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
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”
