06Data Structures---The Stack ADT

The course is mainly about The Stack ADT.Generally covered An array based stack implementation,Uses of Stack,Linked List based Stack Implementation,The Queue ADT,Possible error conditions and so on.

1.The Stack ADT A Stack is a collection of objects inserted and removed according to the Last In First Out (LIFO) principle. Think of a stack of dishes. Push and Pop are the two main operations Browsers, while displaying a new webpage, push the address of the current page into a stack. The address of the previous page can be popped out of the stack. Think of the undo operation of an editor. The recent changes are pushed into a stack, and the undo operation pops it from the stack.

2.An array based stack implementation Main update methods: Push (e) Pop ( ) Additional useful methods Peek () Same as pop, but does not remove the element Empty() Boolean, true when the stack is empty Size () Returns the size of the stack public class Stack { } public Stack { } public Boolean empty() { } public void push (String str) { } public String pop() { { public String peek ( ) { } }


4.Array Based Implementation of Stack public class Stack { int maxSize; int top; String arr[]; } public Stack {int n} { maxsize = n ; arr = new String [maxSize]; top = 0; } public Boolean empty() { if (top == 0) return true; } else { return false; } } public void push (String str) { array[top] = str; top++; } public String pop() { if (top > 0) { return arr[top-1]; arr[top-1] = null; top --; } else { return null } public String peek () { } }

5.public static void main (String args []) { stack myStack = new Stack(7); myStack.push(“cat”); System.out.println(myStack.peek( )); myStack.push(“dog”); System.out.println(myStack.empty( )); myStack.push(“horse”); etc etc Uses of Stack Other than implementing undo and browser back buttons, stacks have many applications. • You can reverse a string using a stack. How? • Checking if the parentheses are well formed [ ( ) ( ) ] is well-formed, but [ ( ( ] ) ) is not. • Expression evaluation by JVM. How will it compute 3+4 = 7 or (3+4)* (6-9) + 18? (More to be discussed in the class)

6.• Activation records at runtime Class Xyz { firstMethod { int b; } secondMethod { int c; } thirdMethod { } } Heap and Stack space allocation to be discussed in the class

7.Advantages of Array-based Implementation Fast – all operations are completed in O(1) time Limitations of Array-based Implementation You have to know the upper bound of growth and allocate memory accordingly. If the array if full and there is another push operation then you encounter an exception. Linked List based Stack Implementation Can we implement a stack using a Linked List? Yes! Do not have to worry about the size when the stack grows. Sky (i.e. the entire memory pool) is the limit. Top of the stack = head of the linked list Bottom of the stack = tail of the linked list Push = add a new head Pop = remove the head

8. HEAD A top A push A HEAD top B B A A push B HEAD top D B D B A A push D Now, push and pop will take O(1) time. However, size ( ) will take O(n) time

9.The Queue ADT Recall the waiting list for courses during registration for courses? When a seat opens up, the first one who joined the waiting list is the first to get a chance to add to the course. This is a queue, works on the first in first out principle. Two access point: front and rear Other examples are: Call centers, printer queue, etc (Taken from http://jcsites.juniata.edu/faculty/kruse/cs240/queues)

10.The queue ADT supports two main update methods: Enqueue (e) Adds element e to the rear Dequeue ( ) Removes and returns the first element from the front. Other useful methods are First ( ) Size ( ) Empty ( ) The obvious implementation uses an array. After several enqueue and dequeue operations, both ends will drift. Queue invariants Acyclic structure Fists In First Out Property

11.Array based implementation of queue

12.public class Queue { public String arr[ ]; int maxSize; int front, rear, numberofItems = 0; public Queue (int n){ maxSize = n arr = new String[maxSize] } public void enqueue (String str) { if (numberofItems + 1 <= maxSize){ array[rear] = input; rear++; numberofItems++} else { System.our.println(“Sorry, the Queue is Full”) } } public void dequeue () { if (numberofItems > 0){ System.our.println(arr[front] + “Was Removed”); front++; numberofItems--} else { System.our.println(“Sorry, the Queue is Empty”) } } public static void main(String[ ] args){ Queue myQueue = new Queue(8); myQueue.enqueue(“alice”) myQueue.enqueue(“bob”); myQueue.enqueue(“clara”) myQueue.dequeue(); etc etc. }

13.Notice any problem with Space management? How will you limit the queue within the allotted space? Observe the cyclic structure

14. Possible error conditions 1. Dequeue from an empty buffer if (numberofItems > 0){ Normal dequeue action } else { System.out.println{“Sorry, the queue is empty’} Also, front must be incremented modulo maxSize 2. Enqueue into a full buffer if (numberofItems < maxSize){ Normal enqueue action } else { System.out.println{“Sorry, the queue is full”} Also, rear must be incremented modulo maxSize