Enumerated type
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In computer programming, an enumerated type is an abstract data type, each of whose values take on exactly one of a finite set of identifiers chosen by the programmer. Enumerated types are typically used for categorical variables (like the suit of a card), which do not have any numerical order. At run-time, enumerated types are often implemented using integers (each identifier has a distinct integer value).
However, compared to using just integers, enumerated types make program source more self-documenting than the use of explicit magic numbers. Depending on the language, the integer representation may not be visible to the programmer; this prevents the programmer from doing absurd things, like performing arithmetic, on enumeration values.
In some languages, the boolean type of truth values is considered a predeclared enumerated type of two values.
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[edit] Pascal
In Pascal, an enumerated type can be implicitly declared by listing the values in a parenthesised list:
var suit: (clubs, diamonds, hearts, spades);
The declaration will often appear in a type synonym declaration, such that it can be used for multiple variables:
type
cardsuit = (clubs, diamonds, hearts, spades);
card = record
suit: cardsuit;
value: 1 .. 13;
end;
var
hand: array [ 1 .. 13 ] of card;
trump: cardsuit;
The order in which the enumeration values are given matters. An enumerated type is an ordinal type, and the prec and succ functions will give the prior or next value of the enumeration, and ord can convert enumeration values to their integer representation. Standard Pascal does not offer a conversion from arithmetic types to enumerations, however. Extended Pascal offers this functionality via an extended succ function. Some other Pascal dialects allow it via type-casts.
[edit] C and syntactically similar languages
The original K&R dialect of the C programming language did not have enumerated types, but they were added in the ANSI standard for C, which became C89. In C, enumerations are created by explicit definitions, which use the enum keyword and are reminiscent of struct and union definitions:
enum cardsuit {
CLUBS,
DIAMONDS,
HEARTS,
SPADES
};
struct card {
enum cardsuit suit;
short int value;
} hand[13];
enum cardsuit trump;
C exposes the small-integer representation of enumeration values directly to the programmer. Integers and enum values can be mixed freely, and all arithmetic operations on enum values are permitted. It is even possible for an enum variable to hold an integer that does not represent any of the enumeration values. In fact, according to the language definition, the above code will define CLUBS, DIAMONDS, HEARTS, and SPADES as constants of type int, which will only be converted (silently) to enum cardsuit if and when they are stored in a variable of that type.
C also allows the programmer to choose the values of the enumeration constants explicitly. For example,
enum cardsuit {
CLUBS = 1,
DIAMONDS = 2,
HEARTS = 4,
SPADES = 8
};
could be used to define a type that allows mathematical sets of suits to be represented as an enum cardsuit by bitwise logic operations.
Typeless languages in the syntactic tradition of C (e.g. perl or JavaScript) do not in general provide enumerations.
[edit] C++
C++ has enumeration types that are directly inherited from C's and work mostly like these, except that an enumeration is a real type in C++, so the "enum" keyword is only used when declaring the enumeration, but the name of the enumeration properly refers to the type thereafter.
[edit] Java
The J2SE version 5.0 of the Java programming language added enumerated types whose declaration syntax is similar to that of C's:
enum Cardsuit { Clubs, Diamonds, Spades, Hearts }
...
Cardsuit trump ;
The Java type system, however, treats enumerations as a type separate from integers, and intermixing of enum and integer values is not allowed (except that you can retrieve the integer out of an enum value using the .ordinal() method). In fact, an enum type in Java is actually a special compiler-generated class rather than an arithmetic type, and enum values behave as global pre-generated instances of that class. Enum types can have instance methods and a constructor (the arguments of which can be specified separately for each enum value). All enum types implicitly extend the Enum abstract class. You are not allowed to instantiate an enum type directly.
[edit] C#
Enumerated types in the C# programming language preserve most of the "small integer" semantics of C's enums. Some arithmetic operations are not defined for enums, but an enum value can be explicitly converted to an integer and back again, and an enum variable can store values that were not declared by the enum definition. For example, given
enum Cardsuit { Clubs, Diamonds, Spades, Hearts };
the expressions Diamonds + 1 and Hearts - Clubs are allowed directly (because it makes sense to step through the sequence of values or ask how many steps there are between two values), but Hearts * Spades is deemed to make less sense and is only allowed if the values are first converted to integers.
[edit] VBA
Enumerated datatypes in VBA are automatically assigned the "Integer" datatype and also become a datatype themselves:
Option Explicit
Enum MyEnumeratedType
myType1 = 1
myType2 = -1
End Enum
Sub EnumExample()
Dim a As MyEnumeratedType
a = myType1
MsgBox a
End Sub
[edit] Algebraic data type in functional programming
In functional programming languages in the ML lineage (e.g., SML, OCaml and Haskell), an algebraic data type with only nullary constructors can be used to implement an enumerated type. For example (in the syntax of SML signatures):
datatype cardsuit = Clubs | Diamonds | Hearts | Spades
type card = { suit: cardsuit; value: int }
val hand : card list
val trump : cardsuit
In these languages the small-integer representation is completely hidden from the programmer, if indeed such a representation is employed by the implementation. However, Haskell has the Enum type class which a type can derive or implement to get a mapping between the type and Int.
[edit] Lisp
Common Lisp uses the member type specifier, e.g.
(deftype cardsuit ()
'(member club diamond heart spade))
which states that object is of type cardsuit if it is #'eql to club, diamond, heart or spade. The member type specifier is not valid as a CLOS parameter specializer, however. Instead, (eql atom), which is the equivalent to (member atom) may be used (that is, only one member of the set may be specified with an eql type specifier, however, it may be used as a CLOS parameter specializer.) In other words, in order to define methods to cover an enumerated type, a method must defined for each specific element of that type.
Additionally,
(deftype finite-element-set-type (&rest elements)
`(member ,@elements))
may be used to define arbitrary enumerated types at runtime. For instance
(finite-element-set-type club diamond heart spade)
would refer to a type equivalent to the prior definition of cardsuit, as of course would simply have been using
(member club diamond heart spade)
but may be less confusing with the function #'member for stylistic reasons.
[edit] Databases
Some databases support enumerated types directly. MySQL provides an enumerated type ENUM with allowable values specified as strings when a table is created. The values are stored as numeric indexes with the empty string stored as 0, the first string value stored as 1, the second string value stored as 2, etc. Values can be stored and retrieved as numeric indexes or string values.
