BUILDING JAVA PROGRAMSCHAPTER 4Conditional Execution

days until the AP Computer Science

test

198

FracCalc Demo

Objectives

•Describe how an if statement works.•List the relational operators.•Define logical AND, OR, and NOT operators.•Compute boolean expressions.

public static void withdraw(int balance, int amount) { balance -= amount; System.out.println(“New balance: “ + balance); return balance;}

What’s wrong?

int balance = 100;balance = withdraw(balance, 10);balance = withdraw(balance, 20);balance = withdraw(balance, 300);

The if statementExecutes a block of statements only if a test

expression is true.

if (<test expression>) { <statement>; ...}

Syntax Yoda

public static void withdraw(int balance, int amount) { if(balance >= amount) { balance -= amount; System.out.println(“New balance:” + balance); }

return balance;}

int balance = 100;balance = withdraw(balance, 10);balance = withdraw(balance, 20);balance = withdraw(balance, 300);

Relational Expressionsif statements and for loops both use logical tests.

for (int i = 1; i <= 10; i++) { ... }if (i <= 10) { ... }

These are boolean expressions and will be covered in Chapter 5.

Tests use relation operators.

Operator Meaning Example Value

== equals 1 + 1 == 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

A note on ==

if (age == 17) {…}

The equality operator (==) is not to be confused with the assignment operator (=).

if (age = 17) {…}

Logical OperatorsTests can be combined using logical operators:

Operator Description

Example Result

&& and (2 == 3) && (-1 < 5) false

|| or (2 == 3) || (-1 < 5) true

! not !(2 == 3) true

Truth TablesTruth tables for each, used with logical 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

Evaluating Logic ExpressionsRelational operators have lower precedence than math; logical operators have lower precedence than relational operators

5 * 7 >= 3 + 5 * (7 – 1) && 7 <= 115 * 7 >= 3 + 5 * 6 && 7 <= 1135 >= 3 + 30 && 7 <= 1135 >= 33 && 7 <= 11true && truetrue

Relational operators cannot be "chained" as in algebra2 <= x <= 10true <= 10 (assume that x is 15)Error!

Instead, combine multiple tests with && or ||2 <= x && x <= 10true && falseFalse

In your notebook…int x = 42;int y = 17;int z = 25;

y < x && y <= zx <= y + z && x >= y + z(x + y) % 2 == 0 || !((z – y) % 2 == 0)

True

True

False

Using boolean• Write a program where user inputs integers until the user no

longer enters a perfect square.

Enter perfect squares: 4 16 9 121 100 4 33 is not a square!

Enter perfect squares: 1 2 82 is not a perfect square!

Using booleanpublic static void main(String args[]) { Scanner console = new Scanner(System.in); System.out.println(“Enter perfect squares: ”); for(boolean shouldExit = false; shouldExit;) { int input = console.nextInt(); int sqrt = (int)(Math.sqrt(input)); if (sqrt * sqrt != input) { System.out.println(input + “ is not a perfect square!”); shouldExit = true; } }}

Homework• Read 3.1• PracticeIt 4.1, 4.2• FracCalc Checkpoint 1

The if/else statementExecutes one block if a test is true, another if false

if (test) { statement(s);} else { statement(s);}

• Example:double gpa = console.nextDouble();if (gpa >= 2.0) { System.out.println("Welcome to Mars University!");} else { System.out.println("Application denied.");}

Syntax Yoda

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!");}...

Nested if/elseChooses between outcomes using many tests

if (test) { statement(s);} else if (test) { statement(s);} else { statement(s);}

• Example:if (x > 0) { System.out.println("Positive");} else if (x < 0) { System.out.println("Negative");} else { System.out.println("Zero");} Syntax Yoda

Let’s Try It!Formula for body mass index (BMI):

• Write a program that produces output like the following:This program reads data for two people andcomputes their body mass index (BMI).

Enter next person's information:height (in inches)? 70.0weight (in pounds)? 194.25

Enter next person's information:height (in inches)? 62.5weight (in pounds)? 130.5

Person 1 BMI = 27.868928571428572overweightPerson 2 BMI = 23.485824normalDifference = 4.3831045714285715

7032

height

weightBMI

BMI Weight classbelow 18.5 underweight18.5 - 24.9 normal25.0 - 29.9 overweight30.0 and up obese

if/else with return// Returns the larger of the two given integers.public static int max(int a, int b) { if (a > b) { return a; } else { return b; }}

• Methods can return different values using if/else• Whichever path the code enters, it will return that value.• Returning a value causes a method to immediately exit.• All paths through the code must reach a return

statement.

All paths must returnpublic static int max(int a, int b) { if (a > b) { return a; } // Error: not all paths return a value}

• The following also does not compile:

public static int max(int a, int b) { if (a > b) { return a; } else if (b >= a) { return b; }}

• The compiler thinks if/else if code might skip all paths, even though mathematically it must choose one or the other.

if/else, return question• Write a method quadrant that accepts a pair of real numbers x and y and returns the quadrant for that point:

• Example: quadrant(-4.2, 17.3) returns 2• If the point falls directly on either axis, return 0.

x+x-

y+

y-

quadrant 1quadrant 2

quadrant 3 quadrant 4

Homework• Read 3.1• Self Check 4.1, 4.2, 4.4, 4.13, • Exercise 4.3, 4.4• FracCalc Checkpoint 1