1. Stacks Chapter 5

2. Chapter 5: Stacks2 Chapter Objectives To learn about the stack data type and how to use its four methods: push, pop, peek, and empty To understand how Java implements a stack To learn how to implement a stack using an underlying array or a linked list To see how to use a stack to perform various applications, including finding palindromes, testing for balanced (properly nested) parentheses, and evaluating arithmetic expressions

3. Chapter 5: Stacks3 Stack Dinner plates at a cafeteria Spring-loaded “stack” of plates “Push” a new plate onto the stack “Pop” an existing plate off of the stack Only the top plate is accessible “Last-In, First-Out”, or LIFO

4. Chapter 5: Stacks4 Stack Pez dispenser also uses a stack Only the top Pez candy can be accessed Can only extract one item at a time “Push” candy onto stack “Pop” candy off of stack Only the top candy is accessible, LIFO, etc.

5. Chapter 5: Stacks5 Stack Stack might seem to be too restrictive LIFO Only one element accessible Only operations are push, pop, peek, empty However, stacks are extremely useful in CS Simple and useful For example, stacks used when executing programs Used like “scratch paper” to keep track of info

6. Chapter 5: Stacks6 Specification of the Stack Abstract Data Type Only the top element of a stack is visible, therefore the number of operations performed by a stack are few The only operations available are Inspect the top element (peek) Retrieve (and remove) the top element (pop) Put a new element on the stack (push) Test for an empty stack (empty)

7. Chapter 5: Stacks7 Specification of the Stack Abstract Data Type (continued)

8. Chapter 5: Stacks8 StackInt Interface /** Stack is a LIFO data structure */ public interface StackInt { /** Pushes an item onto the top of the stack and returns the item pushed. */ E push(E obj); /** Returns the object at the top of the stack without removing it. */ E peek(); /** Returns the object at the top of the stack and removes it. */ E pop(); /** Returns true if the stack is empty; otherwise, returns false.*/ boolean empty();}

9. Chapter 5: Stacks9 Stack Example Suppose we have a stack called names Alice Bob Trudy And we execute names.pop(); What is the return value? “Trudy” and the stack is Alice Bob

10. Chapter 5: Stacks10 Stack Example We now have the stack And we execute names.push(“Charlie”); What is the return value? “Charlie” and stack is Alice Bob Alice Bob Charlie

11. Chapter 5: Stacks11 Stack Example We now have the stack And we execute names.peek(); What is the return value? “Charlie” and stack is Alice Bob Charlie Alice Bob Charlie

12. Chapter 5: Stacks12 Stack Example We now have the stack And we execute names.empty(); What is return value? Returns false and stack is Alice Bob Charlie Alice Bob Charlie

13. Chapter 5: Stacks13 Stack Applications We consider two programs that use stacks Palindrome finder Parentheses matcher First we consider palindrome finder Palindrome: reads the same in either direction For example: “level” and “Able was I ere I saw Elba” Note that we do not ignore spaces and punctuation So, for example, “never odd or even” not considered a palindrome for this exercise (but often it is)

14. Chapter 5: Stacks14 Palindrome Finder Input: string to be tested Output: message indicating whether string is a palindrome or not How to slove this problem? Many different ways to solve this… For example, could create a new string by going from end to beginning of original string Then compare new and original strings If equal, string is a palindrome But we want to use a stack!

15. Chapter 5: Stacks15 Palindrome Finder Using a stack… Push characters of original string onto stack Then pop characters off and append to new string Compare new string and original string If same, then it is a palindrome Why does this work? If we push, then pop, it will reverse string

16. Chapter 5: Stacks16 Palindrome Finder Suppose string is “frank” First, push letters onto stack… f frank r f r f a r f a n r f a n k

17. Chapter 5: Stacks17 Palindrome Finder Then pop letters off of stack and append to new string… r f a n kn r f knar r f a kna f knarf r f a n k k Finally, compare original and new string: We have frank != knarf, so not a palindrome

18. Chapter 5: Stacks18 Class PalindromeFinder

19. Chapter 5: Stacks19 Testing PalindromeFinder What types of strings should be test??? Single character (which is always a palindrome) Single words Both palindromes and non-palindromes Multiple words Both palindromes and non-palindromes Different cases (upper/lower) Even-length strings Odd-length strings Empty string (considered palindrome by default)

20. Chapter 5: Stacks20 Balanced Parenthesis It is important to determine whether an expression is “balanced” with respect to parentheses For example (a+b*(c/(d-e)))+(d/e) is balanced But (a+b*(c/d-e)))+(d/e) is not balanced Problem is more complicated if braces or brackets (square and/or curly) are also used Good solution is to use stacks! Here, we only consider parenthesis

21. Chapter 5: Stacks21 ParenChecker Class Input: String (representing an expression) Output: A message indicating whether expression is balanced with respect to parenthesis or not Of course, we want to use a stack… What is the idea behind the algorithm? Push open parenthesis onto stack When closed parenthesis is encountered Try to pop top element off of stack If no top element (stack is empty), not balanced When done, if stack is not empty, then not balanced

22. Chapter 5: Stacks22 ParenChecker Algorithm Given expression to test… Create an empty stack of characters Set balanced = true (i.e., assume it is balanced) Set index = 0 While balanced is true and index < expression length Get the next character in expression If character in open parenthesis Push open parenthesis onto stack Else if character is closing parenthesis If stack is empty, set balanced to false Else pop element from stack Increment index Return true if balanced is true and stack is empty

23. Chapter 5: Stacks23 ParenChecker Class

24. Chapter 5: Stacks24 ParenChecker Testing What data to test on? Simple valid expressions (one level of matched parens) Simple invalid expressions More complex expressions (valid and invalid) Test too many open parens and too many closed parens Know the correct answer before testing an expression

25. Chapter 5: Stacks25 Implementing a Stack In Java, Stack class extends Vector Could also have been implemented using other Lists Such as ArrayList or LinkedList Could also be implemented using Array We’ll look (briefly) at each of these

26. Chapter 5: Stacks26 Stack as a Vector The Java API includes a Stack class as part of the package java.util The Vector class implements a growable array Elements of a vector accessed using an integer index Size can grow or shrink as needed to accommodate the adding and removing of elements In following examples, push letters of “Java” onto stack: J a v a

27. Chapter 5: Stacks27 Stack as a Vector

28. Chapter 5: Stacks28 Stack as a Vector Java Stack is an extension of Vector Therefore, can use all Vector methods on a Stack Is this a good thing? You can access any element of a Vector So, you can access any element of a Stack If you try to access elements of a Stack, you might get ArrayIndexOutOfBoundsException Best to restrict yourself to stack-specific methods

29. Chapter 5: Stacks29 Stack as a List Can use ArrayList, Vector, LinkedList classes Since all implement the List interface Name of class illustrated in text is ListStack Note that ListStack is an adapter class Gives different names to essentially same operations Also true of Java’s Vector-based implementation For example, java.util.Stack push is code is public E push(E obj) { add(obj); return obj; }

30. Chapter 5: Stacks30 ListStack.java public class ListStack implements StackInt { private List theData; public ListStack() { theData = new ArrayList (); } public E push(E obj) { theData.add(obj); return obj; }...

31. Chapter 5: Stacks31 Stack as an Array Allocate storage for an array with an initial default capacity when creating a new stack object Keep track of the top of the stack, topOfStack For empty stack, topOfStack is set to -1 No size method Array size does not grow/shrink after each push/pop However, must reallocate if more space needed This is very primitive And that’s why I like it best…

32. Chapter 5: Stacks32 Stack as an Array

33. Chapter 5: Stacks33 Stack as an Array public class ArrayStack implements StackInt { E[] theData; int topOfStack = -1; private static final int INITIAL_CAPACITY = 10; public ArrayStack() { theData = (E[])new Object[INITIAL_CAPACITY]; } public E push(E obj) { if (topOfStack == theData.length - 1) { reallocate(); } topOfStack++; theData[topOfStack] = obj; return obj; }...

34. Chapter 5: Stacks34 Stack as a Linked List Can implement a stack using a linked list of nodes See code and details in text

35. Chapter 5: Stacks35 Comparison of Stack Implementations Extending a Vector (as is done by Java) is a poor choice All Vector methods are accessible Then why does Java do it this way? What about ArrayList implementation? Easiest to implement, ArrayList does all of the work… What about Array implementation? Slightly more difficult for programmer What about Linked List implementation? Uses only as much storage as needed However, links are essentially wasted Regardless of method used, push/pop is O(1)

36. Chapter 5: Stacks36 Stack Application We consider problem of evaluating math expressions Postfix and infix notation Expressions normally written in infix form Operators (+,-,*,/, etc.) between the operands Good for humans, not so good for computers Computers prefer postfix notation Operands come first, then operators

37. Chapter 5: Stacks37 Postfix vs Infix

38. Chapter 5: Stacks38 Postfix Advantages Advantage of postfix form include No need for parentheses No need to consider operator precedence

39. Chapter 5: Stacks39 Evaluating Postfix Expressions Problem: Write a class to evaluate postfix expressions. Analysis: Stack is good data structure to store operands until needed Design: Class PostfixEvaluator with methods Method eval: scans postfix expression and processes its tokens Method evalOp: evaluates each operator Method isOperator: determine whether a character is an operator

40. Chapter 5: Stacks40 Evaluating Postfix Expressions

41. Chapter 5: Stacks41 Evaluating Postfix Expressions

42. Chapter 5: Stacks42 Evaluating Postfix Expressions

43. Chapter 5: Stacks43 Converting Infix to Postfix

44. Chapter 5: Stacks44 Converting Infix to Postfix

45. Chapter 5: Stacks45 Converting Infix to Postfix

46. Chapter 5: Stacks46 Converting Infix to Postfix

47. Chapter 5: Stacks47 Converting Infix to Postfix

48. Chapter 5: Stacks48 Chapter Review A stack is a last-in, first-out (LIFO) data structure A stack is a simple but powerful data structure Four operations are empty, peek, pop, and push Stacks are useful to process information in the reverse of the order that it is encountered In Java, Stack is implemented as an extension of the Vector class

49. Chapter 5: Stacks49 Chapter Review We considered 3 different ways to implement a stack: Using the List interface as a container Using an array as a container Using a linked list as a container We applied stacks to evaluate arithmetic expressions