7 - Control Structures

Summary

This chapter describes the three bracketed sequential control structures of Ada. These are the if statement, the case statement and the loop statement. These three statements enable programs to be written with a clear flow of control without using goto statements and labels. However, for pragmatic reasons, Ada does have a goto statement and this is also described in this chapter.

The three control structures exhibit a similar bracketing style. There is an opening reserved word if, case or loop and this is matched at the end of the structure by the same reserved word preceded by end. The whole is, as usual, terminated by a semicolon. So we have

if case loop

… … …

end if; end case; end loop;

In the case of the loop statement the word loop can be preceded by an iteration scheme commencing with for or while.

Corresponding structures for expressions such as if expressions and case expressions which are introduced in Ada 2012 are described in Chapter 9.

If statements

The simplest form of if statement starts with the reserved word if followed by a Boolean expression and the reserved word then. This is then followed by a sequence of statements which will be executed if the Boolean expression turns out to be True. The end of the sequence is indicated by the closing end if. The Boolean expression can, of course, be of arbitrary complexity and the sequence of statements can be of arbitrary length.

A simple example is

if Hungry then

Cook;

Eat;

Wash_Up;

end if;

In this, Hungry is a Boolean variable and Cook, Eat and Wash_Up are various subprograms describing the details of the activities. The statement Eat; merely calls the corresponding subprogram (subprograms are dealt with in detail in Chapter 10).

The effect of this if statement is that if variable Hungry is True then we call the subprograms Cook, Eat and Wash_Up in sequence and otherwise we do nothing. In either case we then obey the statement following the if statement.

Note how we indent the statements to show the flow structure of the program. This is most important since it enables the program to be understood so much more easily. The end if should be underneath the corresponding if and the then is best placed on the same line as the if.