Lesson 127 min read

Structs & Typedef

A struct is like a custom form — you design the fields, C fills them in.

Designing Your Own Data Type

Imagine you're building a student database. Each student has a name, an age, and a GPA. You could juggle three separate arrays — one for names, one for ages, one for GPAs — but that's like storing a person's ID, photo, and address in three different filing cabinets. Messy.

A struct lets you bundle related data into a single package. Think of it as designing a custom form: you pick the fields, and C lets you stamp out as many copies as you need.

Defining and Using a Struct

#include <stdio.h>
#include <string.h>

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

int main() {
    struct Student alice;
    strcpy(alice.name, "Alice");
    alice.age = 20;
    alice.gpa = 3.85;

    printf("Name: %s\n", alice.name);
    printf("Age:  %d\n", alice.age);
    printf("GPA:  %.2f\n", alice.gpa);
    return 0;
}

Output

Name: Alice
Age:  20
GPA:  3.85

typedef — The Shorthand

Typing struct Student everywhere gets old fast. typedef lets you create an alias so you can just write Student instead. It's like giving your form a short name for the label maker.

typedef Makes Life Easier

#include <stdio.h>
#include <string.h>

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

int main() {
    // No need to write "struct Student" anymore!
    Student bob;
    strcpy(bob.name, "Bob");
    bob.age = 22;
    bob.gpa = 3.42;

    printf("%s is %d years old with a %.2f GPA\n",
           bob.name, bob.age, bob.gpa);
    return 0;
}

Output

Bob is 22 years old with a 3.42 GPA

Dot (.) vs Arrow (->)

When you have a struct variable, use the dot operator to access fields: alice.name. But when you have a pointer to a struct, use the arrow operator: ptr->name.

The arrow is just shorthand for (*ptr).name — dereferencing the pointer first, then accessing the field. But the arrow is much cleaner.

Pointer to Struct with Arrow Operator

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

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

void birthday(Person *p) {
    p->age++;  // Arrow operator for pointer-to-struct
    printf("Happy birthday, %s! You're now %d.\n",
           p->name, p->age);
}

int main() {
    Person john;
    strcpy(john.name, "John");
    john.age = 29;

    printf("Before: %s is %d\n", john.name, john.age);
    birthday(&john);  // Pass pointer to avoid copying
    printf("After:  %s is %d\n", john.name, john.age);

    return 0;
}

Output

Before: John is 29
Happy birthday, John! You're now 30.
After:  John is 30

Note: When you pass a struct to a function, C copies the ENTIRE struct — every byte of it. For a struct with a 1000-char array inside, that's 1000+ bytes copied on every call. For large structs, always pass a pointer instead.

Nested Structs

Structs can contain other structs — like a form that has a sub-section. An Address struct inside a Person struct keeps things organized.

Nested Structs

#include <stdio.h>
#include <string.h>

typedef struct {
    char street[100];
    char city[50];
    int zip;
} Address;

typedef struct {
    char name[50];
    Address home;  // Struct inside a struct
} Person;

int main() {
    Person p;
    strcpy(p.name, "Carol");
    strcpy(p.home.street, "123 Main St");
    strcpy(p.home.city, "Springfield");
    p.home.zip = 62704;

    printf("%s lives at %s, %s %d\n",
           p.name, p.home.street,
           p.home.city, p.home.zip);
    return 0;
}

Output

Carol lives at 123 Main St, Springfield 62704

Array of Structs

Need a class roster? An inventory list? Just make an array of structs. Each element is a complete record — like a row in a spreadsheet.

Array of Structs

#include <stdio.h>
#include <string.h>

typedef struct {
    char name[50];
    int score;
} Player;

int main() {
    Player leaderboard[3];

    strcpy(leaderboard[0].name, "Alice");
    leaderboard[0].score = 950;

    strcpy(leaderboard[1].name, "Bob");
    leaderboard[1].score = 870;

    strcpy(leaderboard[2].name, "Carol");
    leaderboard[2].score = 920;

    printf("=== Leaderboard ===\n");
    for (int i = 0; i < 3; i++) {
        printf("%d. %-10s %d pts\n",
               i + 1, leaderboard[i].name,
               leaderboard[i].score);
    }
    return 0;
}

Output

=== Leaderboard ===
1. Alice      950 pts
2. Bob        870 pts
3. Carol      920 pts

Note: Structs in C don't have methods (functions inside them) like classes in C++ or Java. If you want a function that operates on a struct, pass the struct (or a pointer to it) as a parameter.

Challenge

Quick check

What does typedef do when used with a struct?

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