Transcript Lecture 2 Slides

Programming for Beginners
Lecture 2: Variables & Data Types
Martin Nelson
Elizabeth FitzGerald
Revision of Session 1

Differences between:

Procedural and object-oriented languages
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Interpreted and compiled languages
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The basics of Java programming
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How computer programs are constructed
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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
class myprog
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Reserved words
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Identifier
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'class' is a Java reserved word
'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!
Anatomy of a Java program – 3
class myprog
{
}
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Code braces
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Braces { or } usually separate off a block of code
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All programs have several blocks of code
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Braces must be evenly balanced
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Braces are often nested
Anatomy of a Java program – 4
class myprog
{
public static void main (String[ ] args)
{
}
}
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Methods
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Methods contain blocks of functional code
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Methods are named by an identifier
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This is a method called 'main' (applications execute their main
method on starting)
Anatomy of a Java program – 5
class myprog
{
public static void main (String[ ] args)
{
System.out.println(“Hello world”);
}
}
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Statements
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This program contains a single statement
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Statements are terminated by a semi-colon
Anatomy of a Java program – 6
class myprog
{
public static void main (String[ ] args)
{
System.out.println(“Hello world”);
}
}
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println
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This statement calls a 'print' method
Methods can be given data (arguments) which are
contained in brackets
Anatomy of a Java program – 7
class myprog
{
public static void main (String[ ] args)
{
System.out.println(“Hello world”);
}
}

The argument of println here is a string

A string is a sequence of characters
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Java strings are bound in double quotes
Code Presentation
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Add coments to clarify what the code does
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// comments a single line.
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/* and */ comment multiple lines.
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Comments should be brief and helpful!
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Use blank lines to seperate different tasks.
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Indent code inside curly braces
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One tab or three/four spaces.
Session 2 - aims & objectives
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Find out how to declare variables and how to assign
values to them
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Appreciate the main Java variable types:
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char
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byte
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boolean
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String
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integer
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double
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Perform arithmetic using variables
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Introduce concept of decision making
Variables
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Symbolic representation of data of a specific type
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variables are named by an identifier
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the type must be declared before a variable can be used
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e.g. int a
Values can be assigned to a variable
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Java assignment is =
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e.g. a = 10;
b = 5;
c = a + b;
Variables can be modified during program execution
(usually by assignment)
Text-based variable types
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char
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a single ASCII character (all letters, all numbers, all punctuation
marks etc)
bound by single quotes e.g. ‘a’
String
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a series of characters i.e. text, of any length
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note capital S at start of the word String
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bound by double quotes e.g. “some text”
Numeric variable types
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byte
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integer
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whole number in the range -2147483648 to 2147483647
double
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whole number in the range -128 to 127
floating point numbers (15 decimal places)
scientific notation
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The letter 'e' means "times 10 raised to the power"
e.g. 3.45e-3 = 0.00345; 1e6 = 1 000 000
Other variable types
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boolean
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Used for creating true or false variables
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Useful in program control and decision making
e.g.
if condition is true
else
then do this
do something else
Decision making
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Sometimes you will want the program to perform a
function based on a decision
e.g. withdrawing or depositing money into a bank account
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withdrawal - subtract sum from balance
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deposit - add sum to balance.
A decision is required:
if deposit then
add sum to balance
else
subtract sum from balance
Arithmetic Operations
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Addition
x=x+10;
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Subtraction
x=x-10;
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Multiplication
x=x*10;
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Division
x=x/10;
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Increment
x++;
(equivalent to x=x+1)
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Decrement
x--;
(equivalent to x=x-1)
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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
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This statement requires a boolean expression as part of
its code.
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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...
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Flow control!
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How to easily make your code repeat a task
many times.