Unions and Enumerations

🏠 The Magic Toolbox: Unions & Enumerations in C

Imagine you have a special toolbox. But this isn’t just any toolbox—it’s a shape-shifting one!

🎯 What You’ll Learn

• Unions – One space, many faces
• Structure vs Union – Why choose one over the other?
• Enumerations – Giving names to numbers
• typedef – Creating your own nicknames

📦 Chapter 1: Unions — The Shape-Shifting Box

What is a Union?

Think of a union like a magic box that can hold different things—but only ONE thing at a time.

Real-Life Example:

• You have a lunchbox
• It can hold a sandwich OR an apple OR a juice box
• But NOT all three at once!
• The box is always the same size

union LunchBox {
    char sandwich[20];
    int apple_count;
    float juice_ml;
};

How Unions Work

union LunchBox myLunch;

myLunch.apple_count = 3;
// Now the box holds apples!

myLunch.juice_ml = 250.5;
// Poof! Apples gone, now juice!

The Magic Rule: When you put something new in, the old thing disappears. The box only remembers the last thing you put in.

Why Use Unions?

Memory Saver! 🧠

union SmallBox {
    int number;     // 4 bytes
    char letter;    // 1 byte
    float decimal;  // 4 bytes
};
// Total size: 4 bytes (biggest member)

The union is only as big as its largest member. Smart, right?

⚔️ Chapter 2: Structure vs Union — The Big Showdown

The Key Difference

Think of it this way:

Structure: A House with Many Rooms

struct House {
    int bedroom;     // 4 bytes
    char kitchen;    // 1 byte
    float bathroom;  // 4 bytes
};
// Total: 9+ bytes (each has own space)

Union: A Room That Transforms

union MagicRoom {
    int bedroom;     // 4 bytes
    char kitchen;    // 1 byte
    float bathroom;  // 4 bytes
};
// Total: 4 bytes (they share!)

Visual Comparison

Structure Memory:
┌─────────┬─────────┬─────────┐
│ bedroom │ kitchen │bathroom │
└─────────┴─────────┴─────────┘
  4 bytes   1 byte   4 bytes

Union Memory:
┌─────────────────────────────┐
│  bedroom/kitchen/bathroom   │
└─────────────────────────────┘
         4 bytes total

When to Use What?

• Structure: When you need ALL data together (like a person’s name AND age AND height)
• Union: When you need ONLY ONE value at a time (like a value that’s either an integer OR a float)

🏷️ Chapter 3: Enumerations — Names for Numbers

The Problem

int day = 3;  // What day is this? 🤔

Is 3 Tuesday? Wednesday? We don’t know!

The Solution: enum

enum Weekday {
    SUNDAY,    // = 0
    MONDAY,    // = 1
    TUESDAY,   // = 2
    WEDNESDAY, // = 3
    THURSDAY,  // = 4
    FRIDAY,    // = 5
    SATURDAY   // = 6
};

Now we can write:

enum Weekday today = WEDNESDAY;
// Much clearer! 🎉

Custom Starting Values

enum Months {
    JAN = 1,   // Start from 1!
    FEB,       // = 2
    MAR,       // = 3
    APR        // = 4
};

Why Enums Are Awesome

1. Readable Code 📖

   // Confusing:
   if (status == 2) { }
   
   // Clear:
   if (status == SUCCESS) { }
   

2. Prevents Mistakes 🛡️

   • You can’t accidentally use wrong numbers

3. Easy to Change 🔧

   • Update enum, all code updates automatically

Practical Example

enum TrafficLight {
    RED,
    YELLOW,
    GREEN
};

enum TrafficLight signal = RED;

if (signal == RED) {
    printf("Stop!");
} else if (signal == GREEN) {
    printf("Go!");
}

✨ Chapter 4: typedef — Creating Nicknames

What is typedef?

typedef lets you create shorter names for types. Like a nickname!

Real Life:

• Your friend “Christopher” might be called “Chris”
• Same person, shorter name!

Basic typedef

typedef unsigned long int BigNumber;

// Now instead of:
unsigned long int population;

// You can write:
BigNumber population;

typedef with Structures

Without typedef:

struct Student {
    char name[50];
    int age;
};

struct Student alice;  // Must say "struct"

With typedef:

typedef struct {
    char name[50];
    int age;
} Student;

Student alice;  // Cleaner! No "struct"

typedef with Unions

typedef union {
    int whole;
    float decimal;
} Number;

Number value;
value.whole = 42;

typedef with Enums

typedef enum {
    SMALL,
    MEDIUM,
    LARGE
} Size;

Size shirtSize = MEDIUM;

Why Use typedef?

🎨 Putting It All Together

Here’s a real example combining everything:

// Create nickname for enum
typedef enum {
    CIRCLE,
    SQUARE,
    TRIANGLE
} ShapeType;

// Union to store different shape data
typedef union {
    float radius;        // for circle
    float side;          // for square
    float base, height;  // for triangle
} ShapeData;

// Structure combining type and data
typedef struct {
    ShapeType type;
    ShapeData data;
} Shape;

// Usage
Shape myShape;
myShape.type = CIRCLE;
myShape.data.radius = 5.0;

🧠 Quick Memory Tips

UNION = One room, many uses
        (saves space, one at a time)

STRUCT = Many rooms, one house
         (all data together)

ENUM = Names for numbers
       (readable, safe)

TYPEDEF = Nicknames for types
          (shorter, cleaner)

🎯 Key Takeaways

1. Union shares memory—only use one member at a time
2. Structure gives each member its own space
3. Enum makes code readable with named constants
4. typedef creates shorter, cleaner type names

You did it! 🎉 Now you know how to create your own custom types in C!

graph LR
    A["User-Defined Types"] --> B["Union"]
    A --> C["Enum"]
    A --> D["typedef"]
    B --> E["Shares Memory"]
    B --> F["One Value at Time"]
    C --> G["Named Constants"]
    C --> H["Starts at 0"]
    D --> I["Type Nicknames"]
    D --> J["Cleaner Code"]