General LOOP statement

Sometimes it is more natural to test part way through, or at end, of executing statements in the loop body.

• ie may want an indefinite loop with a test not at the top
• in Ada the general indefinite loop is used for this

	loop
		statements_1;
		exit when test;
		statements_2;
	end loop;

• the loop always starts executing statements_1
• at the "exit when" test, if the test is true, the program leaves the loop, otherwise it continues in the loop

Semantics of General loop

Loop structures using the general LOOP statement

Exit at top

• "pre-test": test for termination before executing loop body

While:

	while  test  loop
		statements;
	end loop;

general:

	loop
		exit when not test;
		statements;
	end loop;

Exit in middle

	   loop
		2. read a value from user
		3. if end value entered, leave the loop
		4. do something with value
	   end of loop

Another example is summing numbers up to (but excluding) an exit value:

while:

	PUT("Enter an integer (-1 to exit): "); 
	GET(j); SKIP_LINE;
	tot := 0;
	while j /= -1 loop
		tot := tot + j;
		PUT("Enter an integer (-1 to exit): "); 
		GET(j); SKIP_LINE;
	end loop;
	PUT("Total is "); PUT(tot); NEW_LINE;

general:

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

Note that statements need to be duplicated when using the while loop.

Exit at bottom

• always executes the statement(s) at least once
• (like a Pascal repeat-until loop construct)

	loop
		statement(s);
		exit when test;
	end loop;

Eg read until an acceptable value entered:

while:

	j := -1;
	while (j < 0) loop
		PUT("Enter positive integer: "); 
		GET(j); SKIP_LINE;
	end loop;

general:

		
	loop
		PUT("Enter positive integer: "); 
		GET(j); SKIP_LINE;
		exit when j >= 0;
	end loop;

The code in the general loop is easier to understand.