Difference between revisions of "false and true"

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m (Corrected the status of false and true as of FPC 3.0)
m (replace legacy syntaxhighlight syntax; partially undo revision 133640 (re-introduce Category: Code); add some external references)
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{{false and true}}
 
{{false and true}}
  
The [[Constant|constants]] <syntaxhighlight lang="pascal" enclose="none">false</syntaxhighlight> and <syntaxhighlight lang="pascal" enclose="none">true</syntaxhighlight> are used to define the false and true conditions of a [[Boolean|<syntaxhighlight lang="pascal" enclose="none">boolean</syntaxhighlight>]] [[Variable|variable]].
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The [[Constant|constants]] <syntaxhighlight lang="pascal" inline>false</syntaxhighlight> and <syntaxhighlight lang="pascal" inline>true</syntaxhighlight> are used to define the false and true conditions of a [[Boolean|<syntaxhighlight lang="pascal" inline>boolean</syntaxhighlight>]] [[Variable|variable]].
 
They are [[Manifest constant|manifest constants]] that are defined as part of the [[Standard type|standard data types]] the [[Compiler|compiler]] initially knows about.
 
They are [[Manifest constant|manifest constants]] that are defined as part of the [[Standard type|standard data types]] the [[Compiler|compiler]] initially knows about.
  
 
These constant values must be predefined by the compiler as there is no way to define them in terms of anything else.
 
These constant values must be predefined by the compiler as there is no way to define them in terms of anything else.
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They are defined via [https://svn.freepascal.org/cgi-bin/viewvc.cgi/tags/release_3_2_0/compiler/psystem.pas?view=markup#l114 <tt>compiler/psystem.pas</tt>] as part of the [[System unit|system unit]].
  
As of [[FPC]] 3.0.0 <syntaxhighlight lang="pascal" enclose="none">false</syntaxhighlight> and <syntaxhighlight lang="pascal" enclose="none">true</syntaxhighlight> are '''no longer''' [[Reserved words|reserved words]].
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As of [[FPC]] 3.0.0 <syntaxhighlight lang="pascal" inline>false</syntaxhighlight> and <syntaxhighlight lang="pascal" inline>true</syntaxhighlight> are '''no longer''' [[Reserved words|reserved words]].
 
Thus the following program is valid, compiles and is “usable”:
 
Thus the following program is valid, compiles and is “usable”:
  
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</syntaxhighlight>
 
</syntaxhighlight>
  
When [[Typecast|typecasting]] or interpreting any numeric value as a boolean value, it is important to know, that ''any'' non-zero value means <syntaxhighlight lang="pascal" enclose="none">true</syntaxhighlight> whilst only <syntaxhighlight lang="pascal" enclose="none">0</syntaxhighlight> (zero) is <syntaxhighlight lang="pascal" enclose="none">false</syntaxhighlight>.
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When [[Typecast|typecasting]] or interpreting any numeric value as a boolean value, it is important to know, that ''any'' non-zero value means <syntaxhighlight lang="pascal" inline>true</syntaxhighlight> whilst only <syntaxhighlight lang="pascal" inline>0</syntaxhighlight> (zero) is <syntaxhighlight lang="pascal" inline>false</syntaxhighlight>.
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Confer [http://pascal-central.com/iso7185.html#6.4.2.2%20Required%20simple-types ISO 7185 § “Required simple-types”].
  
 
== See also ==
 
== See also ==
  
 
* [[Boolean]]
 
* [[Boolean]]
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[[Category: Code]]

Revision as of 18:38, 9 June 2020

English (en)

The constants false and true are used to define the false and true conditions of a boolean variable. They are manifest constants that are defined as part of the standard data types the compiler initially knows about.

These constant values must be predefined by the compiler as there is no way to define them in terms of anything else. They are defined via compiler/psystem.pas as part of the system unit.

As of FPC 3.0.0 false and true are no longer reserved words. Thus the following program is valid, compiles and is “usable”:

program falseAndTrue(input, output, stderr);

const
	true = 42;

begin
	writeLn(true);                 // prints 42
	//writeLn(true and false);     // does not compile
	writeLn(system.true and false) // prints FALSE
end.

Internal value

program falseDemo(input, output, stderr);

uses
	typInfo;

begin
	writeLn(false);                            // prints FALSE
	
	// enumerative actions ------------------------------------------
	writeLn(ord(false));                       // prints 0
	writeLn(succ(false));                      // prints TRUE
	// next two statements generate out-of-range compile-time warnings
	writeLn(pred(false));                      // prints TRUE
	writeLn(succ(succ(false)));                // prints TRUE
	
	// data type ----------------------------------------------------
	writeLn(sizeOf(false));                    // prints 1
	writeLn(bitSizeOf(false));                 // prints 8
	writeLn(PTypeInfo(typeInfo(false))^.kind); // prints tkBool
	writeLn(PTypeInfo(typeInfo(false))^.name); // prints Boolean
end.

When typecasting or interpreting any numeric value as a boolean value, it is important to know, that any non-zero value means true whilst only 0 (zero) is false.

Confer ISO 7185 § “Required simple-types”.

See also