Object-oriented Software in Ada 95

25 Ada introduction: Part 1
3 Ada introduction: Part 1
This chapter looks at some simple Ada programs, and presents the basic control structures of the
language. The data types Integer and Character are used to introduce these structures.
3.1 A first Ada program
The first program presented is a simple one that writes the message ‘Hello World’ onto the user’s terminal.
with Text_Io;
use Text_Io;
procedure Hello is
begin
Put("Hello World"); New_Line;
end Hello;
Note:
The example programs in this book are shown with reserved words in bold to aid readability. As the
name suggests, reserved words can only be used for their intended purpose. Strange error messages
can occur when a reserved word is inadvertently used by the programmer as the name of an object in a
program. Reserved words are entered as normal text when writing a program. Section B.1, Appendix B
lists all the reserved words in the Ada programming language.
When compiled and run, this program will display on a user’s terminal the message:
Hello World
In the above program, the reserved words begin and end are used to bracket the body of the procedure
Hello. In Ada, a procedure can be a self-contained program unit that may be independently compiled. In the
above example, the single procedure Hello forms a complete program that may be compiled and run by an
appropriate Ada compiling system.
The statement Put("Hello World"); is responsible for outputting the greeting to the terminal. Used in
conjunction with New_Line, which outputs a new line character to the terminal, these procedures are defined
and implemented by the library package Ada.Text_Io.
Note:
The end keyword is followed by the name of the procedure, in this example helloworld. The
compiler checks for this to ensure that the procedure's extent agrees with the programmer’s view.
One of the important concepts in Ada is the idea of encapsulating items together to form a package which may
be re-used in other programs. The library package Ada.Text_Io is provided on Ada systems to allow the input
and output of textual information to and from the user’s program. This library package is introduced to a
procedure by means of the statements with Ada.Text_Io; use Ada.Text_Io; the details of which will
be explained later.
© M A Smith - May not be reproduced without permission