Difference between revisions of "Basic Pascal Tutorial/Chapter 3/REPEAT..UNTIL"

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{{TYNavigator|WHILE..DO|FOR..IN}}
 
{{TYNavigator|WHILE..DO|FOR..IN}}
  
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REPEAT..UNTIL (author: Tao Yue, state: unchanged)
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REPEAT...UNTIL
  
−
The posttest loop has the following format:
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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:
 
<syntaxhighlight lang=pascal>
 
<syntaxhighlight lang=pascal>
 
repeat
 
repeat
 
   statement1;
 
   statement1;
−
   statement2
+
   // statement2;
 +
  // further statements...
 
until BooleanExpression;
 
until BooleanExpression;
 
</syntaxhighlight>
 
</syntaxhighlight>
  
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In a <tt>repeat</tt> loop, compound statements are built-in -- you don't need to use begin-end. Also, the loop continues until the Boolean expression is <tt>TRUE</tt>, whereas the <tt>while</tt> loop continues until the Boolean expression is <tt>FALSE</tt>.
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A <tt>repeat</tt> loop encloses its executed statements, which means they do not need to be further enclosed in a <tt>begin ... end<tt> block. Note that a repeat loop continues until its controlling Boolean expression is <tt>True</tt>; whereas the <tt>while</tt> loop continues until its Boolean expression is <tt>False</tt>.
 +
 
 +
For instance, the following <tt>repeat</tt> loop executes at least once:
 +
<syntaxhighlight lang=pascal>
 +
repeat
 +
  WriteLn(Node.Text);
 +
  Node := GetNextNode;
 +
until not Assigned(Node);
 +
</syntaxhighlight>
 +
 
 +
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 <tt>while</tt> loop is more defensive, since the needed check is performed before any loop statements are executed:
 +
<syntaxhighlight lang=pascal>
 +
while Assigned(Node) do
 +
  begin
 +
    WriteLn(Node.Text);
 +
    Node := GetNextNode;
 +
  end;
 +
</syntaxhighlight>
  
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This loop is called a posttest loop because the condition is tested after the body of the loop executes. The <tt>REPEAT</tt> loop is useful when you want the loop to execute at least once, no matter what the starting value of the Boolean expression is.
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Use a <tt>repeat</tt> loop when the looping statement(s) must execute at least once, whatever the initial value of the controlling Boolean condition.
  
 
{{TYNavigator|WHILE..DO|FOR..IN}}
 
{{TYNavigator|WHILE..DO|FOR..IN}}

Revision as of 22:49, 20 November 2020

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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.

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