COMP 14:I/O and Boolean Expressions

May 24, 2000

Nick Vallidis

Announcements

Anyone tried the assignment yet?

Review

What are the 2 parts of a program? What are the 2 types of data in Java? What is a data type? What are the 4 primitive data types we’ll

be using in this class? What is a method?

Review part 2

What is a string literal? How do we concatenate things onto a

string? How do we put newline, \, and " in a

string? What are these things called?

Today

Input/Output (I/O) Expressions Boolean Expressions

Input/Output

How do we get data into the program from the keyboard?

How do we display data to the user?

Output

You’ve already seen the basics of this one:

System.out.println("Beefcake!!!");

What about displaying data?

We talked yesterday about concatenation…

int level = 3;

System.out.println("You are on level: " + level);

double price = 2.75;

System.out.println("Please pay: $" + price);

Input

Input is a little more complicated We are not using the "Keyboard" class

described in the book. Let's take a look at the current

assignment...

Part of program P1

BufferedReader stdin = new BufferedReader(new InputStreamReader( System.in ));

int age; // Holds the age of the personString name; // Holds name.

// Ask the user's nameSystem.out.println( "\"Hello\"");System.out.print( "What is your name? -> " );name = stdin.readLine();

Part of program P1

BufferedReader stdin = new BufferedReader(new InputStreamReader( System.in ));

int age; // Holds the age of the personString name; // Holds name.

// Ask the user's nameSystem.out.println( "\"Hello\"");System.out.print( "What is your name? -> " );name = stdin.readLine();

This is some setup code. We create an object stdin. You don't have to understand the details of this.

Part of program P1

BufferedReader stdin = new BufferedReader(new InputStreamReader( System.in ));

int age; // Holds the age of the personString name; // Holds name.

// Ask the user's nameSystem.out.println( "\"Hello\"");System.out.print( "What is your name? -> " );name = stdin.readLine();

Here we actually read in what the user types.

Another piece of program P1

// Ask the user's ageSystem.out.print( "Please enter your age (doesn't

have to be true). -> ");age = Integer.parseInt(stdin.readLine( ) );

And here we read it in and then convert it to an integer

Input Summary

Put this at beginning (on one line)

To read in a String

To read in an int

To read in a double

BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));

<variable> = stdin.readLine();

<variable> = Integer.parseInt(stdin.readLine());

<variable> = (new Double(stdin.readLine())).doubleValue();

Expressions: the revenge

We talked about +, -, *, / and % Just like with arithmetic, there is an

order

* and / have higher precedence than + and -

18 / 5 + 1 is 4, not 3

Using parenthesis

There are many levels of precedence (see pg. 69 in book)

You can force whatever order you want using parenthesis

18 / 5 + 1 is 4, BUT

18 / (5 + 1) is 3

Mixing data types

Avoid this if you can Guidelines

– only mix int and double– the result should be doubledouble total, shoePrice = 54.99;

int numShoes = 4;

total = shoePrice * numShoes;

Should be a double

Methods can be in expressions

If a method returns a value, then it can be in an expression too.

This brings us to a special kind of method...

age = Integer.parseInt(stdin.readLine( )) - 21;

Class Methods

Normally, you have to instantiate an object of a class before you can use its methods

But not if the method has the word static in front of it

Where have you seen this before?

Class (or static) methods

How about main?

public class Simple

{

public static void main(String[] args)

{

System.out.println(“Hello!”);

}

}

Aside: method data types

public class Simple

{

public static void main(String[] args)

{

System.out.println(“Hello!”);

}

}

What's this?

Aside: method data types

public class Simple

{

public static void main(String[] args)

{

System.out.println(“Hello!”);

}

}

It's a data type! It tells you whether the method returns a value (if it can be used in an expression)

This can be very useful

The Math class contains many methods that are useful in expressions:

double result;

// calculate the absolute value

result = Math.abs(-17.2);

// calculate the sin of pi radians

result = Math.sin(3.1416);

Boolean expressions

These are expressions that result in a value of the data type boolean

They almost always result from the use of equality operators, relational operators or logical operators

Equality operators

These are == and !=– == means "equal to"– != means "not equal to"

They go between two expressions:2 == 2 evaluates to true

2 == 5 evaluates to false

15 != 7 evaluates to true

14 != 14 evaluates to false

Equality Operators

Of course you can also use variables and more complex expressions

int myAnswer = 5;

int trueAnswer = 7;

myAnswer == trueAnswer evaluates to false

int myRaise = 0;

int yourRaise = 100000;

myRaise != yourRaise evaluates to true

Relational Operators

These are <, <=, > and >=– < means "less than"– <= means "less than or equal to"– > means "greater than"– >= means "greater than or equal to"

They work the same way as the equality operators

Last bunch of boolean operators

Logical operators: !, && and ||– ! means "NOT"– && means "logical AND"– || means "logical OR"

Truth tablesa !a

false truetrue false

a b a&&b a||bfalse false false falsefalse true false truetrue false false truetrue true true true

Logical operators in action

Much like relational operators, but each side has to be a boolean expression

(3<5) && (6<=7) evaluates to true

!(2!=5) evaluates to false

(3!=3) || (2==2) evaluates to true

Examples

Homework

Read 3.1-3.2, 3.4 Don't forget that P1 is due tomorrow!