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

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 constantvalid_age = 21;
Set type constantVowels = set of (A,E,I,O,U);
Pointer type constantP = NIL;
Real type constante = 2.7182818;
velocity_light = 3.0E+10;
Character type constantOperator = '+';
String type constantpresident = '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 the 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.



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