Break

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The break routine effectively destroys a loop. Its primary application is to exit a loop prior its planned end.

Break, with its special meaning of abandoning a loop, can only be written within loops. It is not a reserved word¹, therefore you could shadow it, but access it by writing the fully qualified identfier system.break at any time, though.

Collatz conjecture

Example: The following program tackles the Collatz problem. The for-loop in collatzIterative uses a break, a) to check for the terminating condition according to Collatz' problem, b) to abort prior reaching the data type's boundaries, and c) while still using the advantage of the for-construct (i.e. automatically incrementing a variable within a specified range).

 1program collatz(input, output, stderr);
 2
 3procedure collatzIterative(n: qword);
 4var
 5	i: qword;
 6begin
 7	for i := 0 to high(i) do
 8	begin
 9		writeLn('step ', i:20, ': ', n);
10		
11		// Collatz conjecture: sequence ends with 1
12		if (n = 1) or (n > (high(n) / 3 - 1)) then
13		begin
14			// leave loop, as next value may get out of range
15			break;
16		end;
17		
18		// n := ifThen(odd(n), 3 * n + 1, n div 2);
19		if odd(n) then
20		// n is odd
21		begin
22			n := 3 * n + 1;
23		end
24		// n is even
25		else
26		begin
27			n := n div 2;
28		end;
29	end;
30end;
31
32var
33	n: longword;
34begin
35	readLn(n);
36	
37	if n < 1 then
38	begin
39		writeLn(stderr, 'not a positive integer');
40		halt(1);
41	end;
42	
43	collatzIterative(n);
44end.

Choosing a for-loop in conjunction with a break is adequate, since the Collatz conjecture hypothesizes that the described function eventually ends in 1, but does not tell for sure. Therefore – mathematically speaking – writing while n <> 1 do does not consider the circumstance, that the problem is an assumption, but would suggest it is determined to eventually result in n = 1.

Other remarks

However, the usage of break is usually considered as bad style, since it “delegitimizes” the loop's condition expression. You have to know a loop's statement block contains a break to determine all abort conditions.

According to the GPC manual, break is a Borland Pascal extension, whereas Mac Pascal has leave. FPC, apart from {$mode macpas}, only knows break, though.

See also

• break in the system unit
• exit to return from routines
• continue to skip the rest of an iteration
• goto

sources

1

compare remarks in the reference manual § “The For..to/downto..do statement” and § “reserved words”