Structures

Big idea

A structure (struct) bundles several related values — of different types — into one variable.

The idea

A student has a name, a roll number, and a mark. Three separate variables per student gets messy fast. A struct makes them one thing:

struct Student {
    char name[20];
    int roll;
    float mark;
};

That defines a new type called struct Student — a template, not a variable yet. (Note the ; after the closing brace.) Now make variables of it:

struct Student s = {"Asha", 12, 88.5};

Reach each member with a dot:

printf("%s\n", s.name);   // Asha
s.mark = 91.0;            // change one member

Structs are copied whole

Unlike arrays, you can assign one struct to another with = — every member is copied, even array members:

#include <stdio.h>

struct Point {
    int x;
    int y;
};

int main(void) {
    struct Point a = {1, 2};
    struct Point b = a;
    a.x = 100;
    printf("a=(%d,%d) b=(%d,%d)\n", a.x, a.y, b.x, b.y);
    return 0;
}

a=(100,2) b=(1,2)

b is its own copy, so changing a afterwards doesn't touch it. The same goes for passing a struct to a function — it gets a copy.

Watch it

Structs and functions

#include <stdio.h>

struct Point {
    int x;
    int y;
};

struct Point midpoint(struct Point a, struct Point b) {
    struct Point m = {(a.x + b.x) / 2, (a.y + b.y) / 2};
    return m;
}

int main(void) {
    struct Point p = {0, 0}, q = {10, 6};
    struct Point m = midpoint(p, q);
    printf("(%d, %d)\n", m.x, m.y);
    return 0;
}

output
(5, 3)

Think of it like

A form with labelled boxes: Name ____, Roll ____, Mark ____. The struct definition is the blank form; each variable is one filled-in copy.

🕷️ Spidey-sense: My case file on every villain: struct Villain { char name[30]; char power[40]; int threat_level; }; One folder, everything I need.

Key terms

• Structure (struct) — A type that groups named members.
• Member — One named field inside a struct.
• . (dot operator) — Reads or writes a member: s.mark.