Constants

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 −

Real type constant

Sr.No	Constant Type & Examples
1	Ordinal(Integer)type constantvalid_age = 21;
2	Set type constantVowels = set of (A,E,I,O,U);
3	Pointer type constantP = NIL;
4	e = 2.7182818;velocity_light = 3.0E+10;
5	Character type constantOperator = ‘+’;
6	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.