1. Arrays, Casting & User Defined Variables
G6DICP - Lecture 10
Arrays, Casting & User Defined Variables

2. Data Structures Data is stored in variables
Closely related data items may conveniently be stored in a “data structure”
Data structures are complex variables that contain more than one item of data
“Metavariables” - variables containing variables
Arrays are a common form of data structure

3. Arrays
Arrays are a data structure that contains a group of data items (elements)
All elements must be of a single type
Elements are indexed by an integer
Arrays are of a pre-determined size
For example:
An array of 3 integers called data
data[0]=3; data[1]=24; data[2]=12;
An array of 3 Strings called day:
day[0]=“Monday”; day[1]=“Tuesday”; day[2]=“Wednesday”

4. Java Arrays Arrays are a variables type, and so must be declared
int[] data; or int data[];
Before they can be used, arrays must be initialized
This determines their size
The new reserved word allocates memory to a data structure, fixing the size and type of an array
data = new int[3];
Once initialized elements of the array may be addressed by index (counting from 0)
data[0]
data[1]
data[2]

5. Casting A simple form of data type conversion
For example if you need to use the double 5.0 as an int then it must be converted to an int
Syntax:
double d=5;
int I = (int) d;
Many types can be cast into many other types
Only if this makes sense!
Information may be lost in the casting process

6. User defined data types
Composite data structures
A data type containing several different data types
Analagous to a record of a flat field database
Record in Pascal
Struct in C
Class in Java

7. A simple Class declaration
class Person {
String name;
String ;
String room;
int phone; }

8. Using a user defined class
Person tim;
tim = new Person();
tim.name = “Tim Brailsford”;
tim. =
tim.room = “C5”;
tim.phone = 14231;