1. Conditional If Week 3

2. Lecture outcomes Boolean operators – == (equal ) – OR (||) – AND (&&) If statements User input vs command line arguments

3. Reading Subject guide: – Introduction to Java and Object Oriented Programming. Volume 1 – Chapters 6 and 7 – Redo all the examples in my web sites. Book – Introduction to Programming in Java – Section 1.2 (Built-in Types of Data) pages (14-46)

4. Basic boolean Operators OperatorMeaningExample == equal(4-2)==(8-6) is true !=Not equal3 !=2 is true but 4!=(6-2) is false >Greater than (3>2) is true >=Greater or equal(3>=2) is true <Less than(3<2) is false <=Less or equal(3<=4) is true

5. Compound Boolean expressions OperatorMeaningExample && AndTrue && true is true (3>2) && (1<=10) is true True && False is False (3>2) && (1>10) is False False && False is flase (3 10) is False ||ORTrue || true is true (3>2) || (1<=10) is true True|| False is True (3>2) || (1>10) is True False || False is false (3 10) is False

6. Compound Boolean Expressions (Cont) True && True = True False && False = False (True && False) = (False && True) = False True || True = True False || False = False (True || False) = (False || True) = True && ||

7. If – The Conditional Statement The if statement evaluates an expression and if that evaluation is true then the specified action is taken if ( x < 10 ) x = 10; If the value of x is less than 10, make x equal to 10 It could have been written: if ( x < 10 ) x = 10; Or, alternatively: if ( x < 10 ) { x = 10; }

8. The if statement Executes a block of statements only if a test is true if ( test ) { statement ;... statement ; } Example: int x = 2; if (x >= 0) { System.out.println(“ x is positive"); }

9. The if/else statement Executes one block if a test is true, another if false if ( test ) { statement(s) ; } else { statement(s) ; } Example: int x = 2; if (x >= 0) { System.out.println(“ x is positive"); } else { System.out.println(“x is negative."); }

10. Example1 What does this program do? // public class IsEven { public static void main(String[] arguments) { int x = Integer.parseInt(args[0]) if (x % 2 == 0) { System.out.println(“YES”); } else { System.out.println(“No”); } //end of program

11. Example2 What does this program do? // public class IsOdd { public static void main(String[] arguments) { int x = Integer.parseInt(args[0]); if (x % 2 == 1) { System.out.println(“YES”); } else { System.out.println(“No”); } //end of program

12. Example3 What does this program do? // public class FirstIsMultipleOfSecond { public static void main(String[] arguments) { int x = Integer.parseInt(args[0]); int y = Integer.parseInt(args[1]); if (x % y == 0) { System.out.println(“YES”); } else { System.out.println(“No”); } //end of program

13. Example4 What does this program do? // public class OneIsMultipleOfOther { public static void main(String[] arguments) { int x = Integer.parseInt(args[0]); int y = Integer.parseInt(args[1]); if ((x % y ==0 ) || (y%x ==0 )) { System.out.println(“YES”); } else { System.out.println(“No”); } //end of program

14. Example5 Nested Ifs // program that takes the exam final mark prints out the corresponding grade. public class Examgrade { public static void main(String[] arguments) { double finalMark = double.parseDouble(args[0]); char grade; if (testscore >= 90) { grade = 'A'; } else if (testscore >= 80) { grade = 'B'; } else if (testscore >= 70) { grade = 'C'; } else if (testscore >= 60) { grade = 'D'; } else { grade = 'F'; } System.out.println("Grade = " + grade); } //end of program

15. Exercise 1 Write a program, DayofTheWeek.java, that takes one command line integer n = 1, 2,...,7. The program prints the following n = 1 it prints Monday n = 2 it prints Tuesday n = 3 it prints Wednesday n = 4 it prints Thursday n = 5 it prints Friday n = 6 it prints Saturday n = 7 it prints Sunday Hints: you can either use Nested ifs or a switch statement.

16. Solution1 ( nested if else) DayofTheWeek.java public class DayofTheWeek { public static void main(String[] args) { int day = Integer.parseInt(args[0]); String today; if(day ==1) { today= "Monday";} else if(day==2) {today= "Tuesda";} else if(day == 3) {today= "Wendesday";} else if(day==4) {today= "Thursday";} else if(day==5) {today= "Friday";} else if(day==6) {today= "Saturday";} else if(day==7) {today= "Sunday";} else {today= "Wrong number Entered";} System.out.println(" Today is " + today); } //end of program

17. The switch Statement switch ( n ) { case 1: // execute code block #1 break; case 2: // execute code block #2 break; default: // if all previous tests fail then //execute code block #4 break; }

18. Solution1 (switch) DayofTheWeek.java public class DayofTheWeek { public static void main(String[] args) { int day = Integer.parseInt(args[0]); String today; switch(day) { case 1: today= "Monday"; break; case 2: today= "Tuesday"; break; case 3: today= "Wednesday"; break; case 4: today= "Thursday"; break; case 5: today= "Friday"; break; case 6: today= "Saturday"; break; case 7: today= "Sunday"; break; default: today = "Wrong number entered"; break; } System.out.println("Today is " + today); }

19. Exercise2 Write a program, MonthofTheyear.java. that takes one command line integer n = 1, 2,...,12. The program prints the following n = 1 it prints January n = 2 it prints February... n = 12 it prints December Hints: you can either use Nested ifs or a switch statement.

20. Lecture3.ppt Console I/O Scanner – A simple text scanner which can parse primitive types and strings – A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace – The resulting tokens may then be converted into values of different types using the various next methods – Resides in the java.util package to read console input (keyboard) by Scanner input = new Scanner(System.in); A Scanner can also be wrapped around a File object to read from a text file… More on this may be later.

21. Lecture3.ppt Keyboard input with Scanner Instantiate a Scanner Scanner myScanner = new Scanner(System.in ); Read an entire line of text String input = myScanner.nextLine(); Read an individual token, i.e., int int i = scanner.nextInt();

22. Lecture3.ppt Scanner Example BothNames.java import java.util.Scanner; Public class BothNames { public static void main(String[] args) { Scanner in =new Scanner(System.in); System.out.println("please input your first name, ending with 'enter'"); String s =in.nextLine(); System.out.println("please input your second name ending with 'enter'"); String t =in.nextLine(); System.out.println("Your name is "+ s +" "+ t ); }

23. Lecture3.ppt Scanner Example BothNamesRev.java import java.util.Scanner; Public class BothNames { public static void main(String[] args) { Scanner in =new Scanner(System.in); System.out.println("please input your first name, ending with 'enter'"); String s =in.nextLine(); System.out.println("please input your second name ending with 'enter'"); String t =in.nextLine(); System.out.println("Your name is "+ t +" "+ S ); }

24. Lecture3.ppt Scanner Example Scanner1.java import java.util.*; public class Scanner1 { public static void main(String[] args) { int age; String name; Scanner in = new Scanner(System.in); System.out.println("Enter first and last name: "); name = in.nextLine(); System.out.println("How old are you? "); age = in. nextInt(); System.out.println(name + '\t' + age); }

25. Lecture3.ppt Scanner Example Scanner2.java import java.util.*; public class Scanner2 { public static void main(String[] args) { int age; String first; String last; Scanner in = new Scanner(System.in); System.out.println("Enter first and last name: "); first = in.next(); last = in.next(); System.out.println("How old are you? "); age = in.nextInt(); System.out.println(first +", " + last + '\t' + age); }

26. Lecture3.ppt Input Errors What happens if the user doesn’t enter an integer when asked for the age? There are a couple of ways to handle it – Look ahead to see if the user entered an integer before we read it or – Read the input and handle the resulting error

27. Lecture3.ppt Look Ahead if (in.hasNextInt()) { age = in.nextInt(); } else { age = 30; } Scanner provide the ability to look at the next token in the input stream before we actually read it into our program –hasNextInt() –hasNextDouble() –hasNext() –etc…

28. Lecture3.ppt Input Exceptions (errors) What happens when we try to read an integer ( myScanner.nextInt() ) and the user enters something different? Java “throws” and exception, i.e., issues and error message. We can “catch” the errors and handle them, thereby preventing the program from crashing try { age = myScanner.nextInt(); } catch(InputMismatchException e) { age = 30; } The InputMismatchException is part of the java.util library so we must import java.util.InputMismatchException or java.util.* in order to catch the exception.

29. Summary Conditional if statement, if else Nested ifs Switch statment Console i/o using scanner

30. Home work Finish all the exercises given in week 2, 3 and 4 Do your assignment