Lecture 5 of Computer Science II

1. Lecture 5 ofComputer Science II Stacks, Queues Instructor: Mr.Ahmed Al Astal

2. Lecture 4 ofComputer Science II Stacks, Queues Instructor: Mr.Ahmed Al Astal

3. STACKS Definition In computer science, a stack is a temporary abstract data type and data structure based on the principle of Last In First Out (LIFO). Stacks are used extensively at every level of a modern computer system.  Page 3

4. STACKS Explanation • Push: pushes an element on the top of the stored elements. • Pop: Pops the last inserted element.  Page 4

5. STACKS Explanation Push Pop 6 6 5 666 5 4 555 4 3 444 3 2 333 2 1 1 0 0  Page 5

6. STACKS Examples of use: • Web browsers. • Undo mechanism in Text browsers. • Function call.  Page 6

7. STACKS Stack ADT • An abstract data type (ADT) is an abstraction of a data structure • An ADT specifies: • Data stored • Operations on the data • Error conditions associated with operations  Page 7

8. STACKS The Stack ADT • Main stack operations: • push(object) inserts an element • object pop() removes and returns the last inserted element  Page 8

9. STACKS The Stack ADT • Auxiliary stack operations: • object top(): returns the last inserted element without removing it • integer size(): returns the number of elements stored • boolean isEmpty(): indicates whether no elements are stored  Page 9

10. STACKS Stack Interface public interface Stack { public int size(); public boolean isEmpty(); public Object top() throws EmptyStackException; public void push(Object o); public Object pop() throws EmptyStackException; }  Page 10

11. STACKS Exception public class StackEmptyException extends RuntimeException { Public StackEmptyException(String err){ super(err); } }  Page 11

12. STACKS Implementing Stacks using Arrays • When the array is empty t is -1, t is initially -1 • Fixed already assigned size, say 100000 • (First entered, Last returned) element is the element 0. • (Last entered, First returned )element is the element t • Stack size is t+1  Page 12

13. STACKS Implementing Stacks using Arrays Algorithm size(): return t+1 public int size() { return (t+1); }  Page 13

14. STACKS Implementing Stacks using Arrays Algorithm isEmpty(): return (t<0) public boolean isEmpty(){ return (t<0); }  Page 14

15. STACKS Implementing Stacks using Arrays Algorithm top(): if isEmpty() then throw a StackEmptyException return S[t] public Object top()throws StackEmptyException{ if(isEmpty()) throw new StackEmptyException(“Stack is Empty”); return S[top]; }  Page 15

16. STACKS Implementing Stacks using Arrays Algorithm push(o): if size() ==N then throw a StackFullException t=t+1 S[t] o publicvoid push(Object obj)throws StackFullException{ if(size()==N) throw new StackFullException(“Stack is Full”); S[++top]=obj; }  Page 16

17. STACKS Implementing Stacks using Arrays Algorithm pop(o): if isEmpty() then throw a StackEmptyException E=S[t] T=t-1 S[t]null Return e public Object pop()throws StackEmptyException{ if(isEmpty()) throw new StackEmptyException(“Stack is Empty”); Object elem=S[t]; S[t--]=null; return elem; }  Page 17

18. STACKS Application reverse publicstatic Integer[] reverse(Integer[] a){ ArrayStack S = new ArrayStack(a.length); Integer[] b= new Integer[a.length]; for(int i=0;i<a.length;i++) S.push(a[i]); for(int i=0;i<a.length;i++) b[i]=(Integer)(S.pop()); return b; }  Page 18

19. QUEUES Definition A container of objects that are inserted and removed according to the first-in first-out FIFO principle.  Page 19

20. QUEUES Basics • Front • Rear • Enqueue: from the rear r • Dequeue: from the front f  Page 20

21. QUEUES Queue ADT Fundamentals methods • enqueue(object): inserts an element at the end of the queue • object dequeue(): removes and returns the element at the front of the queue  Page 21

22. QUEUES Queue ADT Supporting methods • object front(): returns the element at the front without removing it • integer size(): returns the number of elements stored • boolean isEmpty(): indicates whether no elements are stored  Page 22

23. QUEUES Queue Interface in Java public interface Queue { public int size(); public boolean isEmpty(); public Object front() throws EmptyQueueException; public void enqueue(Object o); public Object dequeue() throws EmptyQueueException; }  Page 23