C — Structures

Defining a struct

Group related variables under one name:

#include <stdio.h>

struct Point {
    int x;
    int y;
};

int main() {
    struct Point p1 = {10, 20};
    struct Point p2 = {.x = 5, .y = 15}; // Designated initializers

    printf("p1: (%d, %d)\n", p1.x, p1.y);
    printf("p2: (%d, %d)\n", p2.x, p2.y);

    p1.x = 30; // Modify field
    printf("Modified p1: (%d, %d)\n", p1.x, p1.y);

    return 0;
}

typedef for cleaner syntax

#include <stdio.h>

typedef struct {
    char name[50];
    int age;
    float height;
} Person;

int main() {
    Person alice = {"Alice", 30, 5.7f};
    Person bob;
    bob.age = 25;

    printf("%s is %d years old\n", alice.name, alice.age);
    printf("Bob is %d years old\n", bob.age);

    return 0;
}

Struct arrays

#include <stdio.h>

typedef struct {
    char name[30];
    int score;
} Student;

int main() {
    Student students[] = {
        {"Alice", 95},
        {"Bob", 87},
        {"Charlie", 92}
    };
    int count = sizeof(students) / sizeof(students[0]);

    // Find highest score
    int maxIdx = 0;
    for (int i = 1; i < count; i++) {
        if (students[i].score > students[maxIdx].score) {
            maxIdx = i;
        }
    }
    printf("Top student: %s (%d)\n",
           students[maxIdx].name, students[maxIdx].score);

    return 0;
}

Nested structs

#include <stdio.h>

typedef struct {
    char street[50];
    char city[30];
    int zip;
} Address;

typedef struct {
    char name[50];
    int age;
    Address address;
} Person;

int main() {
    Person p = {
        "Alice",
        30,
        {"123 Main St", "Springfield", 62701}
    };

    printf("%s, age %d\n", p.name, p.age);
    printf("Lives at: %s, %s %d\n",
           p.address.street, p.address.city, p.address.zip);

    return 0;
}

Passing structs

#include <stdio.h>

typedef struct {
    float x, y;
} Point;

// Pass by value (copy)
void printPoint(Point p) {
    printf("(%.1f, %.1f)\n", p.x, p.y);
}

// Pass by pointer (efficient)
void movePoint(Point *p, float dx, float dy) {
    p->x += dx;
    p->y += dy;
}

// Return by value
Point createPoint(float x, float y) {
    return (Point){x, y};
}

int main() {
    Point p = createPoint(1.0f, 2.0f);
    printPoint(p);     // (1.0, 2.0)

    movePoint(&p, 3.0f, 4.0f);
    printPoint(p);     // (4.0, 6.0)

    return 0;
}

Unions

Share memory between different types:

#include <stdio.h>

typedef union {
    int integer;
    float decimal;
    char character;
} Data;

int main() {
    Data d;
    d.integer = 42;
    printf("int: %d\n", d.integer);

    d.decimal = 3.14f;
    printf("float: %.2f\n", d.decimal);
    printf("int (overwritten): %d\n", d.integer); // Garbage

    printf("Size: %zu bytes\n", sizeof(Data)); // 4

    return 0;
}

Enums

Named integer constants:

#include <stdio.h>

typedef enum {
    MONDAY = 1,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
} Day;

typedef enum {
    RED,
    GREEN,
    BLUE
} Color;

int main() {
    Day today = WEDNESDAY;
    Color favorite = BLUE;

    printf("Today: day %d\n", today);    // 3
    printf("Favorite color: %d\n", favorite); // 2

    return 0;
}

Bit fields

Pack multiple values into a single integer:

#include <stdio.h>

typedef struct {
    unsigned int isActive : 1;  // 1 bit
    unsigned int priority : 3;  // 3 bits
    unsigned int userId : 12;   // 12 bits
} Task;

int main() {
    Task t = {.isActive = 1, .priority = 5, .userId = 42};

    printf("Active: %u\n", t.isActive);     // 1
    printf("Priority: %u\n", t.priority);   // 5
    printf("User ID: %u\n", t.userId);      // 42
    printf("Size: %zu bytes\n", sizeof(Task)); // 4

    return 0;
}

Self-referential structs

Build linked lists and trees:

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node *next;
} Node;

Node *createNode(int data) {
    Node *node = malloc(sizeof(Node));
    node->data = data;
    node->next = NULL;
    return node;
}

void printList(Node *head) {
    Node *current = head;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->next;
    }
    printf("NULL\n");
}

void freeList(Node *head) {
    Node *temp;
    while (head != NULL) {
        temp = head;
        head = head->next;
        free(temp);
    }
}

int main() {
    Node *head = createNode(1);
    head->next = createNode(2);
    head->next->next = createNode(3);

    printList(head); // 1 -> 2 -> 3 -> NULL
    freeList(head);

    return 0;
}

Mini Practice

Write C code that:

1. Creates a struct Book with title, author, and pages
2. Passes a struct to a function by pointer to modify it
3. Creates an array of structs and sorts by one field
4. Implements a simple linked list with Node structs

Up Next

In the next lesson, you'll learn about File Handling — reading and writing files in C.