1. Announcements Program 1 due Wed @ noon Lab 1 due Wed @ noon
Michele Weigle - COMP 14 - Spr 04
Announcements
Program 1 due noon
Lab 1 due noon 1

2. Questions?
Last lecture covered conditionals

3. Today in COMP 110
The type boolean
switch statements
Enumerations

4. Review If-Statements Syntax
Used to make decisions or check conditions in a program
E.g. check the value of a variable
Syntax
if(Boolean_Expression)
Statement_1
else
Statement_2
If Boolean_Expression is true, Statement_1 is executed; otherwise Statement_2 is executed

5. If Statement Exercise What is the output? x Output 4 C 5 6 AB 9 10 A
if(x > 5) {
System.out.println(‘A’);
if(x < 10)
System.out.println(‘B’); }
else
System.out.println(‘C’); x
Output 4 C 5 6 AB 9 10 A 11

6. Boolean Expressions
An expression that evaluates to either true or false
Consider ((x > 10) || (x < 100))
Why is this probably not what the programmer intended?
It’s true for any x
Consider ((2 < 5) && (x < 100))
It’s the same as (x < 100)

7. The Type boolean boolean is a primitive type in Java
We can declare variables of type boolean just like we declare an int, double, etc.
boolean ready;
boolean error;

8. Booleans Using booleans can make your programs easier to understand
//a bit difficult to read
if(temp <= 100 && thrust >= && cabinPressure > 30)
System.out.println(“Launch”);
else
System.out.println(“Abort”);

9. Booleans Booleans can be used inside boolean expressions
boolean tempSafe = temp <= 100;
boolean thrustSafe = thrust >= 12000;
boolean pressureSafe = cabinPressure > 30;
boolean systemsGo = tempSafe && thrustSafe && pressureSafe;
//much easier to read
if(systemsGo)
System.out.println(“Launch”);
else
System.out.println(“Abort”);

10. True and False
The words true and false are also reserved words in java
We can use them to initialize boolean variables
boolean ready = false;
boolean initialized = true;

11. Booleans There’s no need to write
if(systemsGo == true)
System.out.println(“Launch”);
The more concise and equivalent way is
if(systemsGo)

12. Naming Booleans
Choose names for boolean variables that sound true when the value of the variable is true
boolean ready; //are we ready?
boolean readingInput; //are we reading input?
boolean errorEncountered; //have we encountered //an error?

13. Precedence
Java uses precedence rules when evaluating boolean expressions
Example
score >= 80 && score < 90
The expressions to the left and right of && are evaluated first

14. Operator Precedence Highest Precedence Lowest Precedence
First: the unary operators +, -, ++, --, !
Second: the binary operators *, /, %
Third: the binary operators +, -
Fourth: the boolean operators <, >, >=, <=
Fifth: the boolean operators ==, !=
Sixth: the boolean operator &&
Seventh: the boolean operator ||
Lowest Precedence

15. Boolean Precedence Example
4 < 3 / 2 – 10 || 4 * -2 > 9
4 < 1 – 10 || 4 * -2 > 9
4 < 1 – 10 || -8 > 9
4 < -9 || -8 > 9
false || -8 > 9
false || false
false

16. Style
It’s usually best to indicate precedence in boolean expressions explicitly with parentheses
(score < ((min / 2) – 10)) || (score > 90)

17. Short-Circuit Evaluation
In some cases, the result of a boolean expression can be determined before all subparts of the expression are evaluated
Example
true || (x >= 60)
This expression is true regardless of the value of x

18. Short-Circuit Evaluation
Java uses what’s called short-circuit evaluation when evaluating boolean expression
If at any point in the evaluation of a boolean expression the outcome is determined, any remaining subparts are not evaluated

19. Short-Circuit Evaluation
Example
We’re computing an average homework score
Print good work if the average is above 60
if(numAssignments > 0 && ((total / numAssignments) > 60)
System.out.println(“Good work!”);
Without short-circuit evaluation, we would divide by zero if numAssignments == 0

20. Reading in Booleans
We can read in booleans from the keyboard just like any other variable
boolean bVar;
Scanner keyboard = new Scanner(System.in);
bVar = keyboard.nextBoolean();

21. Switch Statements
If-statements with many branches can be difficult to read
The switch statement can be used as an alternative to a multi-branch if-statement in certain cases

22. Switch Statement A switch statement begins like this
switch(Controlling_Expression) { … }
Controlling_Expression must have type byte, short, int or char

23. Switch Statements
Inside the body of a switch statement, a number of case labels will appear
The different cases are separated with a break statement
case Case_Label_1:
Statements_1
break; …
case Case_Label_n:
Statements_n

24. Switch Statement Example
int year; …
switch(year) {
case 1:
System.out.println(“You are a freshman”);
break;
case 2:
System.out.println(“You are a sophomore”);
case 3:
System.out.println(“You are a junior”);
case 4:
System.out.println(“You are a senior”);
default:
System.out.println(“This is a default case”); }

25. Switch Statement Syntax
switch(Controlling_Expression) {
case Case_Label:
Statements
break;
case Case_Label : …
default: }

26. Switch Statements
If no break statement is specified, for a case, execution will continue down to the next case
int n; …
switch(n) {
case 1:
System.out.println(“one”);
case 2:
System.out.println(“two”);
break; }
If n == 1, this code prints both one and two

27. Switch Statement Example
char input; …
switch(input) {
case 'y':
case 'Y':
System.out.println(“You entered yes”);
break;
case 'n':
case 'N':
System.out.println(“You entered no”);
default:
System.out.println(“Invalid input”); }

28. Default Case
When using a switch statement you should always provide a default case
This catches any conditions you may not have checked for, such as errors
default:
System.out.println(“This is a default case”);
break;

29. Conversion to If
The previous example can also be written equivalently with an if-statement
if(input == 'y' || input == 'Y')
System.out.println(“You entered yes”);
else if(input == 'n' || input == 'N')
System.out.println(“You entered no”);
else
System.out.println(“Invalid input”);

30. If/Switch How to know whether to use a switch or an if-statement?
Switch statement
Can only be used with type char or byte, short, int
When you want to choose between many, specific values such as 5,6, 'y' etc.
If statement
Can only be used with boolean expressions
When the number of choices is relatively small
When you want to check a range of possibilities, e.g. x > 5, y <= 1000

31. Enumerations
Suppose you wanted to write a computer program that stores different flavors of ice cream
Vanilla, Chocolate, Strawberry, etc.
How would we store them in a computer program?

32. Enumerations
We can give the flavors an underlying numeric representation
Vanilla = 0, Chocolate = 1, Strawberry = 2
We could declare a variable integer to store our flavor, but this is error-prone
int flavor = 0; //vanilla
flavor = 1; //chocolate

33. Enumerations
An enumeration allows us to give unique numeric values to a list of items
enum Flavor {Vanilla, Chocolate, Strawberry}
This statement assigns a unique numeric value to each of {Vanilla, Chocolate, Strawberry}

34. Switch/Enum Example enum Flavor {Vanilla, Chocolate, Strawberry}
Flavor flavor; //declare a variable of type Flavor …
switch(flavor) {
case Vanilla:
System.out.println(“That’s Vanilla!”);
case Chocolate:
System.out.println(“That’s Chocolate!”);
break;
case Strawberry:
System.out.println(“That’s Strawberry!”);
default:
System.out.println(“I don’t recognize that flavor”); }

35. Programming Demo
Write a program that takes two numbers as input from the user
The user should then be able to choose from among the following options by entering the appropriate symbol
Add the two numbers
Subtract the two numbers
Multiply the two numbers
Divide the two numbers

36. Programming Demo
Pseudocode

37. Programming Demo
Programming

38. Wednesday
Loops

39. Programming Demo Sample Pseudocode Ask user to input two numbers
Read in two numbers
Provide the user with a list of arithmetic options
Read in their choice
Perform the operation selected by the user
Output the result