Difference between revisions of "Break"

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{{Break}}
 
{{Break}}
  
The <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> routine effectively destroys a [[Loop|loop]].
+
The <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> (pseudo) [[Routine|routine]] effectively destroys a [[Loops|loop]].
 
Its primary application is to exit a loop prior its planned end.
 
Its primary application is to exit a loop prior its planned end.
  
 
<syntaxhighlight lang="pascal" inline>Break</syntaxhighlight>, with its special meaning of abandoning a loop, can only be written ''within'' loops.
 
<syntaxhighlight lang="pascal" inline>Break</syntaxhighlight>, 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 {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" inline>system.break</syntaxhighlight>}} at any time, though.
+
It is not a reserved word¹, therefore you could shadow it, but access it by writing the fully qualified [[Identifier|identifier]] {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" inline>system.break</syntaxhighlight>}} at any time, though.
  
 
== Collatz conjecture ==
 
== Collatz conjecture ==
 
 
Example:
 
Example:
 
The following program tackles the [https://en.wikipedia.org/wiki/Collatz_conjecture Collatz problem].
 
The following program tackles the [https://en.wikipedia.org/wiki/Collatz_conjecture Collatz problem].
The [[For|<syntaxhighlight lang="pascal" inline>for</syntaxhighlight>-loop]] in <syntaxhighlight lang="pascal" inline>collatzIterative</syntaxhighlight> uses a <syntaxhighlight lang="pascal" inline>break</syntaxhighlight>, 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 <syntaxhighlight lang="pascal" inline>for</syntaxhighlight>-construct (i.e.  automatically [[Inc|incrementing]] a variable within a specified range).
+
The [[For|<syntaxhighlight lang="pascal" inline>for</syntaxhighlight>-loop]] in <syntaxhighlight lang="pascal" inline>collatzIterative</syntaxhighlight> uses a <syntaxhighlight lang="pascal" inline>break</syntaxhighlight>, a)&nbsp;to check for the terminating condition according to Collatz’ problem, b)&nbsp;to abort prior reaching the data type’s boundaries, and c)&nbsp;while still using the advantage of the <syntaxhighlight lang="pascal" inline>for</syntaxhighlight>-construct (i.&#8239;e.  automatically [[Inc|incrementing]] a variable within a specified range).
  
<!-- leave the ifThen expanded, for those who aren't quite familiar with the math unit -->
+
<!-- leave the ifThen expanded, for those who aren’t quite familiar with the math unit -->
 
<syntaxhighlight lang="pascal" line highlight="15">
 
<syntaxhighlight lang="pascal" line highlight="15">
 
program collatz(input, output, stderr);
 
program collatz(input, output, stderr);
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</syntaxhighlight>
 
</syntaxhighlight>
 
Choosing a <syntaxhighlight lang="pascal" inline>for</syntaxhighlight>-loop in conjunction with a <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> is adequate, since the Collatz conjecture ''hypothesizes'' that the described function eventually ends in <syntaxhighlight lang="pascal" inline>1</syntaxhighlight>, but does not tell for sure.
 
Choosing a <syntaxhighlight lang="pascal" inline>for</syntaxhighlight>-loop in conjunction with a <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> is adequate, since the Collatz conjecture ''hypothesizes'' that the described function eventually ends in <syntaxhighlight lang="pascal" inline>1</syntaxhighlight>, but does not tell for sure.
Therefore – mathematically speaking – writing <syntaxhighlight lang="pascal" inline>while n <> 1 do</syntaxhighlight> does not consider the circumstance, that the problem is an ''assumption'', but would suggest it is determined to eventually result in <syntaxhighlight lang="pascal" inline>n = 1</syntaxhighlight>.
+
Therefore – mathematically speaking – writing <syntaxhighlight lang="pascal" inline>while n <> 1 do</syntaxhighlight> does not consider the circumstance, that the problem is an ''assumption'', but would suggest it was ''determined'' to eventually result in <syntaxhighlight lang="pascal" inline>n = 1</syntaxhighlight>.
  
 
== Other remarks ==
 
== Other remarks ==
 
+
However, the usage of <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> is usually considered as bad style, since it “delegitimizes” the loop’s condition expression.
However, the usage of <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> is usually considered as bad style, since it “delegitimizes” the loop's condition expression.
+
You have to ''know'' a loop’s body contains a <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> to determine all abort conditions.
You have to ''know'' a loop's statement block contains a <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> to determine all abort conditions.
 
  
 
According to the [[GNU Pascal|GP]]C manual, <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> is a [[Borland Pascal]] extension, whereas [[Mac Pascal]] has <syntaxhighlight lang="pascal" inline>leave</syntaxhighlight>.
 
According to the [[GNU Pascal|GP]]C manual, <syntaxhighlight lang="pascal" inline>break</syntaxhighlight> is a [[Borland Pascal]] extension, whereas [[Mac Pascal]] has <syntaxhighlight lang="pascal" inline>leave</syntaxhighlight>.
[[FPC]], apart from [[Mode MacPas|<syntaxhighlight lang="pascal" inline>{$mode macpas}</syntaxhighlight>]], only knows <syntaxhighlight lang="pascal" inline>break</syntaxhighlight>, though.
+
[[FPC]], apart from [[Mode MacPas|<syntaxhighlight lang="pascal" inline>{$mode MacPas}</syntaxhighlight>]], only knows <syntaxhighlight lang="pascal" inline>break</syntaxhighlight>, though.
  
 
== See also ==
 
== See also ==
 
* {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" inline>break</syntaxhighlight>}} in the <syntaxhighlight lang="pascal" inline>system</syntaxhighlight> unit
 
* {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" inline>break</syntaxhighlight>}} in the <syntaxhighlight lang="pascal" inline>system</syntaxhighlight> unit
* [[Exit|<syntaxhighlight lang="pascal" inline>exit</syntaxhighlight>]] to return from routines
+
* [[Exit|<syntaxhighlight lang="pascal" inline>exit</syntaxhighlight>]] to return from ''routines''
 
* [[Continue|<syntaxhighlight lang="pascal" inline>continue</syntaxhighlight>]] to skip the rest of an iteration
 
* [[Continue|<syntaxhighlight lang="pascal" inline>continue</syntaxhighlight>]] to skip the rest of an iteration
 
* [[Goto|<syntaxhighlight lang="pascal" inline>goto</syntaxhighlight>]]<!-- an equally disfavored statement as “break” -->
 
* [[Goto|<syntaxhighlight lang="pascal" inline>goto</syntaxhighlight>]]<!-- an equally disfavored statement as “break” -->
  
 
<small>
 
<small>
 
 
== sources ==
 
== sources ==
 
; 1
 
; 1

Latest revision as of 16:12, 20 November 2020

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The break (pseudo) 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 identifier 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 was 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 body 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

sources

1
compare remarks in the reference manual § “The For..to/downto..do statement” and § “reserved words”