Difference between revisions of "Break"
m (syntax highlight line containing “break”) |
(add more explanation) |
||
| Line 9: | Line 9: | ||
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" enclose="none">for</syntaxhighlight>-loop]] in <syntaxhighlight lang="pascal" enclose="none">collatzIterative</syntaxhighlight> uses a <syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight>, a) to abort prior reaching the data type's boundaries, and | + | The [[For|<syntaxhighlight lang="pascal" enclose="none">for</syntaxhighlight>-loop]] in <syntaxhighlight lang="pascal" enclose="none">collatzIterative</syntaxhighlight> uses a <syntaxhighlight lang="pascal" enclose="none">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" enclose="none">for</syntaxhighlight>-construct, that is condition-checking and automatically [[Inc|incrementing]] a variable. |
| + | <!-- leave the ifThen expanded, for those who aren't quite familiar with the math unit --> | ||
<syntaxhighlight lang="pascal" line start="0" highlight="14"> | <syntaxhighlight lang="pascal" line start="0" highlight="14"> | ||
program collatz(input, output, stderr); | program collatz(input, output, stderr); | ||
| Line 22: | Line 23: | ||
writeLn('step ', i:20, ': ', n); | writeLn('step ', i:20, ': ', n); | ||
| + | // Collatz conjecture: sequence ends with 1 | ||
if (n = 1) or (n > (high(n) / 3 - 1)) then | if (n = 1) or (n > (high(n) / 3 - 1)) then | ||
begin | begin | ||
| Line 28: | Line 30: | ||
end; | end; | ||
| + | // n := ifThen(n mod 2 = 0, n div 2, 3 * n + 1); | ||
if n mod 2 = 0 then | if n mod 2 = 0 then | ||
// n is even | // n is even | ||
Revision as of 02:43, 14 February 2018
│
Deutsch (de) │
English (en) │
español (es) │
suomi (fi) │
français (fr) │
русский (ru) │
The break routine effectively destroys a loop.
Its primary application is to exit a loop prior its planned end.
break 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.
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, that is condition-checking and automatically incrementing a variable.
0program collatz(input, output, stderr);
1
2procedure collatzIterative(n: qword);
3var
4 i: qword;
5begin
6 for i := 0 to high(i) do
7 begin
8 writeLn('step ', i:20, ': ', n);
9
10 // Collatz conjecture: sequence ends with 1
11 if (n = 1) or (n > (high(n) / 3 - 1)) then
12 begin
13 // leave loop, as next value may get out of range
14 break;
15 end;
16
17 // n := ifThen(n mod 2 = 0, n div 2, 3 * n + 1);
18 if n mod 2 = 0 then
19 // n is even
20 begin
21 n := n div 2;
22 end
23 // n is odd
24 else
25 begin
26 n := 3 * n + 1;
27 end;
28 end;
29end;
30
31var
32 n: longword;
33begin
34 readLn(n);
35
36 if n < 1 then
37 begin
38 writeLn(stderr, 'not a positive integer');
39 halt(1);
40 end;
41
42 collatzIterative(n);
43end.
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.
see also
sources
- 1
- compare remarks in the reference manual § “The
For..to/downto..dostatement” and § “reserved words”
