Break

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).

program collatz(input, output, stderr);

procedure collatzIterative(n: qword);
var
    i: qword;
begin
    for i := 0 to high(i) do
    begin
        writeLn('step ', i:20, ': ', n);
        
        // Collatz conjecture: sequence ends with 1
        if (n = 1) or (n > (high(n) / 3 - 1)) then
        begin
            // leave loop, as next value may get out of range
            break;
        end;
        
        // n := ifThen(odd(n), 3 * n + 1, n div 2);
        if odd(n) then
        // n is odd
        begin
            n := 3 * n + 1;
        end
        // n is even
        else
        begin
            n := n div 2;
        end;
    end;
end;

var
    n: longword;
begin
    readLn(n);
    
    if n < 1 then
    begin
        writeLn(stderr, 'not a positive integer');
        halt(1);
    end;
    
    collatzIterative(n);
end.

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

• break in the system unit
• exit to return from routines
• continue to skip the rest of an iteration
• goto

sources

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

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.