Enumerated types

The values of an enumerated type are taken from a finite (and usually small) set of values , which are enumerated in the type definition. Each value is represented by an identifier chosen by the programmer e.g.

    {red, orange, yellow, green, blue, indigo, violet}
or
	{Jack, Queen, King, Ace}

As for integer and characters, the ordering sequence of the set is defined and is given by the order in which the identifiers appear in the definition. Thus King > Queen has the value true. Each such value also has a corresponding integer value, which usually starts as zero for the first in the list (i.e. red==0 and Jack==0). Each successive value is one greater than the preceding one (e.g. blue==4 and King==2).

It is possible to alter this sequence by explicitly assigning values to some or all of the names. Thus:

    {StartV=30,SecV,ThirdV,FourV=40,FiveV}

would associate names to values as follows:

    StartV==30,SecV==31,ThirdV==32,FourV==40,FiveV==41

Variables can be declared using this construct as follows:

    enum Colours {red,orange,yellow,green,blue,indigo,violet};

Colours is a typename. It can be used to declare a variable which can (in principle) only be used to hold values defined in the list following. Its type is a sub-set of integer.

    enum Colours MyColour;

declares MyColour as a variable which can have values from the set given in the definition of Colour.

Example of use of enum.

#include <stdio.h>
    void main()
    {
        int Mark, Grade;
        char MedCertificate;

        scanf("%d",&Mark);   /*  Must be in range 0..100 */
        if (Mark<45) Grade = 0; else
        if (Mark<50) Grade = 1; else
        if (Mark<55) Grade = 2; else
        if (Mark<65) Grade = 3; else
        if (Mark<80) Grade = 4; else
                     Grade = 5;

        if((MedCertificate=getchar())=='y'&&Grade<5) Grade++;

        switch (Grade) {
case 0:        printf("Student failed\n"); break;
case 1:        printf("Student obtained ordinary\n"); break;
case 2:        printf("Student obtained third\n"); break;
case 3:        printf("Student obtained two two\n"); break;
case 4:        printf("Student obtained two one\n"); break;
case 5:        printf("Student obtained first\n"); break;
        }
    }

Plain text version to compile.

This is a fairly trivial example. The point to notice is that the numbers used to represent the grades do not (and, indeed, cannot) have any direct meaning in terms of those grades, other than expressing their ordering. It is only by finding the action (printf) corresponding to each in the case statement that some meaning can be inferred.

#include < stdio.h>
    void main()
    {
        enum DegClass {Fail,Ordinary,Third,TwoTwo,TwoOne,First};
        int Mark;
        enum DegClass Grade;
        char MedCertificate;

        scanf("%d",&Mark);   /*  Must be in range 0..100 */
        if (Mark<45) Grade = Fail; else
        if (Mark<50) Grade = Ordinary; else
        if (Mark<55) Grade = Third; else
        if (Mark<65) Grade = TwoTwo; else
        if (Mark<80) Grade = TwoOne; else
                     Grade = First;

        if((MedCertificate=getchar())=='y'&&Grade
Plain text version to compile.


The two examples are equivalent in their effect, but the second is more 
appropriate to the problem being considered.The use of meaningful names 
makes the second example much easier to read and to correct. The notion of 
ordering should not be confused with that of absolute arithmetic.



Exercises on this section.

Next - Making a typedef.

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