Difference between revisions of "If and Then"

From Lazarus wiki
Jump to navigationJump to search
(unify code style; rephrase sentence misusing the term “block” (as it is defined in the article Block); insert →‎Usage)
(10 intermediate revisions by 3 users not shown)
Line 1: Line 1:
{{If}}
+
{{then}}
  
The <code>if</code> [[Keyword|keyword]] precedes a condition, must be followed by <code>[[Then|then]]</code> and a statement. The statement may optionally be followed by <code>[[Else|else]]</code> and another statement.
+
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.
 +
This creates a binary [[Branch|branch]].
  
 
+
== <syntaxhighlight lang="pascal" inline>If then</syntaxhighlight> ==
== <code>If then</code> ==
+
<syntaxhighlight lang="pascal">
 
 
<syntaxhighlight>
 
 
if condition
 
if condition
 
then true_statement
 
then true_statement
else false_statement;
+
else false_statement;
 
</syntaxhighlight>
 
</syntaxhighlight>
  
<code>condition</code> is a [[Boolean|boolean]] expression that evaluates to [[True|true]] xor [[False|false]].
+
<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>]].
<code>true_statement</code> is executed if <code>condition</code> evaluates to <code>true</code>.
+
<syntaxhighlight lang="pascal" inline>true_statement</syntaxhighlight> is executed if <syntaxhighlight lang="pascal" inline>condition</syntaxhighlight> evaluates to <syntaxhighlight lang="pascal" inline>true</syntaxhighlight>.
<code>false_statement</code> is executed if <code>condition</code> evaluates to <code>false</code>.
+
<syntaxhighlight lang="pascal" inline>false_statement</syntaxhighlight> is executed if <syntaxhighlight lang="pascal" inline>condition</syntaxhighlight> evaluates to <syntaxhighlight lang="pascal" inline>false</syntaxhighlight>.
A compile-time error occurs if the type of <code>condition</code> does not evaluate to a <code>boolean</code> value.
+
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.
  
 +
=== Multiple statements in <syntaxhighlight lang="pascal" inline>if then</syntaxhighlight> branch ===
 +
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”).
  
=== Multiple statements in <code>if then</code> branch ===
+
<syntaxhighlight lang="pascal">
 
 
If you need two or more statements for <code>true_statement</code> or <code>false_statement</code>, enclose them within a <code>[[Begin|begin]] … [[End|end]]</code> [[Block]] (compound statement).
 
 
 
<syntaxhighlight>
 
 
if boolean_condition then
 
if boolean_condition then
 
begin
 
begin
Line 31: Line 29:
 
</syntaxhighlight>
 
</syntaxhighlight>
  
 +
== Usage ==
 +
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.
 +
This improves the rate of successful <syntaxhighlight lang="asm" inline>jump</syntaxhighlight> predictions.
  
 
== See also ==
 
== See also ==
 
+
* Official documentation: [https://www.freepascal.org/docs-html/ref/refsu57.html Reference guide: § “The <syntaxhighlight lang="pascal" inline>If..then..else</syntaxhighlight> statement”]
* Official documentation: [https://www.freepascal.org/docs-html/ref/refsu57.html Reference guide: § “The <code>If..then..else</code> statement”]
+
* [[IF]], Tao Yue, Basic Pascal Introduction
* [[IF]], Tao Yue, Object Pascal Introduction
+
* [[;#If statement and semicolon|If statement and semicolon]]
 +
* [[Case|<syntaxhighlight lang="pascal" inline>case</syntaxhighlight>]]
  
 
{{Keywords}}
 
{{Keywords}}
[[Category:Pascal]]
 
[[Category:Control Structures]]
 

Revision as of 07:37, 22 June 2020

Deutsch (de) English (en) français (fr) русский (ru)

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