Difference between revisions of "Break"
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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" enclose="none"> | + | <syntaxhighlight lang="pascal" enclose="none">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" enclose="none">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 identfier {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" enclose="none">system.break</syntaxhighlight>}} at any time, though. | ||
| + | |||
| + | == Collatz conjecture == | ||
Example: | Example: | ||
| Line 18: | Line 20: | ||
var | var | ||
i: qword; | i: qword; | ||
| + | // i: integer; | ||
begin | begin | ||
for i := 0 to high(i) do | for i := 0 to high(i) do | ||
| Line 31: | Line 34: | ||
// n := ifThen(n mod 2 = 0, n div 2, 3 * n + 1); | // n := ifThen(n mod 2 = 0, n div 2, 3 * n + 1); | ||
| − | if n | + | if odd(n) then |
| − | // n is | + | // n is odd |
begin | begin | ||
| − | + | n := 3 * n + 1; | |
| + | |||
end | end | ||
| − | // n is | + | // n is even |
else | else | ||
begin | begin | ||
| − | n := | + | n := n div 2; |
end; | end; | ||
end; | end; | ||
| Line 60: | Line 64: | ||
Choosing a <syntaxhighlight lang="pascal" enclose="none">for</syntaxhighlight>-loop in conjunction with a <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight> is adequate, since the Collatz conjecture ''hypothesizes'' that the described function eventually ends in <syntaxhighlight lang="pascal" enclose="none">1</syntaxhighlight>, but does not tell for sure. | Choosing a <syntaxhighlight lang="pascal" enclose="none">for</syntaxhighlight>-loop in conjunction with a <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight> is adequate, since the Collatz conjecture ''hypothesizes'' that the described function eventually ends in <syntaxhighlight lang="pascal" enclose="none">1</syntaxhighlight>, but does not tell for sure. | ||
Therefore – mathematically speaking – writing <syntaxhighlight lang="pascal" enclose="none">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" enclose="none">n = 1</syntaxhighlight>. | Therefore – mathematically speaking – writing <syntaxhighlight lang="pascal" enclose="none">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" enclose="none">n = 1</syntaxhighlight>. | ||
| + | |||
| + | == Other remarks == | ||
However, the usage of <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight> is usually considered as bad style, since it “delegitimizes” the loop's condition expression. | However, the usage of <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight> is usually considered as bad style, since it “delegitimizes” the loop's condition expression. | ||
| Line 67: | Line 73: | ||
[[FPC]] only knows <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight>, though. | [[FPC]] only knows <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight>, though. | ||
| − | == | + | == See also == |
* {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight>}} in the <syntaxhighlight lang="pascal" enclose="none">system</syntaxhighlight> unit | * {{Doc|package=RTL|unit=system|identifier=break|text=<syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight>}} in the <syntaxhighlight lang="pascal" enclose="none">system</syntaxhighlight> unit | ||
* [[Exit|<syntaxhighlight lang="pascal" enclose="none">exit</syntaxhighlight>]] to return from routines | * [[Exit|<syntaxhighlight lang="pascal" enclose="none">exit</syntaxhighlight>]] to return from routines | ||
Revision as of 12:18, 8 March 2018
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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;
6 // i: integer;
7begin
8 for i := 0 to high(i) do
9 begin
10 writeLn('step ', i:20, ': ', n);
11
12 // Collatz conjecture: sequence ends with 1
13 if (n = 1) or (n > (high(n) / 3 - 1)) then
14 begin
15 // leave loop, as next value may get out of range
16 break;
17 end;
18
19 // n := ifThen(n mod 2 = 0, n div 2, 3 * n + 1);
20 if odd(n) then
21 // n is odd
22 begin
23 n := 3 * n + 1;
24
25 end
26 // n is even
27 else
28 begin
29 n := n div 2;
30 end;
31 end;
32end;
33
34var
35 n: longword;
36begin
37 readLn(n);
38
39 if n < 1 then
40 begin
41 writeLn(stderr, 'not a positive integer');
42 halt(1);
43 end;
44
45 collatzIterative(n);
46end.
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 only knows break, though.
See also
breakin thesystemunitexitto return from routinescontinueto skip the rest of an iterationgoto
sources
- 1
- compare remarks in the reference manual § “The
For..to/downto..dostatement” and § “reserved words”
