Stacks and Queues

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

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

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

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

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

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

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

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

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 Depth First Search—Backtracking 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

Depth First Search—Backtracking (2) • Stack 1 start 2 3 4 5 6 7 8 9 10 11 12 goal Push

Depth First Search—Backtracking (3) • Stack 1 start 2 3 4 5 6 7 8 9 10 11 12 Push goal Push

Depth First Search—Backtracking (4) • Stack 1 start 2 3 4 5 6 7 8 Push 9 10 11 12 Push goal Push

Depth First Search—Backtracking (5) • Stack 1 start 2 3 4 Push 5 6 7 8 Push 9 10 11 12 Push goal Push

Depth First Search—Backtracking (6) • Stack 1 start Push 2 3 4 Push 5 6 7 8 Push 9 10 11 12 Push goal Push

Depth First Search—Backtracking (7) • Stack 1 start Pop 2 3 4 Push 5 6 7 8 Push 9 10 11 12 Push goal Push

Depth First Search—Backtracking (8) • Stack 1 start 2 3 4 Pop 5 6 7 8 Push 9 10 11 12 Push goal Push

Depth First Search—Backtracking (9) • Stack 1 start 2 3 4 5 6 7 8 Pop 9 10 11 12 Push goal Push

Depth First Search—Backtracking (10) • Stack 1 start 2 3 4 5 6 7 8 9 10 11 12 Pop goal Push

Depth First Search—Backtracking (11) • Stack 1 start 2 3 4 5 6 7 8 9 10 11 12 Push goal Push

Depth First Search—Backtracking (12) • Stack 1 start 2 3 4 5 6 7 8 Push 9 10 11 12 Push goal Push

Depth First Search—Backtracking (13) • Stack 1 start 2 3 4 Push 5 6 7 8 Push 9 10 11 12 Push goal Push

DFS Implementation DFS() { stack<location> 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); }

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(); } }

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 heap stack static program memory

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

Queue Model—FIFO • Empty Q animal parade queue

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

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

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

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

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

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

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

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 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

Breadth First Search (2) • Queue Push 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

Breadth First Search (3) • Queue Pop 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

Breadth First Search (4) • Queue Push Push Push 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

Breadth First Search (6) • Queue Push Push 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

Breadth First Search (8) • Queue Push Push 1 start 2 3 4 5 6 7 8 9 10 11 12 goal

BFS Implementation BFS { queue<location> 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); }

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

Stacks and Queues