Enums

Sometimes we have a variable that can only be one of a few named values: a day of the week, a direction, a job state. We could use ints (0 = UP, 1 = DOWN...) but that's unreadable. Enums give names to these constants

#include <stdio.h>

enum Direction { UP, DOWN, LEFT, RIGHT };

int main(void) {
    enum Direction d = UP;

    if (d == UP) {
        printf("Going up!\n");
    }
    return 0;
}

Behind the scenes, UP is 0, DOWN is 1, LEFT is 2, RIGHT is 3. The compiler assigns numbers automatically starting from 0. But we write UP instead of 0, which makes the code much clearer

We can pick our own values if we want

#include <stdio.h>

enum ExitCode {
    SUCCESS = 0,
    READ_ERROR = 1,
    MEMORY_ERROR = 2
};

int main(void) {
    enum ExitCode code = READ_ERROR;
    printf("Exit code: %d\n", code);   // Exit code: 1
    return 0;
}

Enums work great with switch

#include <stdio.h>

enum Color { RED, GREEN, BLUE };

void print_color(enum Color c) {
    switch (c) {
        case RED:   printf("Red\n");   break;
        case GREEN: printf("Green\n"); break;
        case BLUE:  printf("Blue\n");  break;
    }
}

int main(void) {
    print_color(RED);
    print_color(GREEN);
    print_color(BLUE);
    return 0;
}

This is much better than if (c == 0): anyone reading the code knows exactly what RED means

A classic use: the states of a job in the processing queue

#include <stdio.h>

enum JobState { QUEUED, RUNNING, SUSPENDED, DONE };

int main(void) {
    enum JobState state = QUEUED;
    // the operator starts the job
    state = RUNNING;
    // the operator suspends it for a priority job
    state = SUSPENDED;
    printf("%d\n", state);
    return 0;
}

Instead of remembering "was 2 suspended or done?", we simply use the names. The code reads almost like plain English

Mission: The Maintenance Calendar

The computing center runs a quarterly maintenance cycle, one round per season. The operating console needs a function that translates each season code into a readable name on the teletype screen.

1. Define enum Season with values SPRING, SUMMER, AUTUMN, WINTER
2. Write the function void print_season(enum Season s) with a switch statement that prints the season's name
3. In main, call the function for all four seasons, in order

Example

Your program should print

Spring
Summer
Autumn
Winter