1. CS2011 Introduction to Programming I Selections (II)
Chengyu Sun
California State University, Los Angeles

2. Overview Common problems with if/else Logical operators
Conditional operator
Switch statement
Putting it all together
Debugging with Eclipse

3. Common Error #1 if( x > 1 ); System.out.println( "Big X" );
else
System.out.println( "Small X" );

4. Common Error #2 if( radius >= 0 ) area = radius * radius * PI;
System.out.println( area );

5. Common Error #3 if( x > y ) if( y > z ) System.out.println("x");
else
System.out.println("y");

6. Common Error #4 double x = 1.0-0.1-0.1-0.1-0.1-0.1;
System.out.println(x == 0.5);

7. Bad Style #1 boolean even; if( x%2 == 0 ) even = true; else
even = false;

8. Bad Style #2
if( even == true )
System.out.println("it's even");

9. Bad Style #3 if (inState) { tuition = 5000;
System.out.println("The tuition is "
+ tuition); }
else {
tuition = 15000;

10. Bad Style #4 if (x > 10) System.out.println( "Big X" );
System.out.println( "small x" );

11. The Need for More Complex Conditions
A year is a leap year if it is
divisible by 4
but not by 100,
or if it is divisible by 400.
condition 1
condition 2
(with a negation)
condition 3

12. Logical Operators
Operates on boolean values and produces boolean results
Operator
Name !
Negation
&&
And || Or ^
Exclusive or

13. Negation Operator Turns true to false and false to true ! true ??

14. And Operator
The result is true only when both conditions (i.e. operands) are true
true && true ??
true && false
false && true
false && false

15. Or Operator The result is true when either operand is true
true || true ??
true || false
false || true
false || false

16. Exclusive Or Operator
The result is true when the two operands are different
true ^ true ??
true ^ false
false ^ true
false ^ false

17. Logical Operator Examples
Check if a month is in winter (i.e. December, January, or February)
Check if a year is a leap year

18. The Conditional Operator
A shorthand form of an if-else statement
if ( x > 0 )
y = 1;
else
y = -1;
y = x > 0 ? 1 : -1;
boolean
expression
result if expression is true
result if expression is false

19. Conditional Operator Examples
System.out.println((x < y && y < z) ?
"sorted" : "not sorted" );
tax = (income > 10000) ?
income * 0.2 :
income * ;
Check out the Operator Precedence Table

20. Switch Statement Branching based on a number of discreet values Winter1 2
Winter
Example:
month 3
Spring 4
Spring 12
default
Winter
Invalid

21. Switch Statement Syntax
switch ( expression ) {
case value1:
statement(s);
break;
case value2:
...
default: }

22. About Switch Statement Syntax
No need of {} for the statements for a case
break is necessary to separate the cases – sometimes you may intentionally leave out break if multiple cases share the same statement(s)
default is optional
No break is necessary in default (because it's already at the end)

23. Put Everything Together – Lottery Example
Randomly generate a 2-digit number
Prompt the user to enter a 2-digit number
If the user input matches the lottery number in the exact order, the award is $10,000; otherwise
If all digits in the user input match all digits in the lottery number, the award is $3,000; otherwise
If one digit in the user input matches a digit in the lottery number, the award is $1,000.

24. Debug with Eclipse Set break points Debug As … Debug Perspective
Variables view
Switch between perspectives
Common debug commands
Step Over and Step Into
Terminate and Resume

25. Readings
Chapter 3 of the textbook (there will be a quiz next week)