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# Pascal – 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 constant

valid_age = 21;

2 Set type constant

Vowels = set of (A,E,I,O,U);

3 Pointer type constant

P = NIL;

4 e = 2.7182818;

velocity_light = 3.0E+10;

5 Character type constant

Operator = ‘+’;

6 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');

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.