COMP 111. Introduction to Computer Science and Object-Oriented Programming

Transcription

1 COMP 111 Introduction to Computer Science and Object-Oriented Programming 5

2 Decisions, Decisions!

3 Methods - So Far Execute a sequence of statements Declarations Assignment statements Return Maybe an output (e.g. System.out.println ) Execute in order First to last Until last is executed Or a return Or an error With Expressions!

4 But While (as humans) we know how to Prevent overdrawing a bank account Prevent figuring slope of a vertical line Convert 24-hour to 12-hour time We cannot write methods that do these!

5 They All Hinge on a Decision What decisions are needed to Prevent overdrawing a bank account? Prevent figuring slope of a vertical line? Convert 24-hour to 12-hour time?

6 Examples from the World If it is snowing, I don t go to school Whenever I go outside, it is snowing test false true statement if a test is true (then) statement

7 Examples from the World On Saturdays, I get up at 9 and go to Tim Horton s true test statement statement false if a test is true (then) block of statements

8 Examples from the World If it s sunny, I play tennis; otherwise I go home. true statement test false statement if a test is true (then) statement else statement

9 Options In Java We need to test a value, and make a decision Is a statement executed or skipped? true test statement false if (test) if (amount <= balance) balance = balance - amount;

10 Options In Java We need to test a value, and make a decision Is a block of statements executed or skipped? true test statement statement false if (test) { } if (hours > 12) { hours = hours - 1; amorpm = pm ; }

11 In Java We need to test a value, and make a decision Which of two alternatives to execute true statement test false statement if (test) else if (amount <= balance) balance = balance - amount; else balance = balance - OVERDRAFT_PENALTY;

12 What You Need to Know Syntax of Java if and if-else constructs Good style for the constructs How to group statements in a block How to write simple tests (depends on type) How to handle sequences of tests How to handle multi-level tests How to write compound tests

13 Syntax and Style if (condition) if (condition) else Don t forget parens around the condition Indent statements

14 Syntax and Style if (condition) if (condition) else Use a block when there is more than one statement { } A statement block can be used in place of a statement in the above Align braces

15 Simple Numeric Tests Compare the value of two expressions Use one of six operators Operator > >= < <= ==!= Greater than Greater than or equal Less than Less than or equal Equal Not equal Description

16 Consider public int mystery(int x, int y) { int z = 0; if (x > y) x = x - y; z = 2 * x; return z; } What is returned when x is 6 and y is 4? What is returned when x is 3 and y is 3?

17 Gotcha Using = instead of == It may not be a syntax error!

18 Consider double r = Math.sqrt(2); double d = r * r -2; System.out.print("sqrt(2) squared minus 2 is ); if (d == 0) System.out.println( 0"); else System.out.println(d); What gets printed? sqrt(2)squared minus 2 is E-16

19 Tests with Floating Point Same operators But watch roundoff errors! Instead, test if close enough final double EPSILON = ; if (Math.abs(x y) <= EPSILON) This is like Junit assertequals(x, y, );

20 Tests with Strings String is class Use methods of the String class to test String values surname1.equals(surname2) surname2 surname1 dog dog equals equals

21 Testing Equality equals method tests if String values are same Same length Same char in each position Remember A is not the same as a (I.e. case sensitive) surname2 surname1 dog Dog equals equals

22 Gotcha using == to test if two String values are same if (surname1 == surname2) vs if (surname1.equals(surname2)) surname2 surname1 dog dog equals equals

23 Example String surname1 = Smith ; String surname2 = Smith ; String surname3 = Smith ; String surname4 = surname1; if (surname1 == surname2) if (surname1 == Smith ) Which of these tests are true? if (surname1.equals(surname2)) if (surname1.equals(surname3)) if (surname1.equals( smith ) if (surname1.equals(surname4)) if (surname1 == surname4)

24 Other String Tests equalsignorecase( ) surname1.equalsignorecase(surname2)

25 Other String Tests compareto( ) compares in lexographic order (like alphabetic) returns an integer name1.compareto(name2)

26 Order of Char Values

27 Return Values name1.compareto(name2) returns an integer value that is: negative if name1 before name2 in order zero if name1 is the same as name2 positive if name1 after name2 in order

28 Testing Objects Use equals methods to see if values same like String but some classes don t define equals method (like the Line class we worked on) Use == to test for null (null means there is no ref to an object)

29 Examples Line l1; String name1; String name2 = ; String name3 = Smith ; What happens? if (name1 == null) if (name1.equals(null)) if (name1 == ) if (name2 == null) if (name2.equals(null)) if (name2.equals( )) if (name3 == null)

30 Which of the Following Are.. syntactically incorrect? syntactically correct, but logically questionable? String a = "1"; String b = "one"; double x = 1; double y = 3 * (1.0 / 3); a. a == "1" b. a == null c. a.equals("") d. a == b e. a == x f. x == y g. x - y == null h. x.equals(y) Syntactically incorrect: e, g, h. Logically questionable: a, d, f.

31 Sequences of Tests if (age >= 100) s = What is your secret! ; else if (age >= 65) s = Enjoying your golden years? ; else if (age >= 45) s = Feeling some aches and pains? ; else if (age >= 30) s = Oh to be in your prime again ; else s = You young whipper snapper! ;

32 A Note on Syntax if (condition) else if (condition) else if (condition) else if (condition) else

33 Gotcha if (age >= 0) s = You young whipper snapper! ; else if (age >= 30) s = Oh to be in your prime again ;

34 Multi-Level Tests Often referred to as nested ifs Let s look at examples

35 Tax Schedule If your filing status is Single If your filing status is Married Tax Bracket Pctg. Tax Bracket Pctg. $0... $21,450 15% 0... $35,800 15% Amount over $21,450, up to $51,900 28% Amount over $35,800, up to $86,500 28% Amount over $51,900 31% Amount over $86,500 31%

36 Nested Branches Compute taxes due, given filing status and income: 1. branch on the filing status 2. for each filing status, branch on income level The two-level decision process is reflected in two levels of if statements We say that the income test is nested inside the test for filing status

37 Nested Branches