1 Stacks and Queues Sections 3.6 and 3.7 Chapter 3 Lists, Stacks, and Queues Abstract Data Types, Vectors

2 Stack ADT - LIFO Collections: –Elements of some proper type T Operations: –Feature: Last In, First Out –void push(T t) –void pop() –T top() –bool empty() –unsigned int size() –constructor and destructor

3 Stack Model—LIFO Empty stack S –S.empty() is true –S.top() not defined –S.size() == 0 food chain stack

4 Stack Model—LIFO S.push(“mosquito”) –S.empty() is false –S.top() == “mosquito” –S.size() == 1 mosquito food chain stack

5 Stack Model—LIFO S.push(“fish”) –S.empty() is false –S.top() == “fish” –S.size() == 2 fish mosquito food chain stack

6 Stack Model—LIFO S.push(“raccoon”) –S.empty() is false –S.top() == “raccoon” –S.size() == 3 raccoon fish mosquito food chain stack

7 Stack Model—LIFO S.pop() –S.empty() is false –S.top() == “fish” –S.size() == 2 fish mosquito food chain stack

8 Implementations and Uses of Stack ADT Implementations –Any list implementation –list and vector C++ STL –Vector/List ADTs push_back()/pop_back() push_front()/? Uses –Depth first search / backtracking –Evaluating postfix expressions –Converting infix to postfix –Function calls (runtime stack) –Recursion

9 Depth First Search—Backtracking Problem –Discover a path from start to goal Solution –Start from Node start –Stop If node is goal –Go deep If there is an unvisited neighbor, go there –Backtrack Retreat along the path to find an unvisited neighbor, if cannot go deeper Outcome –If there is a path from start to goal, DFS finds one such path start goal

10 Depth First Search—Backtracking (2) Stack start goal 1 Push

11 Depth First Search—Backtracking (3) Stack start goal 2 1 Push

12 Depth First Search—Backtracking (4) Stack start goal Push

13 Depth First Search—Backtracking (5) Stack start goal Push

14 Depth First Search—Backtracking (6) Stack start goal Push

15 Depth First Search—Backtracking (7) Stack start goal Push Pop

16 Depth First Search—Backtracking (8) Stack start goal Push Pop

17 Depth First Search—Backtracking (9) Stack start goal 2 1 Push Pop

18 Depth First Search—Backtracking (10) Stack start goal 1 Push Pop

19 Depth First Search—Backtracking (11) Stack start goal 3 1 Push

20 Depth First Search—Backtracking (12) Stack start goal Push

21 Depth First Search—Backtracking (13) Stack start goal Push

22 DFS Implementation DFS() { stack S; // mark the start location as visited S.push(start); while (S is not empty) { t = S.top(); if (t == goal) Success(S); if (// t has unvisited neighbors) { // choose an unvisited neighbor n // mark n visited; S.push(n); } else { BackTrack(S); } Failure(S); }

23 DFS Implementation (2) BackTrack(S) { while (!S.empty() && S.top() has no unvisited neighbors) { S.pop(); } Success(S) { // print success while (!S.empty()) { output(S.top()); S.pop(); }

24 Runtime Stack Runtime environment –Static Executable code Global variables –Stack Push for each function call Pop for each function return Local variables –Heap Dynamically allocated memories new and delete static stack heap program memory

25 Queue ADT - FIFO Collection –Elements of some proper type T Operations –Feature: First In, First Out –void push(T t) –void pop() –T front() –bool empty() –unsigned int size() –Constructors and destructors

26 Queue Model—FIFO Empty Q animal parade queue

27 Queue Model—FIFO Q.Push(“ant”) ant front back animal parade queue

28 Queue Model—FIFO Q.Push(“bee”) antbee front back animal parade queue

29 Queue Model—FIFO Q.Push(“cat”) antbeecat front back animal parade queue

30 Queue Model—FIFO Q.Push(“dog”) antbeecatdog front back animal parade queue

31 Queue Model—FIFO Q.Pop() beecatdog front back animal parade queue

32 Queue Model—FIFO Q.Pop() catdog front back animal parade queue

33 Queue Model—FIFO Q.Push(“eel”) Q.Pop() eel front back animal parade queue

34 Implementations and Uses of Queue ADT Implementations –Any list implementation push_front()/pop_back() push_back()/? Uses –Buffers –Breadth first search –Simulations –Producer-Consumer Problems

35 Breadth First Search Problem –Find a shortest path from start to goal Solution –Start from Node start –Visit All neighbors of the node –Stop If a neighbor is goal –Otherwise Visit neighbors two hops away –Repeat (Stop/Otherwise) Visiting neighbors N hops away start goal

36 Breadth First Search (2) Queue start goal 1 Push

37 Breadth First Search (3) Queue start goal Pop

38 Breadth First Search (4) Queue start goal 234 Push

39 Breadth First Search (5) Queue start goal Pop 34

40 Breadth First Search (6) Queue start goal 3456 Push

41 Breadth First Search (7) Queue start goal 456 Pop

42 Breadth First Search (8) Queue start goal Push

43 Breadth First Search (9) Queue start goal 5678 Pop

44 Breadth First Search (10) Queue start goal 678 Pop

45 Breadth First Search (11) Queue start goal 78 Pop

46 Breadth First Search (12) Queue start goal 789 Push

47 Breadth First Search (13) Queue start goal 89 Pop

48 Breadth First Search (14) Queue start goal 8910 Push

49 BFS Implementation BFS { queue Q; // mark the start location as visited Q.push(start); while (Q is not empty) { t = Q.front(); for (// each unvisited neighbor n of node t) { Q.push(n); if (n == goal) Success(S); } Q.pop(); } Failure(Q); }

50 Adaptor Class Adapts the public interface of another class Adaptee: the class being used Adaptor: the new class being defined –Uses protected object of the adaptee type –Uses the adaptee’s methods to define adaptor methods Stack and Queue implemented via adaptor classes

51 Stack Adaptor Requirements Stack –push() –pop() –top() –empty() –size() Can use List, Deque –Push(): push_back() –Pop(): pop_back()

52 Class Stack template class Stack { protected: Container c; public: void push(const T & x) { c.push_back(x); } void pop() { c.pop_back(); } T top() const { return c.back(); } int empty() const { return c.empty(); } unsigned int size() const { return c.size(); } void clear() { c.clear(); } }; Declaration –Stack > floatStack; –Stack > intStack; For STL stack container –template > class stack; –stack charStack;

53 Queue Adaptor Requirements Queue –push() –pop () –front() –empty() –size() Can use List, Deque –Push(): push_front() –Pop(): pop_back()

54 Class Queue template class Queue { protected: Container c; public: void push(const T & x) { c.push_back(x); } void pop() { c.pop_front(); } T front() const { return c.front(); } int empty() const { return c.empty(); } unsigned int size() const { return c.size(); } void clear() { c.clear(); } }; Declaration Queue > floatQueue; Queue > intQueue; For STL stack container template > class queue; queue charQueue;

55 Circular Array front back animal parade queue

56 Circular Array Q.Push(“ant”) ant front back animal parade queue

57 Queue Model—FIFO Q.Push(“bee”) antbee front back animal parade queue

58 Queue Model—FIFO Q.Push(“cat”) catantbee front back animal parade queue

59 Queue Model—FIFO Q.Push(“dog”) catdogantbee front back animal parade queue

60 Queue Model—FIFO Q.Pop() catdogbee front back animal parade queue

61 Queue Model—FIFO Q.Pop() catdog front back animal parade queue

62 Expanding the Array abc abc Where are front and back? Why can’t we use all four locations? cabcab c ababc