Variables - Fundamental Programming Techniques

farrightΛογισμικό & κατασκευή λογ/κού

15 Αυγ 2012 (πριν από 4 χρόνια και 11 μήνες)

609 εμφανίσεις

02 Variables

1

March 13

02 Variables

CE00858
-
1:

Fundamental Programming Techniques

02 Variables

2

March 13

Objectives

In this session, we will:


look at integers and doubles


declare and initialise variables in Java applications


perform arithmetic operations on numbers


output values


discuss downloading the JDK and Netbeans






02 Variables

3

March 13

Handling data


most programs need to handle data


to solve a problem, we need to think about:


what data is used


what results are required


how we convert the data into the results


program must store data in memory and access it later


memory is like boxes that can contain values:


20

a

3.141

fred

02 Variables

4

March 13

Numbers


most programs use numbers


may be whole numbers or decimal point numbers


whole numbers:
integer


days in a week


legs on a dog


pages in a book


decimal point numbers:
double


distance between Stafford and Bristol in kilometres


average exam mark for a group of students


person’s reaction time in seconds


02 Variables

5

March 13

Declaring variables


values stored in variables may change as program runs


variables have:


a name


a type


a value


must declare variable before use


int count;

double average;

count

average

02 Variables

6

March 13

Variable names


all names are case sensitive


rules for variable names:


may be any length


must start with a letter _ or $


must not contain spaces, tabs or special characters


must not be keywords (main, void, public, etc)


conventions for variable names:


first letter should be lowercase


each new word starts with a capital


should be meaningful

02 Variables

7

March 13

Initialisation


giving a variable an initial value


good practice to initialise variables


error raised if variable doesn't have a value before it is
used


int count = 10;

double average = 2.5;

10

count

2.5

average

02 Variables

8

March 13

Assignment


storing data in an existing variable


evaluates right hand side of equals sign and
overwrites

contents of variable on left hand side

count = 46;

average = 9.75;

46

count

9.75

average

02 Variables

9

March 13

Arithmetic operators


Java supports the standard arithmetic operators:









all expressions must be given in full

Operator

Meaning

*

multiply

/

divide

%

remainder

+

add

-

subtract

energy = mass * speedOfLight * speedOfLight; //e = mc
2

02 Variables

10

March 13

Operator precedence


operators have precedence which determines the order in
which they are evaluated:






operators with same order of precedence are evaluated
from left to right


brackets can override the order of operation

Operator

Precedence

* / %

carried out before + and
-

+
-

carried out after *, / and %

average = (a + b) / 2;

02 Variables

11

March 13

Displaying variables


contents of variables can be output using calls to
System.out.print and System.out.println


can output more than one thing using concatenator, +:

System.out.println("Count is: " + count + " items");

System.out.print("Answer is: " + ans);

02 Variables

12

March 13

Analysis of calculations


need to consider what data:


type


value


where it comes from


input by user


hard
-
coded in program


calculated by program


what operations


calculations required


output needed



02 Variables

13

March 13

Calculation example


Rectangle


problem:


calculate and output area of rectangle given width of 10 and
height of 20


analysis
:


what data?


width: integer set to 10


height: integer set to 20


area: integer calculated in program


what operations are performed?


area = width
* height


output area

02 Variables

14

March 13

//calculate and output area of rectangle

public class Rectangle

{


public static void main (String[] args)


{



int width = 10;



int height = 20;




//calculate area



int area = width * height;




//output area



System.out.println ("The area is " + area);


}

}



Rectangle.java

02 Variables

15

March 13

Integers and doubles


double variable can store integer values:






integer variable cannot store double values:

double average;

int count = 7;

average = count;

double average;

int count = 7;

count = average;

integer is promoted to a double

potential loss of accuracy

fails to compile


02 Variables

16

March 13

Division


if both operands are
integers

and result is
integer
, integer
division takes place:





if both results are
integer

and result is
double
, integer division
takes place and result is promoted to double:





if either result is
double
, result must be
double

and real
division takes place:



double ave = (1 + 2 + 3 + 4) / 4.0;

//ave = 2.5

double ave = (1 + 2 + 3 + 4) / 4;


//ave = 2.0

int ave = (1 + 2 + 3 + 4) / 4;


//ave = 2

02 Variables

17

March 13

Java Development Kit & NetBeans


we will be using Java Development Kit (JDK)


Java SE


versions are available for Windows and Linux



NetBeans is an integrated development for developing
Java application



JDK and NetBeans are available from Sun


visit the Sun Java website:
http://java.sun.com


download instructions are available from the FPT web site


02 Variables

18

March 13

Summary

In this session we have:


looked at when we use integers and doubles


declared, initialised and used variables in Java applications


performed calculations using numbers


output results of calculations


considered downloading the JDK and Netbeans


In the next session we will:


look at more calculations


input data into programs