enum provides a way to create integer constants. For instance, in Java we might declare:

public static final int GOLD = 1;
public static final int SILVER = 2;
public static final int BRONZE = 3;

..but really what we want is to create a “medal” type that can take on the values GOLD, SILVER, and BRONZE. Moreover, we want GOLD to mean “1”, SILVER to mean “2”, and BRONZE to mean “3”.

(Note: Java does have enums available in its latest version, which are handled very similarly to how they are in C.)

Defining Enums

Here’s the format for defining an enum in C:

enum typeName {
    value1,
    value2,
    ...
    valueN
} objectList;

Here, typeName is the name of the new data type we’re creating. value1, value2, ..., valueN are the different values this type can have. Finally, objectList is a list of variables we want to create of type typeName. In this definition, typeName and objectList are both optional.

Here’s how we would represent medals using an enum:

enum medal {gold, silver, bronze};

Enum Variables

Now, the type of this enum is enum medal. All variables of this type can have a value of either gold, silver, or bronze. Here’s how to declare a variable:

enum medal medal100m;

And here’s how to give this variable the value gold:

medal100m = gold;

We can also use the possible values gold, silver, and bronze in comparisons:

if (medal100m == gold) {
    printf(“You won!\n”);
}

What’s Going On?

When you declare an enum type, what you’re really doing is making the enum values into integer constants. For example, in the medal enum, we’re really creating the integer constants:

gold = 0;
silver = 1;
bronze = 2;

So, for instance, to set medal100m to gold, we could also say:

medal100m = 0;

Then the test:

if (medal100m == gold) {...}

would still evaluate to true, since gold holds the value 0.

Changing Enum Values

Our medal example isn’t quite right. It has gold with the value 0, silver with 1, and bronze with 2. But we would like gold to have the value 1, silver 2, and bronze 3. Fortunately, there’s a way to do that:

enum medal {gold = 1, silver, bronze};

Setting gold to 1 resets the numbering in an enum, so that silver gets set to 2 and bronze to 3. We also could have explicitly set each value:

enum medal {gold = 1, silver = 2, bronze = 3};

Here’s another example:

enum example {val1, val2 = 9, val3, val4 = 12, val5};

Here’s how the integer values were assigned:

val1 = 0;
val2 = 9;
val3 = 10;
val4 = 12;
val5 = 13;

An enum tries to keep numbering in order, unless we explicitly set a value. Then, it uses that value to reset the numbering.

Examples

There is no bool type in C. However, we can create our own bool type using an enum:

enum bool {false, true};

(Notice that false has the value 0 and true has the value 1). Now we can use “bool variables” like we do in C#:

enum bool flag;
flag = true;
if (flag == true) {...}

We can also use an enum to represent grade levels in high school. We’ll start freshman out at 9, and continue the numbering from there:

enum grade {freshman = 9, sophomore, junior, senior};

Now we can use this type:

enum grade class;
class = freshman;
class = 10;         //same as sophomore

Enums inside Structs

Since an enum is a valid type, we can put an enum field inside a struct. For example, suppose we want to create a student struct that has a name and a grade level. Here’s how:

//define the enum type
enum grade {freshman = 9, sophomore, junior, senior};

//define the struct
struct student {
    char name[20];
    enum grade class;
};

This code works, but it’s overkill. Instead of defining the enum separately, we want to be able to define it inside the struct. In fact, ALL we want is a variable of that enum type – we don’t even care about naming the type. The enum name is optional, so we’ll leave it off. Here’s our revised definition:

struct student {
    char name[20];
    enum {freshman = 9, sophomore, junior, senior} class;
};

Now, suppose we want to create a student named Mary who is a junior. Here’s how:

struct student mary;
strcpy(mary.name, "Mary");
mary.class = junior;