Structures in C Tutorial: The Basics

This post serves as an introduction to using Structures in C! This is the first of many C tutorials I plan on writing.

Reading this Tutorial

This tutorial is not meant to be quickly glossed over, unless of course you are already familiar with the subject matter. Each full program (i.e. any snippets beginning with #include) should be hand-typed, compiled, and ran. I also encourage you to try to type the example programs entirely from memory once you understand them; doing so helps me store new-found knowledge into long-term memory, and perhaps you'll receive the same benefits!

What is a Structure?

As said by Neol Kalicharan in Data Structures in C:

"In C, a structure is a collection of one or more variables, possibly of different types, grouped together under a single name for convenient handling."

Structures are pretty simple to understand just by seeing an example in action. Compile and run the following:

// Using structures in C
#include<stdio.h>

// Struct declaration
struct person{
  char *name;
  int age;
  char gender;
};

void print_person(struct person);

int main(){

  // Declaring a variable of type `struct person`
  struct person someguy;
  someguy.name = "Joseph";
  someguy.age = 21;
  someguy.gender = 'M';

  // Print person information
  print_person(someguy);

  return 0;
}

// Print a person's information
void print_person(struct person p){
  printf("Name: %s\n", p.name);
  printf("Age: %d\n", p.age);
  printf("Gender: %c\n", p.gender);
}

Explanation

In the Struct declaration section, we define a person type as a structure. We then define the fields of the structure:

• a pointer to char (to hold a string literal)
• age as an integer
• gender as a char.

Fields of the structure are accessed via the dot operator.

#include

struct product{
  char *label;
  int value;
};

int main(){

  struct product cookie;
  cookie.label = "wonderful cookie";
  cookie.value = 1;

  printf("The %s is $%d\n", cookie.label, cookie.value);

  return 0;
}

After declaring the person structure, we can now do the same things with struct person as we would regular data types, such as int.

// Declaring a variable
struct person someguy;
int x;

// Usage in function parameters
void print_person(struct person p){
  ... // Do things with p
}
void print_number(int num){
  ... // Do things with int
}

// Assignment
struct person someotherguy = someguy;
int y = x;

// Operations
int ageDiff = someotherguy.age - someguy.age;
int z = x - y;

Tidying up with typedef

If you'd like to eliminate the verbosity of using struct person every time you're working with a person structure, you can use typedef.

typedef works like so:

typedef datatype newDataTypeName

For example,

// Using typedef
#include<stdio.h>

typedef int WholeNumber;

int main(){
  WholeNumber x = 10;
  printf("%d is a whole number\n", x);
  return 0;
}

While this example isn't particularly useful, it lets you understand how typedef works.

Note again that struct person behaves like any other data type, so the entire structure declaration can be used as the datatype parameter of typedef. This really cleans up structure usage.

// Using typedef
#include<stdio.h>

typedef struct product{
  char *label;
  int value;
} product_t;

int main(){
  product_t cookie; // Was formerly struct product cookie
  cookie.label = "wonderful cookie";
  cookie.value = 1;

  printf("The %s is $%d\n", cookie.label, cookie.value);
  return 0;
}

Looking again at the usage of typedef (typedef datatype newDataTypeName), we see that struct product{..} is the datatype, and Product is the newDataTypeName. We can now use product_t in place of struct product. This includes variable assignments, function parameters, everything. (Note that product_t is read "product type", and structures generally follow this naming convention.)

Applying typedef

With our newfound knowledge of typedef, lets implement it into our first program. (This is definitely a code snippet I would attempt to type from memory)

// Using structures in C
#include<stdio.h>

// Struct declaration
typedef struct person{
  char *name;
  int age;
  char gender;
} person_t;

void print_person(person_t);

int main(){

  // Declaring a variable of type `person_t`
  person_t someguy;
  someguy.name = "Joseph";
  someguy.age = 21;
  someguy.gender = 'M';

  // Print person information
  print_person(someguy);

  return 0;
}

// Print a person's information
void print_person(person_t p){
  printf("Name: %s\n", p.name);
  printf("Age: %d\n", p.age);
  printf("Gender: %c\n", p.gender);
}

Conclusion

Structures are great for representing collections of related attributes, and they take very little effort to use! We'll cover more involved uses of structures, such as arrays of structures and pointers to structures, in an upcoming post.

July 10, 2012