Break: what changed

Revision 1 to 2. Removed lines have a minus sign; added lines have a plus sign.

Old	New		Line
1	1		The `break` (pseudo) [[Routine\|routine]] effectively destroys a [[Loops\|loop]]. Its primary application is to exit a loop prior its planned end.
2	2
3	3		`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 [[Identifier\|identifier]] [`system.break`](https://www.freepascal.org/docs-html/rtl/system/break.html) at any time, though.
4	4
5	5		## Collatz conjecture
6	6
7	7		Example: The following program tackles the [Collatz problem](https://en.wikipedia.org/wiki/Collatz_conjecture). The [[For\|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 [[Inc\|incrementing]] a variable within a specified range).
8	8
9	9		``` pascal
10	10		program collatz(input, output, stderr);
11	11
12	12		procedure collatzIterative(n: qword);
13	13		var
14	14		i: qword;
15	15		begin
16	16		for i := 0 to high(i) do
17	17		begin
18	18		writeLn('step ', i:20, ': ', n);
19	19
20	20		// Collatz conjecture: sequence ends with 1
21	21		if (n = 1) or (n > (high(n) / 3 - 1)) then
22	22		begin
23	23		// leave loop, as next value may get out of range
24	24		break;
25	25		end;
26	26
27	27		// n := ifThen(odd(n), 3 * n + 1, n div 2);
28	28		if odd(n) then
29	29		// n is odd
30	30		begin
31	31		n := 3 * n + 1;
32	32		end
33	33		// n is even
34	34		else
35	35		begin
36	36		n := n div 2;
37	37		end;
38	38		end;
39	39		end;
40	40
41	41		var
42	42		n: longword;
43	43		begin
44	44		readLn(n);
45	45
46	46		if n < 1 then
47	47		begin
48	48		writeLn(stderr, 'not a positive integer');
49	49		halt(1);
50	50		end;
51	51
52	52		collatzIterative(n);
53	53		end.
54	54		```
55	55
56	56		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 was determined to eventually result in `n = 1`.
57	57
58	58		## Other remarks
59	59
60	60		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 body contains a `break` to determine all abort conditions.
61	61
62	62		According to the [[GNU Pascal\|GPC]] manual, `break` is a [[Borland Pascal]] extension, whereas [[Mac Pascal]] has `leave`. [[FPC]], apart from [[Mode MacPas\|{$mode MacPas}]], only knows `break`, though.
63	63
64	64		## See also
65	65
66	66		- [`break`](https://www.freepascal.org/docs-html/rtl/system/break.html) in the `system` unit
67	67		- [[Exit\|exit]] to return from routines
68	68		- [[Continue\|continue]] to skip the rest of an iteration
69	69		- [[Goto\|goto]]
70	70
71		-	~~<small>~~
72		-
73	71		## sources
74	72
75	73		1
76	74		compare remarks in [the reference manual § “The `For..to`/`downto..do` statement”](https://www.freepascal.org/docs-html/ref/refsu58.html) and [§ “reserved words”](https://www.freepascal.org/docs-html/ref/refsu3.html)
77		-
78		-	~~</small>~~
79	75
80	76		---
81	77		From the Free Pascal wiki, page "Break", as it stood on 2020-11-20 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).
82	78		Category: Code

Back to history