Object-oriented Software in Ada 95

8 Arrays
This chapter introduces arrays that implement a collection facility for objects. With this facility, objects
are stored and retrieved using an index of a discrete type.
8.1 Arrays as container objects
An array is a collection of objects that can be accessed using an instance of a discrete type. For example, the
number of computer terminals in five rooms could be described with the following declaration:
Computers_In_Room : array ( 1 .. 5 ) of Natural;
Note: Computers_In_Room is a collection of Natural numbers and the integer numbers 1 through 5
are used to select a particular object in this collection.
The compiler will check that the subscript is valid. If it cannot be checked directly, code should be
inserted which will perform the check at run-time. The exception Constraint_error is raised if
the index is out of bounds.
The number of terminals in each room can be recorded in the collection Computers_In_Room by using an
array index to select a particular computer room. For example, to set the number of computers in room 1 to 20, 2
to 30, 3 to 25, 4 to 10 and 5 to 15, the following code can be used:
Computers_In_Room(1) := 20;
Computers_In_Room(2) := 30;
Computers_In_Room(3) := 25;
Computers_In_Room(4) := 10;
Computers_In_Room(5) := 15;
This can be visualized diagrammatically as shown in Figure 8.1.
1 2 3 4 5
20 30 25 10 15
Index used to access contents of collection
computers_In_Room
Figure 8.1 Diagrammatic representation of an array.
Once information about the number of computers in each room has been set up, it can be printed with the
following code:
for I in 1 .. 5 loop
Put("Computers in room "); Put( I, Width=>1 ); Put(" is ");
Put( Computers_In_Room(I), Width=>2 ); New_Line;
end loop;
© M A Smith - May not be reproduced without permission