Conditionals (Cont’d)

2 Nested if/else question Formula for body mass index (BMI): Write a program that produces output like the following: This program reads data for two people and computes their body mass index (BMI). Enter next person's information: height (in inches)? 70.0 weight (in pounds)? Enter next person's information: height (in inches)? 62.5 weight (in pounds)? Person 1 BMI = overweight Person 2 BMI = normal Difference = BMIWeight class below 18.5underweight normal overweight 30.0 and upobese

3 Nested if/else answer // This program computes two people's body mass index (BMI) and // compares them. The code uses Scanner for input, and parameters/returns. import java.util.*; // so that I can use Scanner public class BMI { public static void main(String[] args) { printIntro(); Scanner console = new Scanner(System.in); double bmi1 = readInfoAndComputeBmi(console); double bmi2 = readInfoAndComputeBmi(console); // report overall results reportResult(1, bmi1); reportResult(2, bmi2); System.out.println("Difference = " + Math.abs(bmi1 - bmi2)); } // prints a welcome message explaining the program public static void printIntro() { System.out.println("This program reads data for two people and"); System.out.println("computes their body mass index (BMI)."); System.out.println(); }...

4 Nested if/else, cont'd. // reads information for one person, computes their BMI, and returns it public static double readInfoAndComputeBmi(Scanner console) { System.out.println("Enter next person's information:"); System.out.print("height (in inches)? "); double height = console.nextDouble(); System.out.print("weight (in pounds)? "); double weight = console.nextDouble(); System.out.println(); double bodyMass = computeBmi(height, weight); return bodyMass; } // Computes/returns a person's BMI based on their height and weight. public static double computeBmi(double height, double weight) { return (weight * 703 / (height * height)); } // Outputs information about a person's BMI and weight status. public static void reportResult(int number, double bmi) { System.out.println("Person " + number + " BMI = " + bmi); // Complete... }

5 Scanners as parameters If many methods need to read input, declare a Scanner in main and pass it to the other methods as a parameter. public static void main(String[] args) { Scanner console = new Scanner(System.in); int sum = readSum3(console); System.out.println("The sum is " + sum); } // Prompts for 3 numbers and returns their sum. public static int readSum3(Scanner console) { System.out.print("Type 3 numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); return num1 + num2 + num3; }

6 Logical operators Tests can be combined using logical operators: "Truth tables" for each, used with logical values p and q: OperatorDescriptionExampleResult && and (2 == 3) && (-1 < 5)false || or (2 == 3) || (-1 < 5)true ! not !(2 == 3)true pqp && qp || q true false true falsetruefalsetrue false p !p!p truefalse true

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

8 Logical questions 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 && x < z) –(x + y) % 2 == 0 || !((z - y) % 2 == 0) Exercise: Write a program that prompts for information about a person and uses it to decide whether to date them.

9 Factoring if/else code factoring: Extracting 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;

10 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.

11 All paths must return public static int max(int a, int b) { if (a > b) { return a; } // Error: why? } The following also does not compile: why? public static int max(int a, int b) { if (a > b) { return a; } else if (b >= a) { return b; } }

12 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 1 quadrant 2 quadrant 3 quadrant 4

13 if/else, return answer public static int quadrant(double x, double y) { // Determine quadrant! }