C Struct Tutorial | typedef, enum, and Structure Arrays

What is a struct?

Up to now we have declared int, double, and char variables individually. But real-world data often comes as a set of related values — for example, name, age, and height of a person.

A struct lets you bundle several variables of different types into a single new type.

Without a struct...

        char name[20];
int age;
double height;
      

Add another person and you need name1, name2, ... — variables explode.

With a struct

        struct Person {
  char name[20];
  int age;
  double height;
};
      

Everything is bundled into one Person.

Defining and Using a struct

Structs are used in two steps: (1) define the type (the blueprint), then (2) declare a variable (make an instance).

    // (1) Define the struct type (blueprint)

    struct Student {

      char name[20];

      int score;

    };

    // (2) Declare variables and use them

    struct Student s1;

    strcpy(s1.name, "Tanaka");  // access with dot (.)

    s1.score = 85;

    // You can also initialize at declaration

    struct Student s2 = {"Sato", 92};

Members are accessed with the "dot operator (.)": s1.score means "the score field inside s1".

Struct Memory Layout

Members of a struct are laid out in memory in declaration order, consecutively.

Memory layout of struct Student s1;

name[20] takes 20 bytes, score takes 4 bytes → one Student uses at least 24 bytes (padding may apply).

Arrays of Structs

Making an array of a struct lets you manage many records with the same shape. This is where structs really shine.

    struct Student class[3] = {

      {"Tanaka", 85},

      {"Sato", 92},

      {"Suzuki", 78}

    };

    // Print every student's score

    for(int i = 0; i < 3; i++){

      printf("%s: %d points\n", class[i].name, class[i].score);

    }

You combine the array index with the dot operator, as in class[i].score. It reads naturally as "the i-th student's score."

Passing a struct to a Function

Structs can be passed by value (copied), but for large structs the copy cost is high, so passing by pointer is common.

    // Pass by value (the struct is copied)

    void printStudent(struct Student s){

      printf("%s: %d points\n", s.name, s.score);

    }

    // Pass by pointer (refers to the original)

    void addBonus(struct Student *sp, int bonus){

      sp->score += bonus; // equivalent to (*sp).score

    }

    addBonus(&s1, 5); // adds 5 points to s1.score

The arrow operator (->) is the special syntax to access members through a pointer. sp->score is the same as (*sp).score.

typedef — Giving a Type an Alias

With typedef you can give a type a clearer alias. It is especially handy with structs.

Basic usage

// An alias for unsigned int
typedef unsigned int uint;
uint score = 100;  // same as unsigned int score = 100;

Structs and typedef (important)

// Without typedef — you need "struct" every time
struct Point { int x, y; };
struct Point p1 = {3, 7};  // "struct" required

// With typedef — you can drop "struct"
typedef struct { int x, y; } Point;
Point p1 = {3, 7};           // cleaner

In practice, typedef'd structs are the norm. Your code is shorter and more readable.

Enum (enum) — A Group of Named Constants

With enum you can name and group related integer constants — a great way to avoid magic numbers.

enum Color { RED, GREEN, BLUE };  // RED=0, GREEN=1, BLUE=2

enum Color c = GREEN;
printf("%d\n", c);  // 1

Specifying values

enum Month {
    JAN = 1, FEB, MAR, APR, MAY, JUN,
    JUL, AUG, SEP, OCT, NOV, DEC
}; // FEB=2, MAR=3, ... DEC=12 (auto-incremented)

enum + switch

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

enum Direction dir = UP;
switch (dir) {
    case UP:    printf("Up\n");    break;
    case DOWN:  printf("Down\n");  break;
    case LEFT:  printf("Left\n");  break;
    case RIGHT: printf("Right\n"); break;
}

vs #define: You can also write #define RED 0, but an enum is treated as a type, so the compiler can help with type checks.

Method	Type check	Scope	Debugger display
`#define RED 0`	None	Whole file	Number only
`enum { RED }`	Yes	Controllable	Shown by name

Try It Yourself — Structs

Store three students in an array of structs and compute the average score.

struct.c

Output

Click Run...

Quick Quiz

Check your understanding of this lesson.

Q1. Which operator accesses a struct member?

-> (arrow) only

. (dot) only

Both . and -> (depending on context)

Access directly from a variable uses . (dot); access through a pointer uses -> (arrow).

Q2. What happens when you pass a struct to a function?

It is passed by reference

A copy of the value is passed

It is a compile error

In C, structs are passed by value (copied). For large structs, passing a pointer is more efficient.

Q3. What does typedef struct do?

Allocates the struct dynamically

Creates an alias for the struct type

Changes the struct's size

typedef gives a type an alias, letting you write Student s; instead of struct Student s;.

Structs (struct)

What is a struct?

Defining and Using a struct

Struct Memory Layout

Arrays of Structs

Passing a struct to a Function

typedef — Giving a Type an Alias

Basic usage

Structs and typedef (important)

Enum (enum) — A Group of Named Constants

Specifying values

enum + switch

Try It Yourself — Structs

Related lessons

Quick Quiz

Q1. Which operator accesses a struct member?

Q2. What happens when you pass a struct to a function?

Q3. What does typedef struct do?