 24 Carnegie Mellon University Africa

25 Queue Operations Array Linked-List Declare(Q) O(1) O(1) Empty(Q) O(1) O(n) Head(Q) O(1) O(1) Retrieve(First(Q), Q) Enqueue(e, Q) O(1) O(n) why? Insert(e, End(Q), Q) Dequeque(Q) O(n) why? O(1) Retrieve(First(Q), Q) Delete(First(Q), Q) Data Structures and Algorithms for Engineers 25 Carnegie Mellon University Africa

26 Queue Implementation Can you see a particular problem with the linked-list implementation? How would you fix it? Data Structures and Algorithms for Engineers 26 Carnegie Mellon University Africa

27 Queue Implementation Can you see a particular problem with the linked-list implementation? How would you fix it? front rear Data Structures and Algorithms for Engineers 27 Carnegie Mellon University Africa

28 Queue Applications Scheduling/waiting for system service queues Resource queues provide coordinated access to shared resources Message queues (Buffer) Multi-queues and priority queues Data Structures and Algorithms for Engineers 28 Carnegie Mellon University Africa

ADTs Stack and Queue. Outline

ADTs Stack and Queue. Outline Chapter 5 ADTs Stack and Queue Fall 2017 Yanjun Li CS2200 1 Outline Stack Array-based Implementation Linked Implementation Queue Array-based Implementation Linked Implementation Comparison Fall 2017 Yanjun