ics211-lecture13

Transcript

1. Session 13 ICS 211 Stacks
2. Memory Upload • Generic types • Array stacks • Infix and postfix expressions
3. Container Classes • For the rest of the semester, we will be studying container classes, which are classes that contains objects of other classes • We want the container class to be flexible as possible, so we can store any type (class) of data • A class can have a generic type by using the type (class) as a parameter
4. Generic Class • When the program is compiled, the compiler uses text replacement to fill in the name of the actual class • In the class definition, we use the generic type (class) • We use the specific type (class) when we instantiate the container object that uses the particular type
5. Generic Type Syntax • Three steps to make a generic class • Step #1: Use <T> after the name of the class in class definition • public class TwoThings<T, C>{. . .} • Can actually use any letter • In this case, T stands for “Type” • You might use C for “Class”, E for “Element”, etc.
6. Generic Type Syntax • Step #2: Substitute T for the class’s name for object declaration, return type, and parameter type in class definition • Object declaration • private T item1;
7. Generic Type Syntax • Return type • public T getItem1() {return item1;} • Parameter type • public void setItem1(T itemA) { item1 = itemA; }
8. Generic Type Syntax • Step #3: Use <ClassName> as an argument after the name of the generic class for object instantiation • TwoThings<String, Integer> object1 = new TwoThings<String, Integer>("Nami", 25); • TwoThings<Name, Fraction> object2 = new TwoThings<Name, Fraction> (name, years);
9. Generic Type Example • See the very simple container class TwoThings.java on class web page • Generic class TwoThings can store objects of any two classes • In the main() method, object1 stores a String and an Integer, while object2 stores a Name and a Fraction
10. TwoThings Object • Visualize “box and balloon” model TwoThings<String,Integer> object1=new TwoThings<String,Integer>("Nami", 25); object1 item1 item2 Nami 25
11. TwoThings Object • Visualize “box and balloon” model Name name = new Name("Nalu", "Suzuki"); name first last Nalu Suzuki
12. TwoThings Object • Visualize “box and balloon” model Fraction years = new Fraction(5, 2); years numerator denominator 5 2
13. TwoThings Object TwoThings<Name,Fraction> object2=new TwoThings<Name,Fraction>(name, years); name object2 years item1 item2 first last Nalu Suzuki numerator denominator 5 2
14. What Is a Stack? • A stack data structure has a Last-In, First-Out (LIFO) property • The last object pushed (placed) in the top of the stack is the first object popped (taken) out of the stack • Daily life example of a stack is a stack of trays in the school cafeteria
15. Stack Code • See interface StackInterface.java for a list of Stack method prototypes • See class ArrayStack.java for the class definition which includes the method definitions as well as a client program in the main() method
16. Stack Methods • Boolean empty() • Is the stack empty? • T peek() throws EmptyStackException • Retrieve (does NOT remove) an object’s address from top of stack • If no elements on stack, then will throw an EmptyStackException
17. Stack Methods • T pop() throws EmptyStackException • Remove and returns an object from top of stack • If no elements on stack, then will throw an EmptyStackException • void push(T item) • Add an object to the top of stack
18. Stack Implementation • One way to implement a stack is as an array • Push and pop to the end of the array • See class ArrayStack.java on the class web page • The main() method of this class has a driver program that tests all of the methods of the ArrayStack
19. ArrayStack Object • You need to be able to use the “box and balloon” model to visualize an ArrayStack object • Rectangle represents a variable • Arrow represents the address of an object • Oval represents the space where an object’s data is stored
20. ArrayStack Object • The next few slides shows the “box and balloon” model for this code, which creates a stack of Strings • We begin with an empty stack StackInterface<String> stack2 = new ArrayStack<String>();
21. ArrayStack Object array stack2 0 size 1 5 2 top 3 -1 4 indexes addresses null null null null null
22. ArrayStack Object • We push four (4) strings to the stack • This increments the top variable, so it always stores the index of the top element on the stack stack2.push("A"); stack2.push("B"); stack2.push("C"); stack2.push("D");
23. ArrayStack Object null array stack2 0 A size 1 B 2 C 5 top 3 D 4 3 indexes addresses elements
24. ArrayStack Object • If we pop three (3) string objects off the stack, this only changes the top variable, as the three (3) strings are still stored in the array String letter = stack2.pop(); //letter: "D" letter = stack2.pop(); //letter: "C" letter = stack2.pop(); //letter: "B"
25. array stack2 0 A size 1 B 2 C top 3 D 4 ArrayStack Object 5 0 indexes addresses elements null
26. ArrayStack Object • Once we start to push string objects to the stack again, the new string objects will cover up the old string objects stack2.push("E"); stack2.push("F");
27. ArrayStack Object array stack2 0 A size 1 E 2 F top 3 D 4 indexes addresses elements null 5 2
28. ArrayStack Object • Since our stack is generic, it can store objects of any class, such as objects of class ThreeNames StackInterface<ThreeNames> stack3 = new ArrayStack<ThreeNames>(); ThreeNames name = new ThreeNames("A","B","C"); stack3.push(name); name = new ThreeNames("X", "Y", "Z"); stack3.push(name);
29. stack3 0 1 2 3 4 ArrayStack Object first middle last 1 5 array size top null null null first middle last A B C X Y Z
30. Big-O Break • The Big-O for method empty(), peek(), and pop() is O(1) • As you can see in the code for these methods, no looping occurs
31. Big-O Break • The Big-O for method push() is a little complicated • In the worst case, Big-O is O(n) because System.arraycopy() copies all n elements into a new array • However, method push() takes time proportional to n at most once every n times, so on average (amortized) Big-O is O(1)
32. Print Backwards • Because a stack has a LIFO (last-in, first-out) property, we can use a stack to print out a string backwards • See program BackwardLines.java, which takes an input file from the command line and prints each line backwards to the output window
33. Algebraic Expressions • Stacks are used by the Java compiler to evaluate algebraic expressions • In order to understand how this works, we need to review the basics of how to calculate an integer expression
34. Operator Precedence • Precedence means that certain operators are evaluated before others 1. ( ): parentheses have highest precedence 2. *, /, %: multiplication, division, and modulus are next 3. +, -: addition and subtraction have the lowest precedence
35. Operator Precedence • If operators next to each other have the same precedence, the equation is evaluated from left to right • 3+4*5 is evaluated (3+(4*5)) • Because * has higher precedence than + • 3+4-5 is evaluated ((3+4)-5) • Because + and - have the same precedence
36. Operator Precedence • 2*3+4-5%6/7 is evaluated (((2*3)+4)-((5%6)/7)) • Because * has higher precedence than +, + and - have the same precedence, % has higher precedence than -, and % and / have the same precedence
37. Algebraic Expressions • Although humans are good at evaluating infix expressions, computers are good at evaluating postfix expressions • Infix expression: 1 + 2 • Postfix expression: 1 2 +
38. Algebraic Expressions • Computers are better at evaluating postfix expressions, because postfix expressions have no parentheses, and have no precedence rules, so they are very simple to evaluate
39. Algebraic Expressions • The Java compiler (or any computer program) must take two steps to evaluate an algebraic expression • Step #1: use a stack to convert infix expression to a postfix expression (this class) • Step #2: use a stack to evaluate the postfix expression (next class)
40. Infix to Postfix • Computer algorithm to convert infix expression to postfix expression • To simply the algorithm, we will assume that the expression is fully parenthesized, has only single digit integers, and has only the 5 integer operations (*, /, %, +, -)
41. Infix to Postfix Algorithm 1. Input is a string which contains the infix integer expression 2. Loop from the 0th position to the last position in the input string a. Isolate a single character string from the input string b. If the character string is a left parenthesis "(", push it on the stack
42. Infix to Postfix Algorithm c. If the character string is a digit (integer), add it to the end of the output string d. If the character string is an operator (*, /, %, +, -), push it on the stack
43. Infix to Postfix Algorithm e. If the character string is a right parenthesis ")", pop each operator off the stack, and add each operator to the end of the output string, until you pop a left parenthesis "(" off the stack 3. After looping, the output string is the postfix expression
44. Infix to Postfix Trace  Input: "(3+(4*5))"  Loop: 0 • Character: ( • Stack: ( • Output:  Loop: 1 • Character: 3 • Stack: ( • Output: 3  Loop: 2 • Character: + • Stack: +( • Output: 3  Loop: 3 • Character: ( • Stack: (+( • Output: 3
45. Infix to Postfix Trace • Input: "(3+(4*5))" • Loop: 3 • Character: ( • Stack: (+( • Output: 3 • Loop: 4 • Character: 4 • Stack: (+( • Output: 34 • Loop: 5 • Character: * • Stack: *(+( • Output: 34 • Loop: 6 • Character: 5 • Stack: *(+( • Output: 345
46. Infix to Postfix Trace • Input: "(3+(4*5))" • Loop: 6 • Character: 5 • Stack: *(+( • Output: 345 • Loop: 7 • Character: ) • Stack: +( • Output: 345* • Loop: 8 • Character: ) • Stack: • Output: 345*+ • The output string is postfix expression "345*+"
47. Infix to Postfix Trace II  Input: "((3+4)-5)"  Loop: 0 • Character: ( • Stack: ( • Output:  Loop: 1 • Character: ( • Stack: (( • Output: • Loop: 2 • Character: 3 • Stack: (( • Output: 3 • Loop: 3 • Character: + • Stack: +(( • Output: 3
48. Infix to Postfix Trace II  Input: "((3+4)-5)"  Loop: 3 • Character: + • Stack: +(( • Output: 3  Loop: 4 • Character: 4 • Stack: +(( • Output: 34  Loop: 5 • Character: ) • Stack: ( • Output: 34+  Loop: 6 • Character: - • Stack: -( • Output: 34+
49. Infix to Postfix Trace II  Input: "((3+4)-5)"  Loop: 6 • Character: - • Stack: -( • Output: 34+  Loop: 7 • Character: 5 • Stack: -( • Output: 34+5  Loop: 8 • Character: ) • Stack: • Output: 34+5- • The output string is postfix expression "34+5-"
50. Memory Defragmenter • Generic types • Using an array to implement a stack • Converting from infix to postfix expression
51. Task Manager • Before the next class, you need to: 1.Do the assignment corresponding to this lecture 2.Email me any questions you may have about the material 3.Turn in the assignment before the next lecture