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The data type, enum (for enumeration) also allows improvement in program clarity by specifying a list of names, the enumeration constants, which are associated with constant integer values. It is similar to using #define directives to define constant integer values for a set of symbolic names; however, with enum the compiler can generate the values for you, and may check for proper use of enum type variables. A variable of enum type is declared as follows:
enum { FALSE, TRUE } flag;
The variable, flag, is defined here to be of a type
which can take on the two enumerated
constant values, FALSE and TRUE. Normally,
enumeration constants are identifiers whose values start at zero and increase
in sequence: here, FALSE is 0, and TRUE is 1.
However, the
enumeration can have explicit constant values specified in the enumeration:
enum { SUN = 1, MON, TUE, WED, THU, FRI, SAT } day;
Here, SUN is associated with value 1, and
the rest of the names have values in
increasing sequence: MON is 2, TUE is 3,
and so on until SAT which is 7.
The variable, day can hold any of the enumerated values.
An enumeration type can be given a tag, i.e a name which can be used later to declare variables of that tagged enumeration type. For example, we can name an enumeration:
enum boolean { FALSE, TRUE };
where the name boolean can then be used to declare variables of that
enumeration type:
enum boolean flag1, flag2;This declaration defines variables, flag1 and flag2, which are of a type specified by the boolean enumeration; that is, flag1 and flag2 can have values FALSE or TRUE. It is also possible to specify a tag and declare variables in the same declaration:
enum boolean {FALSE, TRUE} done;
enum boolean found;
The first declaration specifies a tag, boolean,
for the enumeration as well as declaring a
variable, done of this type.
The second declaration defines a variable, found, of the
enumeration boolean type.
Here is a function, digitp(), that returns a boolean
value to the calling function.
(The calling function must also declare the
enumeration in order to use the returned value correctly).
enum boolean { FALSE, TRUE };
enum boolean digitp(char c)
{
if (c >= '0' && c <= '9')
return TRUE;
else
return FALSE;
}
Remember, the value of an enum type variable is an integer. An enumerated data type is primarily a convenience for writing the source code; information about the symbolic names are not retained at run time. For example, if we were to execute a statement:
printf("digitp returns %d\n",digitp('0'));
it would print