Basic Pascal Tutorial/Chapter 3/REPEAT..UNTIL

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REPEAT...UNTIL

The repeat .. until construct is termed a post-test loop, because the controlling condition is tested after each iteration of the loop.

It has the following syntax:

repeat
  statement1;
  // statement2;
  // further statements...
until BooleanExpression;

A repeat loop encloses its executed statements, which means they do not need to be further enclosed in a begin ... end block. Note that a repeat loop continues until its controlling Boolean expression is True; whereas the while loop continues until its Boolean expression is False.

For instance, the following repeat loop executes at least once:

repeat
  WriteLn(Node.Text);
  Node := GetNextNode;
until not Assigned(Node);

It assumes that Node is not Nil at the outset. If this assumption is incorrect, the code will fail, and the program may crash.

A while loop is more defensive, since the needed check is performed before any loop statements are executed:

while Assigned(Node) do
  begin
    WriteLn(Node.Text);
    Node := GetNextNode;
  end;

Use a repeat loop when the looping statement(s) must execute at least once, whatever the initial value of the controlling Boolean condition.