Programming for Programming for BeginnersBeginners

Martin Nelson

Elizabeth FitzGerald

Lecture 2: Variables & Data Types

Revision of Session 1

Differences between: Procedural and object-oriented languages Interpreted and compiled languages

The basics of Java programming How computer programs are constructed Statements, comments and basic arithmetic

Anatomy of a Java program – 1

class myprog{

public static void main (String[ ] args)

{System.out.println(“Hello

world!”);}

}

Anatomy of a Java program – 2

Reserved words 'class' is a Java reserved word

Identifier 'myprog' is an identifier This is a word we make up to identify part of the program

(in this case, the program itself) Identifiers must be a single word

Remember - Java is case sensitive!

class myprog

Anatomy of a Java program – 3

Code braces Braces { or } usually separate off a block of code All programs have several blocks of code Braces must be evenly balanced Braces are often nested

class myprog{

}

Anatomy of a Java program – 4

Methods Methods contain blocks of functional code Methods are named by an identifier This is a method called 'main' (applications execute their

main method on starting)

class myprog{

public static void main (String[ ] args)

{}

}

Anatomy of a Java program – 5

Statements This program contains a single statement Statements are terminated by a semi-colon

class myprog{

public static void main (String[ ] args)

{System.out.println(“Hello

world”);}

}

Anatomy of a Java program – 6

println This statement calls a 'print' method Methods can be given data (arguments) which

are contained in brackets

class myprog{

public static void main (String[ ] args)

{System.out.println(“Hello

world”);}

}

Anatomy of a Java program – 7

The argument of println here is a string A string is a sequence of characters Java strings are bound in double quotes

class myprog{

public static void main (String[ ] args)

{System.out.println(“Hello

world”);}

}

Code Presentation

Add coments to clarify what the code does // comments a single line. /* and */ comment multiple lines. Comments should be brief and helpful!

Use blank lines to seperate different tasks.

Indent code inside curly braces One tab or three/four spaces.

Session 2 - aims & objectives

Find out how to declare variables and how to assign values to them

Appreciate the main Java variable types: char byte boolean

Perform arithmetic using variables Introduce concept of decision making

String

integer

double

Variables

Symbolic representation of data of a specific type variables are named by an identifier the type must be declared before a variable can be used e.g. int a

Values can be assigned to a variable Java assignment is = e.g. a = 10; b = 5; c = a + b;

Variables can be modified during program execution (usually by assignment)

Text-based variable types

char a single ASCII character (all letters, all numbers, all

punctuation marks etc) bound by single quotes e.g. ‘a’

String a series of characters i.e. text, of any length note capital S at start of the word String bound by double quotes e.g. “some text”

Numeric variable types

byte whole number in the range -128 to 127

integer whole number in the range -2147483648 to 2147483647

double floating point numbers (15 decimal places)

scientific notation The letter 'e' means "times 10 raised to the power"

e.g. 3.45e-3 = 0.00345; 1e6 = 1 000 000

Other variable types

boolean Used for creating true or false variables Useful in program control and decision making

e.g. if condition is true

then do thiselse

do something else

Decision making

Sometimes you will want the program to perform a function based on a decision

e.g. withdrawing or depositing money into a bank account withdrawal - subtract sum from balance deposit - add sum to balance.

A decision is required:if deposit then

add sum to balanceelse

subtract sum from balance

Arithmetic Operations Addition x=x+10; Subtraction x=x-10; Multiplication x=x*10; Division x=x/10;

Increment x++; (equivalent to x=x+1) Decrement x--; (equivalent to x=x-1)

The modulo operator gives the remainder when dividing x by some number. Useful for deciding if x is odd/even:

x=x%2;

The ‘if’ statement This statement requires a boolean expression as

part of its code. e.g. compare numeric variables a and b

if (a > b){ ...}

if (a > b | b == 0){ ...}

if (a > b){ ...}else{ ...}

Relational operators

> greater than

< less than

== is equal to

!= is not equal to

>= greater or equal to

<= less or equal to

| or

& and

Coming up in Session 3...

Flow control! How to easily make your code repeat a task

many times.