What can we do with Coin dispenser?

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Aldous Thompson
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1 /7/ Outline Part Stacks Stacks? Applications using Stacks Implementing Stacks Relationship between Stacks and Recursive Programming Instructor: Sangmi Pallickara Department of Computer Science Colorado State University Linear, 'me ordered structures Data Structures that reflects a temporal relationship Order of removal based on the order of insertion We will consider First come, first serve First Come, First Out: FIFO (queue) take from the top of the pile Last In, First Out: LIFO (stack) 4 What can we do with Coin dispenser? Stacks push a coin into the dispenser pop a coin from the dispenser see the coin on top check whether this dispenser is empty or not Last In First Out (LIFO) structure Add/Remove from the same end 6

/7/ Stack OperaNons Outline isempty(): determine whether stack is empty push(): add a new item to the stack pop(): remove the item added most recently peek(): retrieve the item added most recently

2 /7/ Stack OperaNons Outline isempty(): determine whether stack is empty push(): add a new item to the stack pop(): remove the item added most recently peek(): retrieve the item added most recently createstack(): create an empty stack removeall(): remove all the items Stacks? Applications using Stacks Implementing Stacks Relationship between Stacks and Recursive Programming 7 8 ApplicaNons Call Stack Call Stack Top of the Stack Stack data structure to store information about active subroutines or computer program Memory allocation for tracking the execution of functions such as a nested function First method that returns is the last one invoked Element of call stack activation record Parameters Local variables Return address: pointer to the next instruction to be executed in calling method Stack Frame For DrawLine subrounne Stack Frame For DrawCircle subrounne Locals of DrawLine Return Address Parameters for DrawLine Locals of DrawCircle Return Address Parameters for DrawCircle Stack Pointer Frame Pointer 9 0 ApplicaNons Backtracking Looking for a solution based on a guess Keeping the guesses (temporary answers) in the stack When the process faces deadend, go back one step (visit the first one in the stack) and repeat the process

3 /7/ Checking for Balanced Braces Peudocode Each time you encounter a }, it matches an already encountered { When you reach the end of the string, you have matched all of the { you have encountered while ( not at the end of the string){! if (the next character is a { ){! astackpush( { )! else if (the character is a } ) {! openbrace = astackpop()! }! 6 String abc{{def}ijm} Peudocode with example a b c { { d e f } while ( not at the end of the string){! i j m if (the next character is a { ){! } astackpush( { )! else if (the character is a } ) {! openbrace = astackpop()! }! { { Algebraic Expressions Evaluating Postfix Expressions Assumptions: String is a syntactically correct postfix expression No unary operator No exponentiation operators Operands are single lowercase letters 7 8

4 /7/ Peudocode 4 + * for ( each character ch in the string){!!if(ch is an operand){!!!push value that operand ch! represents onto stack! } else{!! operand = Pop the top of the stack! operand = Pop the top of the stack! result = operand op operand! push result onto stack! String 4 + * WHY? 4 +4 *(+4) Outline Stacks? Applications using Stacks Implementing Stacks Relationship between Stacks and Recursive Programming 9 0 ImplemenNng Stacks Array- Based ImplementaNon Should perform the stack operations Array-based implementation Reference-based implementation List-based implementation Use an array of Objects (called items) public class StackArrayBased implements StackInterface{! WHY? final int MAX_STACK = 0;! private Object items[];! private int top;! public StackArayBased(){! items = new Object [MAX_STACK];! top = -;! } end default constructor! isempty()! push()! public boolean isempty(){!! return top < 0;! public void push (Object newitem) throws StackException{! if (!isfull()) {! item[++top] = newitem;! }else{! throw new StackException(your_error_message);! 4 4

$/7/ pop()! peek()! public Object pop() throws StackException{! if (!isempty()){! return items[top--];! } else {! throw new StackException(your_error_message);!$ 6 Reference- Based ImplementaNon TOP A Node 0 Your Object To be Stacked 80 A Reference to the next Node 60 private Node top;! public void push (Object newitem){!

6 Reference- Based ImplementaNon TOP A Node 0 Your Object To be Stacked 80 A Reference to the next Node 60 private Node top;! public void push (Object newitem){!

} else { exception handling 7 8 ImplementaNon that uses List List posinon Listsize() 0 80 60 Top of the stack 9 public void push(object newitem){!

5 /7/ pop()! peek()! public Object pop() throws StackException{! if (!isempty()){! return items[top--];! } else {! throw new StackException(your_error_message);! public Object peek() throws StackException{!!if(! isempty()){! return items[top];! }else {! throw new StackException(your_error_message);! 6 Reference- Based ImplementaNon TOP A Node 0 Your Object To be Stacked 80 A Reference to the next Node 60 private Node top;! public void push (Object newitem){!! top = new Node(NewItem,top)! public Object pop() throws StackException{! if (!isempty()){! Node temp = top;! top = topnext;! return tempitem;! } else { exception handling 7 8 ImplementaNon that uses List List posinon Listsize() Top of the stack 9 public void push(object newitem){! listadd(0, newitem);! public Object pop() throws StackException{! if (!listisempty()){! Object temp = listget(0);! listremove(0);! return temp;! } else exception handling! public void popall(){!!listremoveall();! 0

6 /7/ Comparison of ImplementaNon How about javaunlstack? What are the pros and cons for different implementation styles? Array based? Reference based? List based? -- Array based implementation can provide fixed size stack -- List based: Why not just List? Derived from Vector empty(), peek(), pop(), push(), search(object o)! RelaNonship between Stacks and Recursion Next Reading Most implementations of recursion can maintain a stack of activation records Backtracking example Within recursive calls, the most recently executed call is stored at the top of the stack Store and access the same point of the data structure Chap 8 ueues 4 6

8/28/12. Outline. Part 2. Stacks. Linear, time ordered structures. What can we do with Coin dispenser? Stacks

8/28/12. Outline. Part 2. Stacks. Linear, time ordered structures. What can we do with Coin dispenser? Stacks 8/8/ Part Stacks Instructor: Sangmi Pallickara (sangmi@cscolostateedu) Department of Computer Science Linear, time ordered structures Data Structures that reflects a temporal relationship Order of removal