CSS 161 Fundamentals of Compu3ng. Flow control (2) October 10, Instructor: Uma Murthy

Transcription

1 CSS 161 Fundamentals of Compu3ng Flow control (2) October 10, 2012 Instructor: Uma Murthy

2 Outline Reminders: HW 2 due Monday Today: Errata Review condi3onals Boolean expressions (3.2) Loops (3.3) CSS 161: Fundamentals of Compu3ng 2

3 Acknowledgments A subset of the following slides have been either directly taken from or derived from the supplements of the book: Building Java Programs: A Back to Basics Approach, 2nd edi(on by Stuart Reges and Marty Stepp hsp:// CSS 161: Fundamentals of Compu3ng 3

4 Errata: unary operators ++ and -- can be applied to double and float types of variables too double real = 1.2; real++; System.out.println(real); 2.2 In practice, these operators are typically used with int type variables, such as counts of objects. Has the same effect as: real = real + 1; CSS 161: Fundamentals of Compu3ng 4

5 Review if statement if-else statement Nested if-else statement Compound statement: { switch statement Condi3onal operator:? CSS 161: Fundamentals of Compu3ng 5

6 The if statement Executes a block of statements only if a test is true if (test) { statement;... statement; Example: double gpa = console.nextdouble(); if (gpa >= 2.0) { System.out.println("Application accepted."); CSS 161: Fundamentals of Compu3ng 6

7 The if/else statement Executes one block if a test is true, another if false if (test) { else { Example: double gpa = console.nextdouble(); if (gpa >= 2.0) { System.out.println("Welcome to Mars University!"); else { System.out.println("Application denied."); CSS 161: Fundamentals of Compu3ng 7

8 Rela3onal expressions if statements use logical tests. if (i <= 10) {... These are boolean expressions Tests use rela3onal or comparison operators: Operator Meaning Example Value == equals == 2 true!= does not equal 3.2!= 2.5 true < less than 10 < 5 false > greater than 10 > 5 true <= less than or equal to 126 <= 100 false >= greater than or equal to 5.0 >= 5.0 true CSS 161: Fundamentals of Compu3ng 8

9 Compound Statements if (myscore > your Score) { System.out.println("I win!"); wager = wager + 100; else { System.out.println("I wish these were golf scores."); wager = 0; More compact formatting option: if (myscore > your Score) { System.out.println("I win!"); wager = wager + 100; else { System.out.println("I wish these were golf scores."); wager = 0; CSS 161: Fundamentals of Compu3ng 3-9

10 Misuse of if What's wrong with the following code? Scanner console = new Scanner(System.in); System.out.print("What percentage did you earn? "); int percent = console.nextint(); if (percent >= 90) { System.out.println("You got an A!"); if (percent >= 80) { System.out.println("You got a B!"); if (percent >= 70) { System.out.println("You got a C!"); if (percent >= 60) { System.out.println("You got a D!"); if (percent < 60) { System.out.println("You got an F!");... CSS 161: Fundamentals of Compu3ng 10

11 Nested if/else Chooses between outcomes using many tests if (test) { else if (test) { else { Example: if (x > 0) { System.out.println("Positive"); else if (x < 0) { System.out.println("Negative"); else { System.out.println("Zero"); CSS 161: Fundamentals of Compu3ng 11

12 Nested if/else/if If it ends with else, exactly one path must be taken. If it ends with if, the code might not execute any path. Example: if (test) { else if (test) { else if (test) { if (place == 1) { System.out.println("Gold medal!"); else if (place == 2) { System.out.println("Silver medal!"); else if (place == 3) { System.out.println("Bronze medal."); CSS 161: Fundamentals of Compu3ng 12

13 Nested if structures exactly 1 path (mutually exclusive) if (test) { else if (test) { else { 0 or 1 path (mutually exclusive) if (test) { else if (test) { else if (test) { 0, 1, or many paths (independent tests; not exclusive) if (test) { if (test) { if (test) { CSS 161: Fundamentals of Compu3ng 13

14 The switch Statement switch (Controlling_Expression) { case Case_Label_1: Statement_Sequence_1 break; case Case_Label_2: Statement_Sequence_2 break; case Case_Label_n: Statement_Sequence_n break; default: Default_Statement Sequence break;... Controlling_Expression has to evaluate to one of: char, int, short, byte, or String (Java 7) Case_Label same type as Controlling_Expression break; used to end execu3on of switch block 3-14 Copyright 2010 Pearson Addison- Wesley. All rights reserved.

15 Switch examples See Java documenta3on page: hsp://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.htm Use char, int, short, and byte as Controlling_Expression Take cau3on when using a String as the Controlling_Expression as it is available only in Java 7 CSS 161: Fundamentals of Compu3ng 15

16 The Condi3onal Operator The condi;onal operator is a nota3onal variant on certain forms of the if-else statement Also called the ternary operator or arithme;c if The following examples are equivalent: if (n1 > n2) max = n1; else max = n2; vs. max = (n1 > n2)? n1 : n2; The expression to the right of the assignment operator is a condi;onal operator expression If the Boolean expression is true, then the expression evaluates to the value of the first expression (n1), otherwise it evaluates to the value of the second expression (n2) 3-16 Copyright 2010 Pearson Addison- Wesley. All rights reserved.

17 Today s Topics Comparison operators Boolean operators Short- circuit evalua3on Side effects Comparing strings (lexicographic, alphabe3c) Loops CSS 161: Fundamentals of Compu3ng 17

18 Comparison Operators Rela3onal expressions use comparison operators CSS 161: Fundamentals of Compu3ng 18

19 Comparison operators if statements use logical tests. if (i <= 10) {... These are boolean expressions Tests use rela3onal or comparison operators: Operator Meaning Example Value == equals == 2 true!= does not equal 3.2!= 2.5 true < less than 10 < 5 false > greater than 10 > 5 true <= less than or equal to 126 <= 100 false >= greater than or equal to 5.0 >= 5.0 true CSS 161: Fundamentals of Compu3ng 19

20 Boolean (logical) Operators CSS 161: Fundamentals of Compu3ng 20

21 Boolean (logical) Operators CSS 161: Fundamentals of Compu3ng 21

22 Boolean (logical) operators Tests can be combined using boolean operators: Operator Description Example Result && AND (2 == 3) && (-1 < 5) OR (2 == 3) (-1 < 5) false true! NOT!(2 == 3) true "Truth tables" for each, used with boolean values p and q: p q p && q p q true true true true true false false true false true false true false false false false p!p true false false true

23 Precedence and Associa3vity Rules CSS 161: Fundamentals of Compu3ng 23

24 Evalua3ng logic expressions Rela3onal operators have lower precedence than math. 5 * 7 >= * (7-1) 5 * 7 >= * 6 35 >= >= 33 true Rela3onal operators cannot be "chained" as in algebra. 2 <= x <= 10 true <= 10 (assume that x is 15) error! Instead, combine mul3ple tests with && or 2 <= x && x <= 10 true && false false

25 Logical ques3ons What is the result of each of the following expressions? int x = 42; int y = 17; int z = 25; y < x && y <= z x % 2 == y % 2 x % 2 == z % 2 x <= y + z && x >= y + z!(x < y && x < z) (x + y) % 2 == 0!((z - y) % 2 == 0) Answers: true, false, true, true, false Exercise: Write a program that prompts for informa3on about a person and uses it to decide whether to date them.

26 Factoring if/else code factoring: Extrac3ng common/redundant code. Can reduce or eliminate redundancy from if/else code. Example: if (a == 1) { System.out.println(a); x = 3; b = b + x; else if (a == 2) { System.out.println(a); x = 6; y = y + 10; b = b + x; else { // a == 3 System.out.println(a); x = 9; b = b + x; System.out.println(a); x = 3 * a; if (a == 2) { y = y + 10; b = b + x;

27 Short- circuit Evalua3on Evaluate boolean expressions in prescribed order, stop when outcome is inevitable CSS 161: Fundamentals of Compu3ng 27

28 Short- circuit Evalua3on Evaluate boolean expressions in prescribed order, stop when outcome is inevitable int n = 1; if ( n > 0 && n % 2 == 1 ) System.out.println( n + is positive and odd. ); else if ( n > 0 && n % 2 == 0 ) System.out.println( n + is positive and even. ); else if ( n < 0 && n % 2 == 1 ) System.out.println( n + is negative and odd. ); else if ( n < 0 && n % 2 == 0 ) System.out.println( n + is negative and even. ); else System.out.println(n + is zero. ); CSS 161: Fundamentals of Compu3ng 28