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ADTs, Stacks and Queues Readings - Chapter 6 1 Abstract Data Types ADT 2 Abstract Data Types (ADTs) An abstract data type (ADT) is a mathematically specified entity that defines a set of its instances. q An ADT specifies: q What data is stored n Interface - Operations on the data n w manipulation w access n a set of axioms (preconditions and post conditions) that define what the operations do (not how) ADT 3 Why Abstract Data Types (ADTs) serve as specifications of requirements for the building blocks of an algorithm q encapsulate the data structure and the algorithms that works on the data structure q separate issues of correctness and efficiency q ADT 4 ADT Example - Order Book of a Stock Trading System - Dynamic Set q The data stored are Stock n Shares n Price n Buy/Sell n q Element The operations supported are w New(): ADT w Add(B:ADT, e:Element): ADT w Remove(B:ADT, e:Element): ADT w isIn(B:ADT, e:Element): boolean ADT 5 ADT Example - Order Book of a Stock Trading System - Dynamic Set q The data stored are Stock n Shares n Price n Buy/Sell n q Element The operations supported are w New(): ADT Constructor w Add(B:ADT, e:Element): ADT w Remove(B:ADT, e:Element): ADT w isIn(B:ADT, e:Element): boolean ADT 6 ADT Example - Order Book of a Stock Trading System - Dynamic Set q The data stored are Stock n Shares n Price n Buy/Sell n q Element The operations supported are w New(): ADT Constructor w Add(B:ADT, e:Element): ADT Manipulation w Remove(B:ADT, e:Element): ADT Operations w isIn(B:ADT, e:Element): boolean ADT 7 ADT Example - Order Book of a Stock Trading System - Dynamic Set q The data stored are Stock n Shares n Price n Buy/Sell n q Element The operations supported are w New(): ADT Constructor w Add(B:ADT, e:Element): ADT Manipulation w Remove(B:ADT, e:Element): ADT Operations w isIn(B:ADT, e:Element): boolean Access ADT 8 ADT Example - Order Book of a Stock Trading System - Dynamic Set q Axioms that define the methods isIn(New(), e) - false n isIn(Add(B, e), e) - true n isIn(Add(B, f), e) - isIn(B, e) if e≠f n isIn(Remove(B, e), e) - false n isIn(Remove(B, f), e) = isIn(B, e), if e≠f n ADT 9 The Stack ADT q q Container of abstract objects that are inserted and removed according to the last-in-first-out (LIFO) principle Objects can be inserted at any time, but only the most-recently inserted object can be removed n n Inserting an object - push removing an object - pop Stacks 10 The Stack ADT - Specification Data Object q Operations q new():ADT - creates a new stack n push(S:ADT, o:Object):ADT - Inserts object o onto the top of stack S n pop(S:ADT):ADT - removes the top object of stack S; if the stack is empty an error occurs n Stacks 11 The Stack ADT - Specification (contd) q Auxiliary operations n n n q top(S:ADT):Object - returns the top object of stack S without removing it; if the stack is empty, an error occurs size(S:ADT):integer returns the number of objects in stack S isEmpty(S:ADT):boolean - indicates if stack S is empty Axioms n n pop(push(S, o)) = S top(push(S, o)) = o Stacks 12 Stack - Example Stacks 13 Stack ADT specification in JAVA? Interfaces q Interface - collection of method declarations with no data and no bodies write down the method names and the parameters (essentially the types) n when a class implements an interface, it must implement all the methods declared in the interface. n Separating interface and implementation is a useful programming technique. n Stacks 14 Stack Interface in Java q java.util has a built-in stack data structure class - we will define our own interface and not use the built-in class public interface Stack<E> {! // accessor methods! public int size(); // returns the number of objects in the stack! public boolean isEmpty(); // returns true if the stack is empty, false otherwise! public E top(); throws StackEmptyException; // returns the top object of the stack! ! // manipulation/update methods! public void push(E o); // inserts the object at the top of the stack! public E pop(); throws StackEmptyException; // removes and returns the object at the top of the stack! } Stacks 15 Stack ADT specification in JAVA? Exceptions q unexpected events that occur during the execution of a program n useful for handling errors when an error (or an exceptional event is encountered, the Java code throws an exception q Upon throwing an exception, the control flow exits from the current method and goes to the parent calling method q n responsibility of handling the error can be delegated to the parent method Stacks 16 Exceptions - Example public void UpdateStudentScore(Student s) throws StudentNotFoundException {! ...! if (s is not part of CSL201)! throw new StudentNotFoundException(s+"is not a student in CSL201");! ...! }! ! public void UpdateClassScore() {! ...! try {! CSL201.UpdateStudentScore(s);! }! catch (StudentNotFoundException e) {! system.out.println(e);! }! ...! } Stacks 17 More Exceptions - Try and Catch try - guarded fragment of code that might thrown an exception when executed. q catch - block of code to which control jumps on catching an exception. The block contains code to analyze the exception and apply an appropriate solution. q If an exception is never caught in a method, it will propagate upwards along the sequence of method calls until the user sees it. q Exceptions are essentially Java Classes q n Section 2.4 Stacks 18 Stack Interface in Java - Finally q java.util has a built-in stack data structure class - we will define our own interface and not use the built-in class public interface Stack<E> {! // accessor methods! public int size(); // returns the number of objects in the stack! public boolean isEmpty(); // returns true if the stack is empty, false otherwise! public E top() throws StackEmptyException; // returns the top object of the stack! ! // manipulation/update methods! public void push(E o); // inserts the object at the top of the stack! public E pop() throws StackEmptyException; // removes and returns the object at the top of the stack! } Stacks 19 Array-Based Stack Implementation in Java q Simple way to implement a stack n specify the maximum size N of our stack Add elements left to right q variable t keeps track of the top element of the array stack S q … S 0 1 2 q t array indices start at 0, so we initialize t to -1 Stacks 20 Array-Based Stack Implementation in Java - 1 public class ArrayStack<E> implements Stack<E> {! /** Default array capacity. */! public static final int CAPACITY=1000; // default array capacity! ! /** Generic array used for storage of stack elements. */! private E[] data; // generic array used for storage! ! /** Index of the top element of the stack in the array. */! private int t = -1; // index of the top element in stack! ! /** Constructs an empty stack using the default array capacity. */! public ArrayStack() { this(CAPACITY); } // constructs stack with default capacity! ! /**! * Constructs and empty stack with the given array capacity.! * @param capacity length of the underlying array! */! public ArrayStack(int capacity) { // constructs stack with given capacity! data = (E[]) new Object[capacity]; ! } Stacks 21 Array-Based Stack Implementation in Java - 2 /**! * Returns the number of elements in the stack.! * @return number of elements in the stack! */! public int size() { return (t + 1); }! ! /**! * Tests whether the stack is empty.! * @return true if the stack is empty, false otherwise! */! public boolean isEmpty() { return (t == -1); }! ! /**! * Inserts an element at the top of the stack.! * @param e the element to be inserted! * @throws StackFullException if the array storing the elements is full! */! public void push(E e) throws StackFullException {! if (size() == data.length) throw new StackFullException("Stack is full");! data[++t] = e; // increment t before storing new item! } Stacks 22 Array-Based Stack Implementation in Java - 3 /**! * Returns, but does not remove, the element at the top of the stack.! * @return top element in the stack (or null if empty)! * @throws StackEmptyException if the array storing the stack is empty! */! public E top() {! if (isEmpty()) throw new StackEmptyException ("Stack is empty”);! return data[t];! }! ! /**! * Removes and returns the top element from the stack.! * @return element removed! * @throws StackEmptyException if the array storing the stack is empty! */! public E pop() {! if (isEmpty()) throw new StackEmptyException ("Stack is empty”);! E answer = data[t];! data[t] = null; // dereference to help garbage collection! t--;! return answer;! } Stacks 23 Array-Based Stack Performance and Limitations q Performance n implementation is simple and efficient w methods performed in O(1) n q Space complexity - O(N) Limitations n maximum capacity of the array has to be specified a priori w too large - wastage of space w too small - insufficient for a given application n n StackFullException is specific to this implementation StackEmptyException is required by the interface Stacks 24 A Growable Array-Based Stack q On StackFullException, replace the array S with a larger one and continue processing the push operations Algorithm push(o) if size()=N then A new array of length f(N) for i ç 0 to N-1 A[i] ç S[i] S ç A t ç t+1 S[t] ç o q How large should the new array be? n n tight strategy - add a constant f(N) = N+c growth strategy - double up f(N) = 2N Stacks 25 Comparing Tight and Growth Strategies q cost model n regular push - when array capacity is not exceeded; adds one element w costs 1 unit n special push - when a new array is created and elements of the old array are copied before adding one element. w costs f(N)+N+1 units Stacks 26 Tight Strategy (c=4) q start with an array size of 0 a 4 + 1 a b 1 a b c 1 a b c d 1 a b c d e 8 + 4 + 1 a b c d e f 1 a b c d e f g 1 a b c d e f g h 1 a b c d e f g h i 1 2 + 8 + 1 a b c d e f g h i j a b c d e f g h i j k 1 a b c d e f g h i j k l 1 cost of special push 2n+5 for a total of n pushes cost is O(n^2/c) 1 Stacks 27 Growth Strategy (double the size) q start with an array size of 0 a 1 + 0 + 1 a b 2 + 1 + 1 a b c 4 + 2 + 1 a b c d 1 a b c d e 8 + 4 + 1 a b c d e f 1 a b c d e f g 1 a b c d e f g h 1 a b c d e f g h i . 1 6 + 8 + 1 a b c d e f g h i j . 1 a b c d e f g h i j k . 1 a b c d e f g h i j k l . 1 Stacks cost of special push 3n+1 for a total of n pushes cost is O(n) 28 Linked List-Based Stack Implementation in Java - 1 public class LinkedStack<E> implements Stack<E> {! ! /** The primary storage for elements of the stack */! private SinglyLinkedList<E> list = new SinglyLinkedList<>(); // an empty list! ! /** Constructs an initially empty stack. */! public LinkedStack() { } // new stack relies on the initially empty list! ! /**! * Returns the number of elements in the stack.! * @return number of elements in the stack! */! public int size() { return list.size(); }! ! Stacks 29 Linked List-Based Stack Implementation in Java - 2 /**! * Tests whether the stack is empty.! * @return true if the stack is empty, false otherwise! */! public boolean isEmpty() { return list.isEmpty(); } ! /**! * Inserts an element at the top of the stack.! * @param element the element to be inserted! */! public void push(E element) { list.addFirst(element); }! ! ! Stacks 30 Linked List-Based Stack Implementation in Java - 3 /**! * Returns, but does not remove, the element at the top of the stack.! * @return top element in the stack! * @throws StackEmptyException if the list storing the stack is empty! */! public E top() {! if (isEmpty()) throw new StackEmptyException ("Stack is empty”);! return list.first(); ! }! ! /**! * Removes and returns the top element from the stack.! * @return element removed ! * @throws StackEmptyException if the list storing the stack is empty! */! public E pop() {! if (isEmpty()) throw new StackEmptyException ("Stack is empty”);! return list.removeFirst(); ! } Stacks 31 Applications of Stacks q Direct applications Page-visited history in a Web browser n Undo sequence in a text editor n Chain of method calls in the Java Virtual Machine n q Indirect applications Auxiliary data structure for algorithms n Component of other data structures n © 2014 Goodrich, Tamassia, Goldwasser Stacks 32 Stack Applications Chain of Method Calls in JVM q q q q The Java Virtual Machine (JVM) main() { keeps track of the chain of int i = 5; active methods with a stack foo(i); When a method is called, the } JVM pushes on the stack a frame containing foo(int j) { n Local variables and return value int k; n Program counter, keeping track of the statement being executed k = j+1; When a method ends, its frame bar(k); is popped from the stack and } control is passed to the method on top of the stack bar(int m) { Allows for recursion … } © 2014 Goodrich, Tamassia, Goldwasser Stacks bar PC = 1 m=6 foo PC = 3 j=5 k=6 main PC = 2 i=5 33 Stack ApplicationsEvaluating Arithmetic Expressions 14 – 3 * 2 + 7 = (14 – (3 * 2) ) + 7 Slide by Matt Stallmann included with permission. Operator precedence * has precedence over +/– Associativity operators of the same precedence group evaluated from left to right Example: (x – y) + z rather than x – (y + z) Idea: push each operator on the stack, but first pop and perform higher and equal precedence operations. © 2014 Goodrich, Tamassia, Goldwasser Stacks 34 Algorithm for Evaluating Expressions Two stacks: q opStk holds operators q valStk holds values q Use $ as special “end of input” token with lowest precedence Algorithm doOp() x ← valStk.pop(); y ← valStk.pop(); op ← opStk.pop(); valStk.push( y op x ) Algorithm repeatOps( refOp ): while ( valStk.size() > 1 ∧ prec(refOp) ≤ prec(opStk.top()) doOp() © 2014 Goodrich, Tamassia, Goldwasser Slide by Matt Stallmann included with permission. Algorithm EvalExp() Input: a stream of tokens representing an arithmetic expression (with numbers) Output: the value of the expression while there’s another token z if isNumber(z) then valStk.push(z) else repeatOps(z); opStk.push(z) repeatOps($); return valStk.top() Stacks 35 Algorithm on an Example Expression Slide by Matt Stallmann included with permission. 14 ≤ 4 – 3 * 2 + 7 Operator ≤ has lower precedence than +/– 4 14 – ≤ 3 4 14 * – ≤ $ 7 -2 14 + 2 3 4 14 * – ≤ 2 3 4 14 + * – ≤ © 2014 Goodrich, Tamassia, Goldwasser 6 4 14 – ≤ Stacks -2 14 $ $ + ≤ 5 14 F ≤ + ≤ 36 Computing Spans (not in book) q q q Using a stack as an auxiliary data structure in an algorithm Given an an array X, the span S[i] of X[i] is the maximum number of consecutive elements X[j] immediately preceding X[i] and such that X[j] ≤ X[i] Spans have applications to financial analysis n E.g., stock at 52-week high © 2014 Goodrich, Tamassia, Goldwasser Stacks 7 6 5 4 3 2 1 0 0 X S 1 6 1 3 1 2 3 4 2 5 3 4 2 1 37 Quadratic Algorithm Algorithm spans1(X, n) Input array X of n integers Output array S of spans of X # S ← new array of n integers n for i ← 0 to n - 1 do n s←1 n while s ≤ i ∧ X[i - s] ≤ X[i] 1 + 2 + …+ (n - 1) s←s+1 1 + 2 + …+ (n - 1) S[i] ← s n return S 1 © 2014 Goodrich, Tamassia, Goldwasser Stacks 38 Computing Spans with a Stack q q We keep in a stack the indices of the elements visible when “looking back” We scan the array from left to right n n n n Let i be the current index We pop indices from the stack until we find index j such that X[i] < X[j] We set S[i] ← i - j We push i onto the stack © 2014 Goodrich, Tamassia, Goldwasser Stacks 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 39 Efficient Algorithm q Each index of the array q q q q Is pushed into the stack exactly once Is popped from the stack at most once The statements in the while-loop are executed at most n times Algorithm spans2 runs in O(n) time © 2014 Goodrich, Tamassia, Goldwasser Algorithm spans2(X, n) # S ← new array of n integers n A ← new empty stack 1 for i ← 0 to n - 1 do n while (¬A.isEmpty() ∧ X[A.top()] ≤ X[i] ) do n A.pop() n if A.isEmpty() then n S[i] ← i + 1 n else S[i] ← i - A.top() n A.push(i) n return S 1 Stacks 40 Efficient Algorithm 7 6 5 4 3 2 1 0 0 X S 1 6 1 3 1 2 3 4 2 5 3 4 2 1 © 2014 Goodrich, Tamassia, Goldwasser Algorithm spans2(X, n) # S ← new array of n integers n A ← new empty stack 1 for i ← 0 to n - 1 do n while (¬A.isEmpty() ∧ X[A.top()] ≤ X[i] ) do n A.pop() n if A.isEmpty() then n S[i] ← i + 1 n else S[i] ← i - A.top() n A.push(i) n return S 1 Stacks 41 Java File I/O using Buffered and File Readers import java.io.*;! import java.util.Scanner;! ! ! public class FileScan{! public static void main(String[] args) throws IOException {! Scanner s = null;! try {! s = new Scanner(new BufferedReader(new FileReader(“test.txt”)));! ! while (s.hasNext()) {! System.out.println(s.next());! }! } finally {! if (s != null) {! s.close();! }! }! }! } Java File I/O 42 Queues Readings - 6.2 and 6.3 The Queue ADT q q The insertion and removal routines of the Queue ADT follow the first-in-first-out (FIFO) principle. Elements may be inserted at any time, but only the element which has been in the queue the longest may be removed. n n Inserting an object (at the rear) – enqueue Removing an object ( from the front) - dequeue Tickets Queues 44 The Queue ADT - Specification q q Data Object The queue supports the following fundamental methods: n n n n New():ADT – creates an empty queue Enqueue(Q:ADT, o:element):ADT – inserts the object o at the rear of the queue Dequeue(Q:ADT):element – removes the object from the front of the queue; an error occurs if the queue is empty First(Q:ADT):element – returns, but does not remove the object at the front of the queue; an error occurs if the queue is empty. Queues 45 The Queue ADT - Specification q Auxiliary methods n n q Size(Q:QDT):integer – returns the number of elements in the queue isEmpty(Q:ADT):boolean – indicates whether the queue is empty Axioms n n n n Front(Enqueue(New(), v)) = v Dequeue(Enqueue(New(), v)) = New() Front(Enqueue(Enqueue(Q,w),v)) = Front(Enqueue(Q,w)) Dequeue(Enqueue(Enqueue(Q,w), v)) = Enqueue(Dequeue(Enqueue(Q,w),v) Queues 46 Example Operation enqueue(5) enqueue(3) dequeue() enqueue(7) dequeue() first() dequeue() dequeue() isEmpty() enqueue(9) enqueue(7) size() enqueue(3) enqueue(5) dequeue() © 2014 Goodrich, Tamassia, Goldwasser – – 5 – 3 7 7 null true – – 2 – – 9 Output Q (5) (5, 3) (3) (3, 7) (7) (7) () () () (9) (9, 7) (9, 7) (9, 7, 3) (9, 7, 3, 5) (7, 3, 5) Queues 47 Queue ADT specification in JAVA public interface Queue<E> {! // accessor methods! int size(); // returns the number of elements in the queue! boolean isEmpty(); // returns true if the queue is empty, false otherwise! ! //manipulation methods! void enqueue(E e); // adds the element to the rear of the queue! E first() throws QueueEmptyException; // returns the first element of the queue! E dequeue() throws QueueEmptyException; // removes and returns the first element of the queue! }! Queues 48 Array-based Queue q q Use an array of size N in a circular fashion Two variables keep track of the front and size f index of the front element sz number of stored elements q When the queue has fewer than N elements, array location r = (f + sz) mod N is the first empty slot past the rear of the queue normal configuration Q 0 1 2 f r wrapped-around configuration Q 0 1 2 r f © 2014 Goodrich, Tamassia, Goldwasser Queues 49 Queue Operations q We use the modulo operator (remainder of division) Algorithm size() return sz Algorithm isEmpty() return (sz == 0) Q 0 1 2 f 0 1 2 r r Q © 2014 Goodrich, Tamassia, Goldwasser f Queues 50 Queue Operations (cont.) q Operation enqueue throws an exception if the array is full – Implementation specific exception Algorithm enqueue(o) if size() = N then throw QueueFullException else r ← (f + sz) mod N Q[r] ← o sz ← (sz + 1) Q 0 1 2 f 0 1 2 r r Q © 2014 Goodrich, Tamassia, Goldwasser Queues f 51 Queue Operations (cont.) q Note that operation dequeue throws QueueEmptyException if the queue is empty Algorithm dequeue() if isEmpty() then throw QueueEmptyException else o ← Q[f] f ← (f + 1) mod N sz ← (sz - 1) return o Q Q 0 1 2 f 0 1 2 r © 2014 Goodrich, Tamassia, Goldwasser r f Queues 52 Array-based Implementation public class ArrayQueue<E> implements Queue<E> {! // instance variables! public static final int CAPACITY = 1000; // private E[] data; // private int f = 0; // private int sz = 0; // default array capacity! generic array used for storage! index of the front element! current number of elements! ! // constructors! public ArrayQueue() {this(CAPACITY);} // constructs queue with default capacity! ! public ArrayQueue(int capacity) { data = (E[]) new Object[capacity]; }! // constructs queue with given capacity! // safe cast; compiler may give warning! ! // methods! /**! * Returns the number of elements in the queue.! * @return number of elements in the queue! */! public int size() { return sz; }! ! public boolean isEmpty() { return (sz == 0); } // Tests whether the queue is empty.! Queues 53 Array-based Implementation (2) ! /**! * Inserts an element at the rear of the queue.! * This method runs in O(1) time.! * @param e new element to be inserted! * @throws QueueFullException if the array storing the elements is full! */! public void enqueue(E e) throws QueueFullException {! if (sz == data.length) throw new QueueFullException ("Queue is full");! int avail = (f + sz) % data.length; // use modular arithmetic! data[avail] = e;! sz++;! }! ! /**! * Returns, but does not remove, the first element of the queue.! * @return the first element of the queue! * @throws QueueEmptyException if the array storing the elements is empty! */! public E first() throws QueueEmptyException {! if (isEmpty()) throws new QueueEmptyException() ;! return data[f];! }! Queues 54 Array-based Implementation (3) /**! * Removes and returns the first element of the queue.! * @return element removed ! * @throws QueueEmptyException if the queue is empty)! */! public E dequeue() throws QueueEmptyException {! if (isEmpty()) throw new QueueEmptyException ();! E answer = data[f];! data[f] = null; // dereference to help garbage collection! f = (f + 1) % data.length;! sz--;! return answer;! } Queues 55 Applications of Queues q Direct applications Waiting lists, bureaucracy n Access to shared resources (e.g., printer) n Multiprogramming n q Indirect applications Auxiliary data structure for algorithms n Component of other data structures n © 2014 Goodrich, Tamassia, Goldwasser Queues 56 Application: Round Robin Schedulers We can implement a round robin scheduler using a queue Q by repeatedly performing the following steps: q 1. 2. 3. e = Q.dequeue() Service element e Q.enqueue(e) Queue Dequeue Enqueue Shared Service © 2014 Goodrich, Tamassia, Goldwasser Queues 57 Double-Ended Queue - Deque q q Deque supports insertion and deletion from the front and back of the queue The deque ADT supports the following fundamental methods n n n n n n InsertFirst(Q:ADT, e:element):ADT – inserts e at the beginning of the deque InsertLast(Q:ADT, e:element):ADT – inserts e at the end of the deque RemoveFirst(Q:ADT):ADT – removes the first element RemoveLast(Q:ADT):ADT – removes the last element First(Q:ADT): element – returns the first element Last(Q:ADT):element – returns the last element Queues 58 Deque implementations q Singly linked list q Doubly linked list A n Tail B C constant time operations Queues 59