1. The if-else statement

2. The if-else statement in Java The if-else statement is the second conditional statement in Java The if-else statement selects one of the two possible statements to be executed based on a given condition Example: if ( condition is true ) then execute this statement; otherwise execute the other statement;

3. Syntax and meaning of the if-else-statement Syntax of the if-else-statement : if ( CONDITION ) ONE-statement else ONE-statement The keyword if announces (to the Java compiler ) that we started an if- else-statement A conditional clause ( CONDITION ) follows the keyword if Following the condition clause, you can write (only) one statement Following the then-part, you must specify the keyword else followed by (only) one statement

4. Programming example: find maximum of 2 numbers (1)

5. Programming example: find maximum of 2 numbers (2) import java.util.Scanner; public class Max01 { public static void main(String[] args) { double a, b, max; Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b if ( a >= b ) max = a; else max = b; System.out.println( "max value = " + max ); }

6. Program example: find maximum of 3 numbers (1)

7. Program example: find maximum of 3 numbers (2) import java.util.Scanner; public class Max01 { public static void main(String[] args) { double a, b, max; Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c if ( a >= b ) // Find max(a,b) max = a; else max = b; if ( c > max ) // Check c > max ? max = c; System.out.println( "max value = " + max ); }

8. Programming example: leap year (1) In the Gregorian calendar, the current standard calendar in most of the world, most years that are evenly divisible by 4 are leap years. Years that are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years Year Leap year ? Reason ---------- --------------- ================= 1904 Yes Divisible by 4 1900 No Divisible by 100 2000 Yes Divisible by 400

9. Programming example: leap year (2)

10. Programming example: leap year (3) import java.util.Scanner; public class LeapYear01 { public static void main(String[] args) { int year; boolean leap; Scanner in = new Scanner(System.in); // Construct Scanner object year = in.nextInt(); // Read in year if ( year % 4 == 0 ) leap = true; else leap = false; if ( year % 100 == 0 ) leap = false; if ( year % 400 == 0 ) leap = true; System.out.println("Year is leap year ? " + leap); }

11. Programming example: leap year (4) import java.util.Scanner; public class LeapYear02 { public static void main(String[] args) { int year; boolean leap; Scanner in = new Scanner(System.in); // Construct Scanner object System.out.print("Enter year:"); year = in.nextInt(); // Read in year if ( (year % 4 == 0) && !(year % 100 == 0) || (year % 400 == 0) ) { System.out.println("It is a leap year"); } else { System.out.println("It is NOT a leap year"); }

12. Comparing floating point values on equality (and inequality) (1) When are 2 values equal to each other: Two values are equal if they are equal in all digits Consequently: 4.00000000000001 != 3.9999999999999

13. Comparing floating point values on equality (and inequality) (2) public class FloatEq1 { public static void main(String[] args) { double a, b; int i; a = 4.0; b = 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0; System.out.println("a = " + a); System.out.println("b = " + b); if ( a == b ) { System.out.println("a is equal to b"); } else { System.out.println("a is NOT equal to b"); }

14. Testing equality within a given tolerance (1) When we want to test if 2 values a and b are approximately equal to each other, we use this test: if ( absoluteValue( b − a ) < some-very-small-value ) { a and b are equal } else { a and b are not equal (too far apart) }

15. Testing equality within a given tolerance (2) public class FloatEq2 { public static void main(String[] args) { double a, b; int i; a = 4.0; b = 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0 + 4.0/7.0; System.out.println("a = " + a); System.out.println("b = " + b); if ( Math.abs( b - a ) < 0.000000001 ) { System.out.println("a is (approximately) equal to b"); } else { System.out.println("a is NOT (approximately) equal to b"); }

16. Nested conditional statements

17. A conditional statement (i.e., an if-statement or an if-else- statement) is also a statement We can use an if-statement or an if-else-statement in the then- part (and in the else-part) of a conditional statement !!! Nested conditional statement = a conditional statement where the then-part and/or the else-part contains another conditional statement

18. Programming example: determine the price for a hair cut (1)

19. Programming example: determine the price for a hair cut (2)

20. Programming example: determine the price for a hair cut (3)