Difference between revisions of "false and true"

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(True and False ar NOT reserved words in fpc.)
(past tense statement concerning reserved words)
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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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In [[FPC]] 3.0.0 they were <!-- documented as --> [[Reserved word|reserved words]], but they are not anymore.
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Thus the following program is valid, compiles and is “usable”:
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<syntaxhighlight lang="pascal">
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program falseAndTrue(input, output, stderr);
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 +
const
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true = 42;
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begin
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writeLn(true);                // prints 42
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//writeLn(true and false);    // does not compile
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writeLn(system.true and false) // prints FALSE
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end.
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</syntaxhighlight>
  
 
== internal value ==
 
== internal value ==

Revision as of 23:19, 4 May 2018

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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.

In FPC 3.0.0 they were reserved words, but they are not anymore. 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.