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In computer programming, an enumerated type (also called enumeration or enum, or factor in the R programming language, and a categorical variable in statistics) is a data type consisting of a set of named values called elements, members or enumerators of the type. The enumerator names are usually identifiers that behave as constants in the language. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value. In other words, an enumerated type has values that are different from each other, and that can be compared and assigned, but which are not specified by the programmer as having any particular concrete representation in the computer's memory; compilers and interpreters can represent them arbitrarily. For example, the four suits in a deck of playing cards may be four enumerators named ''CLUB'', ''DIAMOND'', ''HEART'', ''SPADE'', belonging to an enumerated type named ''suit''. If a variable ''V'' is declared having ''suit'' as its data type, one can assign any of those four values to it. Although the enumerators are usually distinct, some languages may allow the same enumerator to be listed twice in the type's declaration. The names of enumerators need not be semantically complete or compatible in any sense. For example, an enumerated type called ''color'' may be defined to consist of the enumerators ''RED'', ''GREEN'', ''ZEBRA'', ''MISSING'', and ''BACON''. In some languages, the declaration of an enumerated type also intentionally defines an ordering of its members; in others, the enumerators are unordered; in others still, an implicit ordering arises from the compiler concretely representing enumerators as integers. Some enumerator types may be built into the language. The Boolean type, for example is often a pre-defined enumeration of the values ''FALSE'' and ''TRUE''. Many languages allow the user to define new enumerated types. Values and variables of an enumerated type are usually implemented as fixed-length bit strings, often in a format and size compatible with some integer type. Some languages, especially system programming languages, allow the user to specify the bit combination to be used for each enumerator. In type theory, enumerated types are often regarded as tagged unions of unit types. Since such types are of the form , they may also be written as natural numbers. ==Rationale== Some early programming languages did not originally have enumerated types. If a programmer wanted a variable, for example ''myColor'', to have a value of red, the variable red would be declared and assigned some arbitrary value, usually an integer constant. The variable red would then be assigned to ''myColor''. Other techniques assigned arbitrary values to strings containing the names of the enumerators. These arbitrary values were sometimes referred to as magic numbers since there often was no explanation as to how the numbers were obtained or whether their actual values were significant. These magic numbers could make the source code harder for others to understand and maintain. Enumerated types, on the other hand, made the code more self-documenting. Depending on the language, the compiler could automatically assign default values to the enumerators thereby hiding unnecessary detail from the programmer. These values may not even be visible to the programmer (see information hiding). Enumerated types can also prevent a programmer from writing illogical code such as performing mathematical operations on the values of the enumerators. If the value of a variable that was assigned an enumerator were to be printed, some programming languages could also print the name of the enumerator rather than its underlying numerical value. A further advantage is that enumerated types can allow compilers to enforce semantic correctness. For instance: can be forbidden, whilst is accepted, even if ''TRIANGLE'' and ''RED'' are both internally represented as ''1''. Conceptually, an enumerated type is similar to a list of nominals, since each possible value of the type is assigned a distinctive natural number. A given enumerated type is thus a concrete implementation of this notion. When order is meaningful and/or used for comparison, then an enumerated type becomes an ordinal type. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「enumerated type」の詳細全文を読む スポンサード リンク
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