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'); 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.