Inside Delphi 2006 (Wordware Delphi Developers Library)

The repeat-until loop, like the while loop, uses a condition to determine the number of times it needs to execute a group of statements. One difference between the while and repeat-until loops is that the while loop executes statements while the condition is True, and the repeat-until loop executes statements while the condition is False (until the condition is met).

The syntax of the repeat-until loop is:

repeat statement_1; statement_n; until condition;

Let's see how to display the numbers 1 to 10 using the repeat-until loop.

Listing 4-9: A simple repeat-until loop

program Project1; {$APPTYPE CONSOLE} uses SysUtils; var i: Integer; begin i := 1; { Display numbers 1 to 10. } repeat WriteLn(i); i := i + 1; until i > 10; ReadLn; end.

Another major difference between the while and repeat-until loops is that the repeat-until loop tests the condition after it executes the statements in the block. This means that the repeat-until loop executes the statements at least once, regardless of the condition. This can lead to subtle errors in your applications (see Listing 4-10 and Figure 4-5).

Figure 4-5: An erroneous repeat-until loop

Listing 4-10: An erroneous repeat-until loop

program Project1; {$APPTYPE CONSOLE} uses SysUtils; var i: Integer; begin i := 25; repeat WriteLn('Number ', i, ' is smaller than number 10.'); i := i + 1; until i >= 10; ReadLn; end.

Unlike other loop constructs, the repeat-until loop acts as a block, so you can automatically write a larger number of statements inside the repeat-until loop without having to define a block.