Inside Delphi 2006 (Wordware Delphi Developers Library)

Loop execution is normally governed by the condition of the while and repeat-until loops or the start and end values of the for loop. In case we have to exit from the loop earlier, we can use the Break procedure (you can read more about procedures in Chapter 5). The following example shows how to exit from a loop after it executes five times.

Listing 4-11: Breaking out of a loop

program Project1; {$APPTYPE CONSOLE} uses SysUtils; var i: Integer; begin for i := 1 to 1000 do begin WriteLn(i); if i = 5 then Break; end; // for i ReadLn; end.

Infinite Loops

As the title suggests, you can write loops that continue to execute forever. In Delphi, only the while and repeat-until loops can be infinite, but in C++ and C# the for loop can also be infinite.

Infinite loops are really easy to write. Since a while loop continues to execute as long as the condition evaluates to True, a simple infinite while loop looks like this:

while True do WriteLn('This is an infinite while loop.');

An infinite repeat-until loop is equally easy to write since it continues to execute as long as the condition evaluates to False:

repeat WriteLn('This is an infinite repeat-until loop.'); until False;

The following example utilizes an infinite while loop to read user names. To exit from the loop, the user has to enter the word "end."

Listing 4-12: An infinite while loop

program Project1; {$APPTYPE CONSOLE} uses SysUtils; var UserName: string; UsrCount: Integer; begin UsrCount := 0; while True do begin Write('Username: '); ReadLn(UserName); if UserName = 'end' then begin WriteLn('Thank you.'); ReadLn; Break; end else begin UsrCount := UsrCount + 1; WriteLn(UsrCount, '. ', UserName); end; // if end; // while end.

Another way of controlling loop execution is by skipping certain iterations. To skip an iteration, you can use the Continue procedure. The Continue procedure causes the loop to skip to the next iteration without executing the statements that follow it in the block. The following example displays only odd numbers by skipping even iterations.

Listing 4-13: The Continue procedure

program Project1; {$APPTYPE CONSOLE} uses SysUtils; var i: Integer; begin i := 0; while i < 20 do begin i := i + 1; { Skip even numbers. } if i mod 2 = 0 then Continue; WriteLn('Number ', i, ' is odd.'); end; ReadLn; end.

The WriteLn statement is only called when the variable i contains an odd number. If the variable i contains an even number, the Continue procedure executes and returns to the first line of the while loop that increments the i variable, thus skipping an iteration.