|Fortran Language Reference Manual, Volume 1 - S-3692-51|
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An enumeration defines the name of a group of related values and the name of each value within the group. For example, the group COLOR and the colors within the group: RED, BLUE, and GREEN. The enumerator is a member of the group (for example, RED, BLUE, and GREEN). These names can enhance the readability and maintainability of your code.
In Fortran, an enumeration is an integer type-alias. The enumerator is a named integer constant of the same type. Since enumerators are all of the same integer type, kind values can be specified for them as a group. Hence, enumerators and integers of the same kind are interchangeable.
Fortran enumeration also allows you to access enumeration defined in C and use them as you would normally use a Fortran enumeration.
This is the form for enum_alias_def (enumerations):
These notes apply to enumerations:
kind_selector. If it is not specified, the compiler uses the default integer kind. For more information about valid kind values, see the Section 5.1.1 section.
type_alias_name is the name you assign to the group. This name is treated as a type alias name.
If named_constant is initialized, a double colon must appear after ENUMERATOR.
scalar_int_initialization_expr, see Section 18.104.22.168 for more information about initialization expressions.
The type_alias_name after END ENUM must be identical to the name in the enum_def_stmt.
If you declare an enumeration that uses the C interoperability feature (BIND(C)), the compiler ensures that the integer type that you select is compatible with the integer type used by the corresponding C enumeration. Refer toFortran Language Reference Manual, Volume 2 for more information about interoperable enumerations.
The format of an enumeration when a kind is specified is shown below:
ENUM kind_selector :: type_alias_name ENUMERATOR [ :: named_constant = scalar_int_initialization_expr ] ... END ENUM [ type_alias_name ]
Enumerators are treated as though they were declared with type type_alias_name and the PARAMETER attribute.
If you do not explicitly assign each enumerator a value, the compiler assigns a value according to these rules:
The first enumerator in the enumerator_list is assigned a zero value.
If the enumerator is not the first enumerator, it is given a value that is one greater than the preceding enumerator.
This example shows how to declare enumerations:
IMPLICIT NONE ENUM (KIND=1) :: COUNT ENUMERATOR :: ZERO, ONE, TWO END ENUM COUNT ENUM :: COLORS ENUMERATOR :: PURPLE ENUMERATOR :: RED = 2, BLUE = 5 ENUMERATOR GREEN END ENUM TYPE (COUNT) :: COUNTING INTEGER (KIND=1) :: NUMBER NUMBER = TWO
The enumeration definitions for COUNT and COLOR show that enumerators can be defined in a single statement or in multiple statements. The order of the enumerators are important if the values are not assigned explicitly. For example, if GREEN was defined first, its value would be zero rather than six.
The above example also shows that enumerators and integers of the same kind can be used interchangeably (NUMBER = TWO).
Enumerations are equivalent to type aliases of the same type specification. For example, these type alias statements are equivalent to the COUNT enumeration definition:
TYPEALIAS :: COUNT => INTEGER (KIND = 1) TYPE (COUNT), PARAMETER :: ZERO = 0, ONE = 1, TWO = 2 TYPE (COUNT) :: NUMBER