Week 3 - Friday What did we talk about last time? Wrapper classes - - PowerPoint PPT Presentation

Week 3 - Friday

 What did we talk about last time?  Wrapper classes  Examples  if statements

 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!

 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!"); }

The if part Any boolean expression Executable statements

if( condition ){ statements; }

 A very natural if-then sort of relationship  If the condition is true, then do something  For example:

• If I win a million dollars,
• Then I’ll yodel like an insane Swiss monkey

 Write a program that prompts the user for the secret

password

 If they enter the word "eggplant", congratulate them for

knowing the password

 Any statement that evaluates to a boolean is legal  Examples:

• x == y
• true
• Character.isDigit('r')
• s.equals("Help me!") && (z < 4)

 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

 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, 0.33333333 is not equal to

0.33333332

int x = 3; if( x == 4 ) { System.out.println("This doesn't print"); }

 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) )

 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

 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"); }

 Just like less than or equal to, except the opposite  Note that (because of the All-Powerful Math Gods) the

• pposite of <= is > and the opposite of >= is <

 Thus,

• !( x <= y ) is equivalent to ( x > y )
• !( x >= y ) is equivalent to ( x < y )

 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 COMP 1600,

▪ Then I throw a kegger to celebrate

• Otherwise,

▪ I punch Dr. Wittman in the face

 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

Two different

• utcomes

if( condition ) { statements1; } else { statements2; }

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); }

 You can have one or many statements inside the braces of an if or an else  However, you can optionally leave off the braces if there's only a single

statement

 It's probably a good idea to use the braces until you really know what you're

doing

 Some programmers always use them

if( x == 4 ) { System.out.println("I hate 4"); } if( x == 4 ) System.out.println("I hate 4"); // braces not needed

 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

if( condition1 ){ statement1;

if( condition2 ) {

if( condition3 ) statement2; …

}

}

 For the next example, recall the 4 quadrants of the Cartesian

coordinate system

x

• x

y

• y

(0,0) 1

2 3 4

 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"); } else { if( y >= 0.0 ) System.out.println("Quadrant 2"); else System.out.println("Quadrant 3"); }

 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");

 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 { if( … ) statement2; else statement3; }

=

 Examples  Pitfalls with if statements  switch statements

 Keep reading Chapter 4 of the textbook  Keep working on Project 1 (due tonight!)