The Queue

1. The Queue Whenever you are asked if you can do a job, tell 'em, "Certainly, I can!" Then get busy and find out how to do it. Theodore Roosevelt

2. Queue • a list (initially empty) of items (of some type) to which items may be added at one end (called the rear) and from which items may be removed at the other end (called the front) • examples • waiting lines • print queues • process queue • behaviour • FIFO ordering • error conditions: • underflow • overflow

3. Queue Interface • generic (Stores objects of type E) • no requirements • operations: • enter (enqueue, add, insert) • leave (dequeue, remove, delete) • front (head, first) • length (count, size) • empty • exceptions • NoItemException • NoSpaceException

4. Queue ADTContiguous Implementation • based on variable-sized array • two indices: front & rear • add at rear, remove at front • queue moves towards rear • repositioning on delete: O(n) • circular array • at end of array reuse front • index modulo array size

5. Queue ADT. • implementation • instance variables • count • constructors • empty state • methods • enter • overflow • increment • leave, front • underflow • length, empty • compute? • empty vs full

6. Queue ADTLinked Implementation • sequentially-linked structure of items • deletion from front • insertion at end • keep pointer to rear O(1) • length? • keep count else O(n) • comparison with contiguous • all operations O(1) • space tradeoffs

7. The End