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Pascal - Constants

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A constant is an entity that remains unchanged during program execution. Pascal allows only constants of the following types to be declared:

  • Ordinal types
  • Set types
  • Pointer types (but the only allowed value is Nil).
  • Real types
  • Char
  • String

 

1. Declaring Constants

Syntax for declaring constants is as follows:

const
identifier = constant_value;
 

The following table provides examples of some valid constant declarations:

Constant TypeExamples
Ordinal(Integer)type constant valid_age = 21;
Set type constant Vowels = set of (A,E,I,O,U);
Pointer type constant P = NIL;
Real type constant e = 2.7182818;
velocity_light = 3.0E+10;
Character type constant Operator = '+';
String type constant president = 'Johnny Depp';

 

The following example illustrates the concept:

program const_circle (input,output);
const
PI =3.141592654;
var
r, d, c : real;   //variable declaration: radius, dia, circumference
begin
   writeln('Enter the radius of the circle');
   readln(r);
   d :=2* r;
   c :=  PI * d;
   writeln('The circumference of the circle is ',c:7:2);
end.
 

When the above code is compiled and executed, it produces following result:

Enter the radius of the circle
23
The circumference of the circle is 144.51

Observe the formatting in the output statement of the program. The variable c is to be formatted with total number of digits 7 and 2 digits after the decimal sign. Pascal allows such output formatting with the numerical variables.