Stacks and Queues

1. Stacks and Queues

2. Stacks and Queues • Not really “data structures” • More of an enforcement of policy • Can be implemented using an array or linked list • Can store just about any kind of data • Queues • First In, First Out (FIFO) • Like waiting in line • Stacks • First In, Last Out (FILO) • Like a stack of trays

3. Stacks • Three primary operations • Push() – put new data on the top of the stack • Pop() – remove data from the top of the stack • Peek() – get a copy of the data on the top of the stack • Useful • our function stack! • stack of cards, tiles, loot, etc…

4. Example(pushing) 5 Note: could use a linked list also

5. Example(pushing) 11 5

6. Example(pushing) -6 11 5

7. Example(pushing) 45 -6 11 5

8. Example(current stack) 45 -6 11 5

9. Example(peeking) 45 45 -6 11 5

10. Example(popping) 45 -6 11 5

11. Example(pushing) 31 -6 11 5

12. Example(popping) 31 -6 11 5

13. Example(popping) -6 11 5

14. Example(popping) 11 5

15. Example(popping) FILO 5

16. Queues • Two primary operations • Enqueue() – put new data at the end of the queue • Dequeue() – remove data from the beginning of the queue • Peek() – yes, it’s still there… • Useful • Enforcing fairness (waitlist at SPSU) • Player turns during a round

17. Example

18. Example(enqueue a 5) 5

19. Example 5

20. Example(enqueue an 11) 11 5

21. Example 5 11

22. Example(enqueue a -6) -6 5 11

23. Example 5 11 -6

24. Example(dequeue) 5 11 -6

25. Example(dequeue) 5 11 -6

26. Example(dequeue) 11 5 -6 FIFO

27. Example(dequeue) 11 -6

28. Summary • Not really “data structures” • More of an enforcement of policy • Can be implemented using an array or linked list • Queues are FIFO • Stacks are FILO • Which data structure is LILO? • Which one is LIFO?