p03 - variables.pdf

7/31/2019 P03 - Variables.pdf

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Computer Studies 2013 Syllabus

Mr. A. Gatt Page 1 of 8

Variables (03)

Variables and their data types.

Introduction

In most cases, whilst a program is running, it requires memory storage for calculations or text manipulation.

Most of the time, the RAM (random access memory) is used to store these temporary values; these small

memory locations are called variables.

When coding a program, the programmer can create variables of a particular data type which can storevalues throughout the execution of the program. Each variable must have a unique name so that it can be

used (written to or read from). If we imagine the RAM as a set of boxes, in order to use one of these boxes,

a label must be applied to it and then the data which it can store must be also declared.

For instance, if we need to store three whole numbers we can create two integer (int) variables called N1,

N2 and tot.

After these are created they can be used to store data which in this case is limited to whole numbers. Note

that a variable can only store one value.

N1

N2

int int

tot

int


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Declaring a Variable

Before using the variable it must be declared. When declaring a variable its name must be specified along

with its data type in the following format.

typevariable_name;

Type must be replaced with the correct data type (seen later on) and variable_name must be set by the

user. For instance:

intN1;

intN2;

inttot;

In this case the three variables are declares as integers (int) which can only store whole numbers. There are

other data types which can be used to store real numbers and text.

class VariablesExample {

public static void main (String args[]){

//variables are declared

int N1;

int N2;

int tot;

//assign values to variables

N1 = 50;

N2 = 13;

//the total of variables N1 and N2

//is stored in tot

tot = N1 + N2;

//Finally, we can show the result

System.out.println(tot);

}

}

The memory locations for three

variables are created. These can

only store whole numbers.

A number is stored in the variable

using the = operator. So in this case

50 is placed in N1 and 13 in N2.

Instead of storing a particular value

in tot, the total of N1 and N2 is

stored.


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Overwriting Values

If a variable already stores a value and it is re-assigned a value, the previous value will be lost. For instance if

N1 is assigned 50:

N1 = 50;

Then during the course of the program it is re-assigned such as:

N1 = 33;

The fifty is lost and replaced with 33.

Direct Initalisation

Variables can also be assigned values during their declaration. So the above program can be changed to

class VariablesExample {


//variables are declared are assigned

int N1 = 50;

int N2 = 13;

int tot;

//the total of variables N1 and N2

//is stored in tot

tot = N1 + N2;

//Finally, we can show the result

System.out.println(tot);

}

}

33

N1

int

50

N1

int

N1 and N2 are declared as int and

assigned a value immediately.


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Data Types

There are eight primitive data types which are byte, short, int, long, char, float, double and

boolean.

Integers

Integers allow only whole numbers. So no fractions can be stored in them. There are four types of integers.

Type Number of Bits Used Range

byte 8 -128 to 127

short 16 -32,768 to 32,767

int 32 -2,147,483,648 to 2,147,486,647

long 64 -9,223,372,036,854,775,808 to 9,223,375,036,854,775,807

Variables can be declared as previously specified. So if a byte is required it can be declared as:

byte month;

Reals

Reals can store any type of number including fractions as well.


float 32 1.4012984643248170710-45

to 3.402823466385288601038

double 64 4.94065645841246544x10-324 to 1.79769313486231570x10308

Others

Apart from numbers we can also store characters (a single character) and Boolean values (true or false).


char 16 0 to 65,535

boolean 1 true or false


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When assigning a value to a char variable, the character to be assigned must be enclosed in single quotes.

For example:

class CharEx {


char label;

label = B;

}

}

On the other hand a boolean variable can only store true or false; these are not to be placed in single

quotes.

class BooleanEx {


boolean canDrive;

canDrive=false;

}

}

Display contents of a Variable

In order to print the contents of a variable (what the variable is storing) it is important that the variable

name is not placed inside the quotes otherwise the name of the variable will appear. Consider the following

example.

Code Output

char letter;

letter = C;

System.out.println(letter);

letter

char letter;

letter = C;

System.out.println(letter);

C


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Naming a Variable

When a name is given to a variable there are some rules which must be followed and some pointers which

are good practice. These are some of the things to remember:

it must start with a lowercase letter, it cannot be a keyword used in Java, cannot be a Boolean literal (true or false) or null, it must be unique; so throughout its scope different variables cannot have the same name, it cannot contain blank spaces; however if a variable is made up of two different words they are

usually combined together using CamelCase; for example fullNameor totalSum,

it must reflect the contents that it will store, so that just by looking at it you know what it contains.

Strings

In most programs text needs to be stored in a program as well. The chardata type cannot be used since it

can only store one character. A Stringcan store characters, numbers, a mixture of both and also symbols.

Strings can be declared as the other variable types, for instance:

String fullName = Alex Gerada;

In order to display the String, the usual println()method can be used:

System.out.println(fullName);

However if the Stringis to be combined with additional text, the + operator must be used, for example:

System.out.println (The next person is +fullName);


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Strings can also be combined together by using the + operator. For example:

class StringExample {


String firstName = John;

String lastName = Grech;

String fullName = firstName+lastName;

System.out.println(My name is +fullName);

}

}

Activities

1. Explain what a variable is and why its used in a program.

2. Instead of first declaring a variable and then assigning it a value, direct initialisation can be used.

Show the difference between these two methods by using Java examples.

3. By using a Java example, show how a variable is overwritten.

4. State a suitable data type for the following values:

a. Xb. 67c. -9.45d. The clouds are gathering.e. 32000

5. Which of the following are suitable variable names? (if not state a reason why)

a. Total Sumb. TotalSUMc. totalSumd. fffe. true


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6. 1 class StringExample {

2 public static void main (String args[]){

3 String firstName = John;

4 String lastName = Grech;

5 String fullName = firstName+lastName;

6 System.out.println(My name is +fullName);

7 }

8 }

Copy this program and run it. Check what happens. Try to solve the problem by only changing line 5.

7. Write a program that uses three variables to store your date of birth in this format 3 February 1999.

Then display the date of birth on screen using only one println() statement.

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p03 - variables.pdf

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