Enumerated type and subrange type
Ordinal type
The predefined types integer, char and boolean are so-called ordinal types.
There is a correspondence between an ordinal type and a range of natural integers.
Example: there is a correspondence between the char type and the range of natural integers [0 .. 255] representing the character codes in ASCII.
An ordinal type has the following properties:
- It is countable and ordered. This property allows to define functions and procedures operating on this type such as
ord,pred,succ,dec,inc. - Ordinal values admit a lower bound and an upper bound.
dec(c) : decrements c : assigns to c the predecessor of c.inc(c) : increments c : assigns to c the successor of c.
Enumerated type
The enumerated type is a user-defined ordinal data type.
Its declaration enumerates values in a list.
Syntax: declaring an enumerated type
type
identifier = (element1, element2, . . .);
Example: declaring an enumerated type
type
color = (green, yellow, red);
The values enumerated in the list are identifiers.
An enumerated type is ordinal, and the order in which values are listed in the type declaration determines their order in the type.
The enumerated values correspond to the values 0, 1, 2, etc. which define the order of the values in the list. The first value in the list always corresponds to the value 0.
So in the example above, odr(yellow) evaluates to 1.
Only assignment operators and relational operators are allowed on an enumerated type.
Subrange Type
The subrange type is a user-defined ordinal data type.
It allows to define an range of consecutive values of a host ordinal type.
This range of values is defined by a lower bound and an upper bound.
Syntax: declaring a subrange type
type
identifier = lower_bound .. upper_bound;
Example: declaring a subrange type
type
marks = 0 .. 10; { host type is integer }
digit = '0' .. '9' { host type is char }
A subrange type can be defined from a subset of a previously defined enumerated type.
type
month = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec);
spring = Apr .. Jun;