Loop control

1. initialisation of loop parameter(s)
   • automatic in a for loop
   • before a while loop
   • before a general loop, or in the statements before the exit test
2. test
   • automatic in a for loop
   • while: test for continuation
   • general: test for termination
3. update loop parameter
   • not allowed inside for loop
   • must update inside indefinite loop

Loop Design

Approaches to loop design:

• analyse the individual application
• recognise standard situations, use standard loops for those situations (eg sentinel-controlled loops, flag-controlled loops)
• use loop templates
• definite or indefinite?
• when to test for termination?
• what is appropriate initialisation etc?

Sentinel-controlled loops

One solution is to define a sentinel value

• a unique value that indicates end-of-data
• it cannot be a value that could occur as data

Code template:

	sentinel : constant := ???;	

	loop
		read (item);
		exit when item = sentinel;

		process the item;

	end loop;

Example: add integers that the user enters, up to but not including -1 for end of data

	tot := 0;
	loop
		PUT("Enter an integer (-1 to exit): "); 
		GET(j); SKIP_LINE;
		exit when (j = -1);
		tot := tot + j;
	end loop;

Flag-controlled loops

• a value of FALSE indicates that the desired event has not yet occurred
• TRUE indicates that it has occurred

Code template:

	flag : BOOLEAN;

	flag := FALSE;
	while not flag loop

		do some processing;

		if desired event has happened then
			flag := TRUE;
		end if;

		do some processing;

	end loop;

Example: read characters until a digit is typed

	DigitRead := FALSE;
	while not DigitRead loop
		get (c);
		if (c >= '0') and (c <= '9') then
			DigitRead := TRUE;
		end if;
	end loop;