C Enums

An enumeration (enum) is a type whose values are a fixed list of names, such as the days of the week, the levels of a game or the states of a traffic light. Names make the code easier to read than plain numbers.

Declaring an enum

#include <stdio.h>

enum Level { LOW, MEDIUM, HIGH };

int main(void) {
    enum Level fan = MEDIUM;
    printf("LOW = %d, MEDIUM = %d, HIGH = %d\n", LOW, MEDIUM, HIGH);
    if (fan == MEDIUM) {
        printf("The fan is at medium speed.\n");
    }
    return 0;
}

LOW = 0, MEDIUM = 1, HIGH = 2
The fan is at medium speed.

Each name stands for a whole number. The first is 0 and each next one is 1 more, unless you choose the values yourself.

Choosing the values

#include <stdio.h>

enum Month { JAN = 1, FEB, MAR, APR };
enum HttpStatus { OK = 200, NOT_FOUND = 404, SERVER_ERROR = 500 };

int main(void) {
    printf("MAR is month %d\n", MAR);
    printf("Not found is %d\n", NOT_FOUND);
    return 0;
}

MAR is month 3
Not found is 404

Enums with switch

An enum and a switch fit together well. With -Wall the compiler even warns when a switch forgets one of the enum's names:

#include <stdio.h>

typedef enum { RED, YELLOW, GREEN } Light;

const char *action(Light l) {
    switch (l) {
        case RED:    return "stop";
        case YELLOW: return "get ready";
        case GREEN:  return "go";
    }
    return "?";
}

int main(void) {
    for (Light l = RED; l <= GREEN; l++) {
        printf("%d: %s\n", l, action(l));
    }
    return 0;
}

0: stop
1: get ready
2: go

Printing the names

C cannot print an enum's name directly; printf sees only the number. A common trick is an array of strings in the same order:

#include <stdio.h>

typedef enum { MON, TUE, WED, THU, FRI, SAT, SUN } Day;
static const char *dayNames[] = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};

int main(void) {
    Day today = WED;
    printf("Today is %s.\n", dayNames[today]);
    printf("The weekend starts on %s.\n", dayNames[SAT]);
    return 0;
}

Today is Wednesday.
The weekend starts on Saturday.

enum or #define?

A list of related constants could also be written with #define RED 0, #define YELLOW 1 and so on. An enum is usually better:

• the names are grouped under one type, so a function parameter of type Light tells the reader which values make sense;
• the compiler numbers the values for you, so adding one in the middle cannot create duplicates by mistake;
• the compiler can warn when a switch misses a value, and debuggers show the names.

Keep in mind that C does not stop you from storing any number in an enum variable: Light l = 7; compiles. The type documents your intention; it does not enforce it.

Practice exercises

1. Make an enum for the four seasons and print a sentence for each with a switch.
2. Make an enum for coffee sizes with prices as values (SMALL = 120, MEDIUM = 150, LARGE = 180) and add up an order.
3. Remove one case from the traffic light switch and read the warning.