Understanding Conditional Statements in Java Programming

1. Decisions, Decisions, Decisions Conditional Statements In Java

2. Conditional Statements in Java • How would you explain to the Java compiler how to do the absolute value function? • Seems simple enough in math • All programming languages have some form of decision structure based on logic. • The absolute value logic is: • If the value is negative, multiply it by -1 • In Java it would be written as: if( value < 0 ) { value = -1 * value; }

3. The if statement • The syntax for an if statement in Java is: if( condition ) { programming statements… } • If the condition is true, java begins executing the code in the {}. Otherwise, the code in the {} is skipped • Note the style!!! Indent Braces line up

4. Conditional Operators Common mistake is to use = (assignment) when you meant == (test if equal)

5. What will the code print? int testValue = 5; if( testValue > 1 ) { System.out.println( “The testValue is > 1” ); } System.out.println( “End of Program” ); • The condition, testValue > 1 is true • Code in the {} will be executed • This code prints: The testValue is >1 End of Program

6. What will the code print? int testValue = 5; if( testValue < 5 ) { System.out.println( “The testValue is < 5” ); } System.out.println( “End of Program” ); • The condition, testValue > 1 is false • Code in the {} will be skipped • This code prints: End of Program

7. What is the output?

8. Exercises with if • Copy and paste the code into a new project called IfTest1: public class IfTest1 { public static void main( String[] args ) { System.out.println( “Enter a your age” ); Scanner input = new Scanner( System.in ); int age = input.nextInt(); final int LISCENCE_AGE = 16; if( age >= LISCENCE_AGE ) { System.out.println( “You are old enough to get your driver’s license” ); } System.out.println( “Thank you” ); } } • Run the program and enter some test values. • Alter the code to print “You are old enough to vote” if the age is greater than or equal to 18 • Alter the code to print “You may pay the child rate” if the age is less than 13

9. if … else statements • Another programming feature with decisions is the if… else statement • Suppose you’re charging admission for a theatre and the ticket prices are for children or adults. • Adult: age >= 18 • Child: not an adult • We can write this as the following sentence: • If age is 18 or higher, charge adult price, otherwise charge child price • The new keyword in our problem is otherwise which translates in Java to else

10. if … else statements • In java the code for the movie theatre is: final double ADULT_PRICE = 10.75; final double CHILD_PRICE = 8.25; int ticketPrice; if( age >= 18 )//this person is an adult { ticketPrice = ADULT_PRICE; } else//this person is not an adult, therefore a child { ticketPrice = CHILD_PRICE; } System.out.println( “Your ticket price is: “ + ticketPrice );

11. if … else • The syntax for if … else in java is: if( condition ) { programming statements if true } else { programming statements if false } • If the original condition is true, then the code in the {} associated with the if statement is executed • Otherwise, the code is ignored and the code in the {} associated with the else statement is executed

12. What Will the Code Print? public class IfElseTest { public static void main( String[] args ) { int hoursPastMidNight = 11; if( hoursPastMidNight <= 12 )//entered a negative! { System.out.println( “It is ” + hoursPastMidNight + “am” ); } else { hoursPastMidNight -= 12;//convert to 12 hour time System.out.println( “It is ” + hoursPastMidNight + “pm” ); } System.out.println( “End of program” ); } } • The first statement is true • The code in the {} attached to the if is executed • The code in the {} attached to the else is ignored • The result is: It is 11am End of Program

13. What Will the Code Print? public class IfElseTest { public static void main( String[] args ) { int hoursPastMidNight = 21; if( hoursPastMidNight <= 12 )//entered a negative! { System.out.println( “It is ” + hoursPastMidNight + “am” ); } else { hoursPastMidNight -= 12;//convert to 12 hour time System.out.println( “It is ” + hoursPastMidNight + “pm” ); } System.out.println( “End of program” ); } } • The first statement is false • The code in the {} attached to the if is ignored • The code in the {} attached to the else is executed • The result is: It is 9pm End of Program

14. What is the output?

15. Example • Copy and paste the program into a new project public class IfElseTest { public static void main( String[] args ) { System.out.print( “Please enter a positive integer: “ ); Scanner input = new Scanner( System.in ); int userInput = input.nextInt(); if( userInput < 0 )//entered a negative! { System.out.println( “I said a POSITIVE!” ); System.out.println( “What’s the matter with you!” ); } else { System.out.println( “Thank you for following the instructions” ); } System.out.println( “End of program” ); } } • Compile and run the program • Alter the code to see if a person is old enough to drive, if not, calculate how many years they have to wait and tell them to come back in that many years • Alter the code to check how many times a person has been late to class this year. Give them a mean message if it’s more than 5 times!

16. Block Statements • In our class I will always use block statements for conditions and loops. • A condition or loop with only one statement in the body does not require a block statement: • Ex. if( age > 16 ) { System.out.println( “You may get your liscence” ); } • Ex. if( age > 16 ) System.out.println( “You may get your liscence” ); • Can you spot the error? • You can omit the block statements if you wish, but it is likely to result in more errors

17. Block Statements • Another advantage of block statements is clarity. • Which if does the else belong to? • If we add braces it is clearer which it belongs to

18. Fixing Errors

19. Fixing Errors

20. Tracing by hand

21. Tracing by hand

22. And finally, if…else if…else • What about when there are several conditions that are related? • In order to ensure that only one statement is executed, we need to chain the if’s together. • The next two slides show an example where the limitations of only having if are illustrated

23. A Whole Bunch of if’s False if( richterScale < 1 ) { System.out.println( “Didn’t even feel it” ); } if( richterScale < 4 ) { System.out.println( “All my dishes broke :(” ); } if( richterScale < 7 ) { System.out.println( “Whole lotta shakin going on!” ); } if( richterScale < 10 ) { System.out.println( “Run for the hills!” ); } • What does the code print if richterScale = 8? Run for the hills! • What does the code print if richterScale = 5? Whole lotta shakin going on! Run for the hills! False True False True

24. Fixing the previous example False if( richterScale < 1 ) { System.out.println( “Didn’t even feel it” ); } else if( richterScale < 4 ) { System.out.println( “All my dishes broke :(” ); } else if( richterScale < 7 ) { System.out.println( “Whole lotta shakin going on!” ); } else { System.out.println( “Run for the hills!” ); } • What does the code print if richterScale = 8? Run for the hills! • What does the code print if richterScale = 5? Whole lotta shakin going on! False True, ignore anything else in the group False No if’s were true, so automatically go here

25. if...else if…else • Java evaluates EVERY if statement. • The only exception is when they are joined by the reserved word else • Java evaluates a sequence of if/else if/else statements until the FIRST one is true. • If none are true then the else is evaluated • Only one set of statements in the group will be evaluated

26. if...else if…else syntax if( condition ) { statements if true } else if( condition ) { statements if true } … … else if( condition ) { statements if true } else { statements if all others in the group were false }

27. What is Printed? int age = 21; if( age <= 12 )//12 or younger { System.out.println( "Your cost is $5.50" ); } else if( age <=19 ) //older than 12 but 19 or younger { System.out.println( "Your cost is $8.25" ); } else if( age <= 64 ) //older than 19 but 64 or younger { System.out.println( "Your cost is $13.75" ); } else //older than 64 { System.out.println( "Your cost is $5.50" ); } • Your cost is $13.75 False False True, everything else below ignored