Difference between revisions of "If and Then"

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This [[Keyword|keyword]] preciedes a condition, is required to be followed by [[Then|then]] and may be followed by [[Else|else]].
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{{then}}
  
== If then ==
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The <syntaxhighlight lang="pascal" inline>if</syntaxhighlight> [[Keyword|keyword]] precedes a condition, must be followed by <syntaxhighlight lang="pascal" inline>then</syntaxhighlight> and a [[statement]].
 +
The statement may optionally be followed by [[Else|<syntaxhighlight lang="pascal" inline>else</syntaxhighlight>]] and another statement.
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This creates a binary [[Branch|branch]].
  
<delphi>
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== <syntaxhighlight lang="pascal" inline>If then</syntaxhighlight> ==
  if condition
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<syntaxhighlight lang="pascal">
    then action1
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if condition
    else action2;
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then true_statement
</delphi>
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else false_statement;
 +
</syntaxhighlight>
  
Condition is an expression that evaluates to [[True|true]] or [[False|false]].
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<syntaxhighlight lang="pascal" inline>condition</syntaxhighlight> is a [[Boolean|<syntaxhighlight lang="pascal" inline>Boolean</syntaxhighlight>]] expression that evaluates to [[True|<syntaxhighlight lang="pascal" inline>true</syntaxhighlight>]] xor [[False|<syntaxhighlight lang="pascal" inline>false</syntaxhighlight>]].
Action1 is executed if condition evaluates to true.
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<syntaxhighlight lang="pascal" inline>true_statement</syntaxhighlight> is executed if <syntaxhighlight lang="pascal" inline>condition</syntaxhighlight> evaluates to <syntaxhighlight lang="pascal" inline>true</syntaxhighlight>.
Action2 is executed if condition evaluates to false.
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<syntaxhighlight lang="pascal" inline>false_statement</syntaxhighlight> is executed if <syntaxhighlight lang="pascal" inline>condition</syntaxhighlight> evaluates to <syntaxhighlight lang="pascal" inline>false</syntaxhighlight>.
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A [[compile-time error]] occurs if the type of <syntaxhighlight lang="pascal" inline>condition</syntaxhighlight> does not evaluate to a <syntaxhighlight lang="pascal" inline>Boolean</syntaxhighlight> value.
  
If you need two or more statements in "if then" statement. In that case, you need the [[Begin|begin]] ... [[End|end]] around the statements.
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=== Multiple statements in <syntaxhighlight lang="pascal" inline>if then</syntaxhighlight> branch ===
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If you need two or more statements for <syntaxhighlight lang="pascal" inline>true_statement</syntaxhighlight> or <syntaxhighlight lang="pascal" inline>false_statement</syntaxhighlight>, enclose them within a [[Begin|<syntaxhighlight lang="pascal" inline>begin</syntaxhighlight>]]&nbsp;…&nbsp;[[End|<syntaxhighlight lang="pascal" inline>end</syntaxhighlight>]] frame (“compound statement”).
  
<delphi>
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<syntaxhighlight lang="pascal">
  if boolean_condition then
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if boolean_condition then
    begin
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begin
      statement_one;
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statement_zero;
      statement_two;
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statement_one;
      statement_three;
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statement_two;
    end;
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end;
</delphi>
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</syntaxhighlight>
  
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== Usage ==
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In order to optimize for speed in an <syntaxhighlight lang="pascal" inline>if … then … else</syntaxhighlight> branch, try to write your expression so that the <syntaxhighlight lang="pascal" inline>then</syntaxhighlight>-part gets executed most often.
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This improves the rate of successful <syntaxhighlight lang="asm" inline>jump</syntaxhighlight> predictions.
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== See also ==
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* Official documentation: [https://www.freepascal.org/docs-html/ref/refsu57.html Reference guide: § “The <syntaxhighlight lang="pascal" inline>If..then..else</syntaxhighlight> statement”]
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* [[IF]], Tao Yue, Basic Pascal Introduction
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* [[;#If statement and semicolon|If statement and semicolon]]
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* [[Case|<syntaxhighlight lang="pascal" inline>case</syntaxhighlight>]]
  
 
{{Keywords}}
 
{{Keywords}}

Latest revision as of 08:37, 22 June 2020

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The if keyword precedes a condition, must be followed by then and a statement. The statement may optionally be followed by else and another statement. This creates a binary branch.

If then

if condition
	then true_statement
else false_statement;

condition is a Boolean expression that evaluates to true xor false. true_statement is executed if condition evaluates to true. false_statement is executed if condition evaluates to false. A compile-time error occurs if the type of condition does not evaluate to a Boolean value.

Multiple statements in if then branch

If you need two or more statements for true_statement or false_statement, enclose them within a begin … end frame (“compound statement”).

if boolean_condition then
begin
	statement_zero;
	statement_one;
	statement_two;
end;

Usage

In order to optimize for speed in an if  then  else branch, try to write your expression so that the then-part gets executed most often. This improves the rate of successful jump predictions.

See also


Keywords: begindoelseendforifrepeatthenuntilwhile