0xC0Lesson 13 of 15

Structs: your own types

Group related values into one type with struct, name it with typedef, and reach through pointers with ->.

14 min 5-question quiz 2 code exercises

By the end of this lesson you can

Define a struct and create values of it
Name struct types with typedef
Access members through a pointer with ->

Real data comes in bundles: a point has an x and a y; a student has a name, an id and a grade. A struct groups several values - its members - into one new type, so you can pass the bundle around as a single thing.

point.c

1#include <stdio.h>
2
3struct Point {
4    int x;
5    int y;
6};
7
8int main(void) {
9    struct Point p = {3, 4};
10    p.x = p.x + 10;
11    printf("(%d, %d)\n", p.x, p.y);
12    return 0;
13}

Output

(13, 4)

The definition creates a type called struct Point; it does not create any variable. struct Point p = {3, 4}; then makes one, with members initialized in order. The dot operator reaches a member: p.x. You can also name members in the initializer, which is clearer: struct Point p = {.x = 3, .y = 4};.

typedef

Writing struct everywhere gets tedious. typedef gives a type a shorter name:

student.c

1#include <stdio.h>
2
3typedef struct {
4    char name[32];
5    int id;
6    double gpa;
7} Student;
8
9int main(void) {
10    Student s = {.name = "Grace", .id = 1906, .gpa = 3.9};
11    printf("%s (#%d): %.1f\n", s.name, s.id, s.gpa);
12    return 0;
13}

Output

Grace (#1906): 3.9

Structs and functions

Structs are passed by value like any other variable: the function gets a copy of the whole struct. That is fine for small structs, but for large ones - or when the function must modify the original - pass a pointer. C has a special operator for members through a pointer: p->x means (*p).x.

bank.c

1#include <stdio.h>
2
3typedef struct {
4    char owner[32];
5    long balance_cents;
6} Account;
7
8void deposit(Account *account, long cents) {
9    account->balance_cents += cents;
10}
11
12void print_account(const Account *account) {
13    printf("%s: $%ld.%02ld\n", account->owner,
14           account->balance_cents / 100, account->balance_cents % 100);
15}
16
17int main(void) {
18    Account a = {.owner = "Linus", .balance_cents = 1050};
19    deposit(&a, 275);
20    print_account(&a);
21    return 0;
22}

Output

Linus: $13.25

Arrays of structs

Structs combine naturally with arrays - an array of Student is a tiny database. Loop over it just like any other array, and use . on each element: students[i].gpa.

Quick check

Predict the output

What does this print?

1#include <stdio.h>
2
3typedef struct {
4    int x;
5    int y;
6} Point;
7
8void move_right(Point p) {
9    p.x += 5;
10}
11
12int main(void) {
13    Point p = {1, 1};
14    move_right(p);
15    printf("%d\n", p.x);
16    return 0;
17}

Output

Key takeaways

A struct groups related members into one type; typedef gives it a convenient name.
Use . on a struct and -> through a pointer to a struct (p->x is (*p).x).
Structs are copied when passed; pass T * to modify and const T * to read large structs cheaply.

Lesson quiz

5 questions · pass with 4 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write real C

These run for real. Write the program, press Run tests, and read what the compiler tells you - learning to read compiler messages is half of learning C.

Exercise 1

Distance between points

+25 XP

Define a Point type with double members x and y, and write double distance(Point a, Point b) returning the straight-line distance, sqrt((ax - bx)² + (ay - by)²). Read four numbers (x1 y1 x2 y2) and print the distance with 2 decimals.

For 0 0 3 4 print 5.00.

3-4-5 triangle
Same point
Negative coordinates

main.c

Compiler Explorer

Loading editor…

Your code is compiled with gcc 14 (-Wall -Wextra) and run on Compiler Explorer (godbolt.org), a free public service.

Exercise 2

Top of the class

+25 XP

The input starts with a count n (at most 50), then n lines of name score. Store them in an array of a Student struct and print the name and score of the student with the highest score (the first one wins ties):

Top: Ada 97

Three students
Tie keeps the first

main.c

Compiler Explorer

Loading editor…

Your code is compiled with gcc 14 (-Wall -Wextra) and run on Compiler Explorer (godbolt.org), a free public service.

Questions about this lesson

Stuck? Ask. Figured something out? Share it. Explaining is one of the best ways to learn.

Loading posts…