2.5 System Structures: General Form of an Ada Program

To summarize what we have learned so far, the programs shown earlier all have the general form described in Figure 2.1:

Each program begins with one or more context clauses followed by a program heading.
The last line of each program begins with the reserved word END.
The program heading is followed by declarations, if any, which may appear in any order.
The reserved word BEGIN signals the start of the sequence of executable statements part of the program. The sequence of executable statements consists of the program statements that are translated into machine language and executed. The program statements seen so far consist of statements that perform computations and input/output operations. These executable statements are described in the next section.
The last line in a program has the form
END pname;
where pname is the name of the program.
Each declaration and statement in an Ada program ends with a semicolon.
An Ada statement can extend over more than one line; such a statement cannot be split in the middle of an identifier, a reserved word, a number, or a string. Also, we can write more than one statement on a line, although we will not do so in this book and do not recommend it.

One of the main functions of a computer is to perform arithmetic computations and display the results of computations. Such operations are specified by the sequence of executable statements that appear in the program body following the reserved word BEGIN. Each statement is translated by the Ada compiler into one or more machine-language instructions, which are copied to the object file and later executed. Declarations, on the other hand, describe to the compiler the meaning and purpose of each user-defined identifier. They result in the allocation of some memory space to hold the data values.

Figure 2.1
General Form of an Ada Program

WITH package1;
WITH package2;
 ...
WITH packageN;
PROCEDURE pname IS

  declarations (variables, constants, etc.)

BEGIN

  program statement;
  ...
  program statement;

END pname;

PROGRAM STYLE
Use of Blank Space

The consistent and careful use of blank spaces can significantly enhance the style of a program. A blank space is required between words in a program line (e.g., between PROCEDURE and Distance in Program 2.5). Because extra blanks between words and symbols are ignored by the compiler, you may insert them as desired to improve the style and appearance of a program.

Always leave a blank space after a comma and before and after operators such as *, -,and =.

Indent by two or more spaces all lines except for the first and last lines of the program and the line BEGIN.

Finally, use blank lines between sections of the program.

All of these measures are taken for the sole purpose of improving the style and hence the clarity of the program. They have no effect whatever on the meaning of the program as far as the computer is concerned; however, they make it easier for people to read and understand the program. Be careful not to insert blank spaces where they do not belong. For example, there cannot be a space between the characters : and = that comprise the assignment symbol :=. Also, you cannot put a blank in the middle of an identifier.