Module 34 · Interactive lesson

Structures

Your own record types

A structure bundles related values — a name, an age, some marks — into one variable of a type you define yourself. In the lab below you create and copy a struct, keep a whole class of students in an array of structures, and look inside memory to see where each member sits and why sizeof is sometimes bigger than you expect.

structgroups values of different types
p.xone member of p
sizeofmay include padding

x y

main.c

Ready

Statements run 0 Lines printed 0

Ready

Keyboard: ← → step · Space play / pause · R reset · Esc exit classroom mode

How it works

The ideas behind the lab

Exactly what you watched happen, in plain steps.

Define a type, then make variables of it

struct Name { members }; defines a new type — a blueprint that lists the members and their types. It uses no memory by itself. Then struct Name v; creates a variable with all those members side by side. Reach a member with the dot: v.member. Structs can be initialised with { }, copied with =, put in arrays and nested inside other structs.

struct Point {          // the type (no memory yet)
    int x;
    int y;
};                      // note the semicolon!

struct Point p = {3, 4};   // a variable: 8 bytes
p.x = 10;                  // one member
struct Point q = p;        // copies both members
struct Point pts[10];      // an array of 10 points

struct S { char c; int i; char d; } — where the members go
Member	Size	Offset	Why
char c	1	0	The first member starts at offset 0
(padding)	3	1–3	int must start at a multiple of 4
int i	4	4
char d	1	8
(padding)	3	9–11	The size is rounded up to a multiple of 4
sizeof(struct S)	12	—	Reordered as int, char, char it is only 8

Defining and creating

struct Student { char name[20]; int age; double gpa; }; — end with a semicolon.
The definition usually goes above main, so every function can use the type.
struct Student s; creates one variable; its members are garbage until set.
Initialise in order: struct Student s = {"Ann", 19, 8.5};
Or by name (C99): struct Student s = {.age = 19, .name = "Ann"};

Using members

s.age = 20; — the dot selects one member.
A member is an ordinary variable: s.age++, scanf("%d", &s.age).
Text members are char arrays: use strcpy(s.name, "Bob"), not =.
Whole structs can be assigned: t = s; copies every member, arrays too.
Structs cannot be compared with == — compare the members one by one.

Arrays, nesting and size

struct Student cls[30]; — cls[i].age is the age of student i.
A member can be another struct: b.published.year.
Members are placed in order, each at a multiple of its own size (alignment).
The gaps are padding; the total is rounded up too.
Ordering members from largest to smallest often saves memory. Always use sizeof.

Walkthrough

Books with a nested date

Every line, explained.

1 #include <stdio.h>

2 #include <string.h>

4 struct Date {

5 int day, month, year;

6 };

8 struct Book {

9 char title[30];

10 double price;

11 struct Date published; A struct inside a struct.

12 };

14 int main(void)

15 {

16 struct Book b = {"C Basics", 299.50, {15, 8, 2024}}; Nested braces for the nested struct.

17 struct Book shelf[2]; An array of structs.

18 shelf[0] = b; Copies the whole book, title included.

19 strcpy(shelf[1].title, "Pointers"); A char array member needs strcpy.

20 shelf[1].price = 349.00;

21 shelf[1].published.day = 1; Two dots: book → date → day.

22 shelf[1].published.month = 1;

23 shelf[1].published.year = 2025;

24 for (int i = 0; i < 2; i++)

25 printf("%s %.2f %d\n", shelf[i].title, shelf[i].price, shelf[i].published.year); C Basics 299.50 2024 / Pointers 349.00 2025

26 printf("%zu bytes per book\n", sizeof(struct Book)); 30 + 2 padding + 8 + 12 = 52 → rounded to 56

27 return 0;

28 }

Common mistakes

Spot the bug

Mistakes almost every beginner makes — and how to fix them.

Missing semicolon after the definition

✗ Wrong

struct Point { int x; int y; }
int main(void) { … }

✓ Right

struct Point { int x; int y; };
int main(void) { … }

Without ; the compiler reads the next line as part of the struct declaration — confusing errors follow.

Forgetting the word struct

✗ Wrong

Point p;            // error in C

✓ Right

struct Point p;     // or use typedef (Module 35)

In C the type's full name is "struct Point".

Assigning text with =

✗ Wrong

s.name = "Ann";

✓ Right

strcpy(s.name, "Ann");

name is a char array, and arrays cannot be assigned. (Copying the whole struct with = is fine.)

Comparing structs with ==

✗ Wrong

if (p == q) …

✓ Right

if (p.x == q.x && p.y == q.y) …

C has no == for structs; compare the members.

Assuming sizeof is the sum of the members

✗ Wrong

// char + int + char = 6 bytes?

✓ Right

printf("%zu", sizeof(struct S));   // 12

Padding is added for alignment. Never hard-code a struct's size.

Practice

Check your understanding

Answer each question — you will see the explanation straight away.

0 / 5 correct

Question 1

struct P { int x; int y; }; struct P a = {1, 2}; — what is a.y?

Question 2

struct P a = {1, 2}; struct P b = a; b.x = 9; — what is a.x?

Question 3

Which operator selects a member of a struct variable?

Question 4

struct S { char c; int i; }; — what is sizeof(struct S) on a typical system?

Question 5

struct Student cls[30]; — how do you get the age of the 5th student?

Assessment

Structures quiz

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