COP3538 – Data Structures Using OOP Chapter 4 – Stacks and Queues

Abstract Data Structures An array is a concrete structure  The Java language defines Structure Rules for storing and retrieving values Limitations  Uses of an array are limitless Limited only by the programmers imagination  Sometimes we require better-defined structures Abstract data structures  Improve the structure  Additional storage and retrieval rules  Additional limitations 1/20/2014 Jim Littleton - COP35382

Abstract Data Structures Abstract data structures  Commonly implemented using arrays  Can be implemented using other structures Abstract data structure examples  Stacks  Queues  Priority queues Each abstract data structure  Solves a particular set of problems  Has a unique set of advantages and disadvantages 1/20/2014 Jim Littleton - COP35383

Access (interface) Arrays  Directly accessed via an index (immediate)  Search through values sequentially or logically  Only one item can be accessed at a time Abstract data structures  An interface controls access to the values Direct access is NOT possible by user (programmer)  Implementation of the interface controls Access to values How values are stored, retrieved and deleted 1/20/2014 Jim Littleton - COP35384

Stacks Stack properties  Access to only one item at a time  Follows the Last In First Out (LIFO) logical process Think of any natural “stack” of items  Stack of dishes  Stack of coins  Stack of papers  Four basic operations (know these!) Push – place an item on top of the stack Pop – remove an item off the top of the stack Overflow – procedure to follow when the stack is full Underflow – procedure to follow when the stack is empty 1/20/2014 Jim Littleton - COP35385

Stacks Code example pubic class Stack { private int maxSize; private int top = -1; private int[] array; public Stack(int size) { maxSize = size; array = new int[maxSize]; } public int pop() { return array[top--]; // Returns the topmost item } public void push(int item) { array[++top] = item; // Places item on top of the stack } public boolean isEmpty() { // Underflow operation return top == -1; // Test if stack is empty } public boolean isFull() { // Overflow operation return top == maxSize – 1; // Test if stack is full } 1/20/2014 Jim Littleton - COP35386

Stacks Important notes  Always call isEmpty() before calling pop() Avoids ArrayIndexOutOfBounds exception  Always call isFull() before calling push() Avoids ArrayIndexOutOfBounds exception Extremely useful in programming  Following a sequence that requires backtracking  Evaluating parenthesized expressions Arithmetical expressions 1/20/2014 Jim Littleton - COP35387

Stacks Example use of a stack  Reverse the letters of a word Non-stack example private class NonStackApp { public static void main(String[] args) { String s = “COP3538”; char[] c = new char[s.length()]; for(int x = 0; x < c.length; x++) { c[x] = s.charAt(x); } for(int x = c.length – 1; x >= 0; x--) { System.out.print(c[x]); } 1/20/2014 Jim Littleton - COP35388

Stacks Example use of a stack  Reverse the letters of a word Stack example private class StackApp { public static void main(String[] args) { Stack stack = new Stack(); String s = “COP3538”; for(int x = 0; x < s.length(); x++) { stack.push(s.charAt(x)); } while(!stack.isEmpty()) { System.out.print(stack.pop()); } 1/20/2014 Jim Littleton - COP35389

Stacks A stack is a very useful data structure  Don’t have to keep track of array indices  Limited access to the elements in the array Elements are accessible only via the push and pop methods Stack efficiency  Push and pop take O(1) time A constant value. Why??  Not dependent on the number of items in the stack  No comparisons or swaps necessary 1/20/2014 Jim Littleton - COP353810

Queues A First In First Out (FIFO) data Structure  Remember: A Stack is a LIFO data structure Can be implemented using several structures  Array  Linked-list  Etc. Very useful for many different applications  Print queues  Job queues  Ready queues  Keyboard queues Jim Littleton - COP /20/2014

Queues Queue Terminology (Know the terms!!)  Front Points to the first element in the queue  Rear Points to the last element in the queue  Linear queue Front pointer always exists before the rear pointer  Circular queue The front and rear pointers can wrap around the queue  Deques (double-ended queue) Inserts and removals can occur on either end  Overflow Attempting to add an element to a full queue  Underflow Attempting to remove an element from an empty queue Jim Littleton - COP /20/2014

Queues Queue methods  Insert() Method used to store data at the rear of the queue  Remove() Method used to retrieve data from the front of the queue  IsEmpty() Tests whether the queue is empty  The queue is considered empty if the front and rear are together  IsFull() Tests whether the queue is full  The queue is considered full if the rear equals the size of the queue  Not necessarily true for Circular queues Jim Littleton - COP /20/2014

Linear Queue public class LinearQueue { private int maxSize, front, rear, numElems; private int[] array; public Queue(int size) { maxSize = size; array = new int[maxSize]; front = numElems = 0; rear = -1; } public void insert(int value) { array[++rear] = value; numElems++; } public boolean isEmpty() { return numElems == 0; } Jim Littleton - COP /20/2014 public boolean isFull() { return numElems == maxSize; } public int remove() { numElems--; return array[front++]; } public int size() { return numElems; }

Circular Queue public class CircularQueue { private int maxSize, front, rear, numElems; private int[] array; public Queue(int size) { maxSize = size; array = new int[maxSize]; front = numElems = 0; rear = -1; } public void insert(int value) { if(rear == maxSize - 1) { rear = -1; // Wrap around } array[++rear] = value; numElems++; } public boolean isEmpty() { return numElems == 0; } Jim Littleton - COP /20/2014 public boolean isFull() { return numElems == maxSize; } public int remove() { if(front == maxSize) { front = 0; // Wrap around } numElems--; return array[front++]; } public int size() { return numElems; }

Queues Queue efficiency  Insert() and remove() occur in O(1) time Simply insert or remove at front and rear pointers  Minor adjustment of front and rear values  Even in the case when the pointers wrap the queue (circular queue) Not dependent on the number of items in the array No comparisons or swaps necessary Jim Littleton - COP /20/2014

Priority Queue A priority queue (pQueue)  Similar to a regular queue Insert in rear (becomes inconsequential) Remove from front  Significant differences Items are organized by a key field  i.e., Last name, N#, etc. New items are inserted in their correct positions  The position of existing items in the queue may change  Insertion time is slower for a pQueue compared to a regular queue  Remove time is the same for both queue types If the queue is full  An existing item must be removed before a new item is added Jim Littleton - COP /20/2014

Priority Queue Several useful applications  Scheduling queues Shortest job first Fewest resources first  Print queues Fewest pages first Jobs sent to faster printers first A pQueue is no longer a FIFO queue!  The FO item is based on the specified priority Jim Littleton - COP /20/2014

Priority Queue pQueue Code Jim Littleton - COP /20/2014 public class PriorityQueue { private int maxSize; private int numElems = 0; private int[] array; public PriorityQueue(int size) { maxSize = size; array = new int[maxSize]; } public void insert(int item) { int x; if(numElems == 0) { array[numElems++] = item; } else { x = numElems; while(x > 0 && item > array[x - 1]) { array[x] = array[x - 1]; x--; } array[x] = item; numElems++; } public boolean isEmpty() { return numElems == 0; } public boolean isFull() { return numElems == maxSize; } public int remove() { return array[--numElems]; } This block of code is identical to the code that performs the copies in the Insertion Sort!

Priority Queue pQueue Efficiency  Remove occurs at O(1) time (immediate)  Insert occurs at O(n) time Items have to be copied to new locations  To make room for the item being inserted Jim Littleton - COP /20/2014

Questions Jim Littleton - COP /20/2014