1. Week 4 - Monday
CS 121

2. Last time
What did we talk about last time?
if statements
Lab 3

3. Questions?

4. Project 1

5. Conditional Execution Review

6. Conditional execution
So far we have only considered Java programs that do one thing after another, in sequence
Our programs have not had the ability to choose between different possibilities
Now, they will!

7. Behold! The if-statement:
x is small will only print out if x is less than 5
In this case, we know that it is, but x could come from user input or a file or elsewhere
int x = 4;
if( x < 5 )
System.out.println("x is small!");

8. if( condition ) statement; Anatomy of an if Any boolean expression
The if part
if( condition )
statement;
Any single executable statement

9. Conditions

10. Conditions in the if
Any statement that evaluates to a boolean is legal
Examples:
x == y
true
Character.isDigit('r')
s.equals("Help me!") && (z < 4)

11. Comparison
The most common condition you will find is a comparison between two things
In Java, that comparison can be:
== equals
!= does not equal
< less than
<= less than or equal to
> greater than
>= greater than or equal to
These are called relational operators

12. Equals
You can use the == operator to compare any two things of the same type
Different numerical types can be compared as well (3 == 3.0)
Be careful with double types, is not equal to
int x = 3;
if( x == 4 )
System.out.println("This doesn't print");

13. Not Equals
Any place you could have used the == operator, you can use the != operator
If == gives true, the != operator will always give false, and vice versa
If you want to negate a condition, you can always use the ! as a not
is the same as
if( x != 4 )
if( !(x == 4) )

14. = != ==
Remember, a single equal sign (=) is the assignment operator (think of a left-pointing arrow)
A double equals (==) is a comparison operator
int y = 10;
if( y = 6 ) //compiler error!
boolean b = false;
if( b = false ) //no error but wrong

15. Less Than (or Equal To) Inequality is very important in programming
You may want to take an action as long as a value is below a certain threshold
For example, you might want to keep bidding at an auction until the price is greater than what you can afford
Watch for strict inequality (<) vs. non-strict inequality (<=)
if( x <= 4 )
System.out.println("x is less than 5");

16. Greater Than (or Equal To)
Just like less than or equal to, except the opposite
Note that (because of the All-Powerful Math Gods) the opposite of <= is > and the opposite of >= is <
Thus,
!( x <= y ) is equivalent to ( x > y )
!( x >= y ) is equivalent to ( x < y )

17. Else

18. Either/Or Sometimes you have to make a decision
If a condition is true, you go one way, if not, you go the other
For example:
If I pass CS121,
Then I throw a kegger to celebrate
Otherwise,
I punch Dr. Wittman in the face

19. Exclusivity Notice the nature of this kind of condition
Both outcomes cannot happen
Either a kegger gets thrown or Dr. Wittman gets punched in the face
For these situations, we use the else construct

20. Two different outcomes
Anatomy of an if-else
if( condition )
statement1;
else
statement2;
Two different outcomes

21. else example Scanner in = new Scanner(System.in);
int balance = in.nextInt();
if( balance < 0 )
System.out.println("You are in debt!");
else
System.out.println("You have $" + balance);

22. Multiple Statements and Nesting

23. What if you need to do several things conditionally?
No problem
Use braces to treat a group of statements like a single statement
if( x == 4 ) {
System.out.println("I hate 4");
System.out.println("Let's change x.");
x = 10; }

24. An if with multiple statements
if( condition ) {
statement1;
statement2; …
statementn; }
A whole bunch of statements

25. Nesting
Sometimes you want to make one set of decisions based on another set of decisions
if-statements can be nested inside the bodies of other if-statements
You can put if-statements inside of if-statements inside of if-statements… going arbitrarily deep

26. Nested ifs if( condition1 ) { statement1; if( condition2 )… }

27. An example using quadrants
For the next example, recall the 4 quadrants of the Cartesian coordinate system y 2 1
(0,0) -x x 3 4 -y

28. Nesting example Find which quadrant the point (x,y) is in
if( x >= 0.0 ) {
if( y >= 0.0 )
System.out.println("Quadrant 1");
else
System.out.println("Quadrant 4"); }
System.out.println("Quadrant 2");
System.out.println("Quadrant 3");

29. if and else if
You can list a sequence of exclusive possibilities using nesting:
if( index == 1 )
System.out.println("First");
else if( index == 2 )
System.out.println("Second");
else if( index == 3 )
System.out.println("Third");
else
System.out.println(index + "th");

30. else-if doesn’t actually exist
A block of code is treated just like one statement
A whole if-else is treated the same
if( … )
statement1;
else if( … )
statement2;
else
statement3;
if( … ) {
statement1; }
else
statement2;
statement3; =

31. Pitfalls

32. Watch out!
Now you are controlling the flow of execution in your program
There is a wider range of mistakes you can make when giving instructions
Huge chunks of code can be executed or skipped by mistake
Here are a few things to watch out for

33. Empty statements
Remember that an if-statement is not an executable statement
It does not end with a semicolon
if( balance < 0 ); // empty statement
{ // this block always runs
System.out.println("You owe a fee!");
balance -= 15; }

34. Confusing indentation
In some languages, indentation actually matters
Java ignores whitespace
"Fight!" prints no matter what
if( enemies > 2 ) System.out.println("Run away!");
else
defense = true;
System.out.println("Fight!");

35. Imprecise conditions
It’s easy to make logical errors when writing conditions
If an airline allows two or fewer bags on the plane, someone might code that as:
But this is too restrictive. It should be:
if( bags < 2 ) // only allows 1 or 0
boarding = true;
if( bags <= 2 )
boarding = true;

36. Reversed conditions Sometimes it’s easy to get a condition backwards
Try not to assume you wrote the condition correctly
Always double check
if( number % 3 == 0 ) System.out.println("Not divisible by 3!");
else
System.out.println("Divisible by 3!");

37. if Examples

38. Speed limit Sometimes you probably break the speed limit
But, there's one speed limit you can never break
The speed of light c is about 3 x 108 m/s
Given a variable named speed of type double, what's an if-statement that will print an error message if speed is larger than c?

39. Upcoming

40. Next time…
Examples
switch statements

41. Reminders Keep reading Chapter 4 of the textbook
Keep working on Project 1 (due this Friday)
Exam 1 is next Monday