CIS241

System-Level Programming and Utilities

C - structs

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Object-oriented programming in C

(somewhat)

We can structure our data

• We're not limited to raw ints, chars, arrays, etc

structs

Structs allow us to group together pieces of data

typedef struct Coord {
    double x;
    double y;
} Coord;

We can treat Coord like a new type!

To create and use an instance of a struct:

Coord c;
c.x = 5.0;
c.y = 10;

structs

Structs can container raw data types, pointers, arrays, even other structs!

We can access members of a struct with .

• E.g., c.x = 5; double d = c.y;

structs

Structs still work with pointers!

We can create a pointer to a struct:

• Coord* p = &c;

We can access members with ->

• p->x = 1.0;

This is the same as (*p).x = 1.0;

We can allocate structs on the stack or the heap!

What's missing?

That OO thing...

Methods! (member functions)

C does not support methods by default.

• You can build them, but it’s complicated (function pointers)

More on structs (declaring)

typedef struct Coord {
    double x;
    double y;
} Coord;
...
Coord c;

Also can do:

struct Coord2 {
    double x;
    double y;
}
...
struct Coord2 d;

typedef

typedef defines a new type!

• typedef type name;

This is not limited to structs:

typedef int* int_pointer;
int x = 40;
int_pointer p = &x;
printf("p: %p, *p: %d\n", p, *p);

With structs, this prevents us from typing struct Coord over and over again!

Practice!

1. Create a struct that represents a player (name, health, room ID)
2. Set the defaults in a function
3. Use l and r as inputs to move them around (l moves left and r moves right)

Skeleton (curly braces on same line for space)

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

typedef struct Player {
} Player;

void setupPlayer(?) {
}

int main() {
    return 0;
}

The struct

typedef struct Player {
  int roomID;
  int health;
  char* name;
} Player;

The setup function

void setupPlayer(Player* p) {
  p->roomID = 0;
  p->health = 10;
  p->name = "Erik";
}

More main

int main() {
  // create and setup player
  Player player;
  setupPlayer(&player);

  bool done = false; // forever loop variable
  char ch;           // user input

  // forever loop
  while (!done) {

  }

  printf("Done.\n");
  return 0;
}

and the loop

  // forever loop
  while (!done) {
    printf("---\n");
    printf("%d %d %s\n", player.roomID, player.health, player.name);
    printf("Waiting for input [l, r, q]: ");

    // read a character - NOTE THE SPACE
    int res = scanf(" %c", &ch);
    if(res == EOF) done = true; // Ctrl+D pressed
    else {
      if (ch == 'l') player.roomID--;
      else if (ch == 'r') player.roomID++;
      else if (ch == 'q') done = true;
    }
    printf("---\n\n");
  }