C struct

In C programming, a struct (or structure) is a collection of variables (can be of different types) under a single name.

Define Structures

Before you can create structure variables, you need to define its data type. To define a struct, the struct keyword is used.

Syntax of struct

struct structureName {
  dataType member1;
  dataType member2;
  ...
};

For example,

struct Person {
  char name[50];
  int citNo;
  float salary;
};

Here, a derived type struct Person is defined. Now, you can create variables of this type.

Create struct Variables

When a struct type is declared, no storage or memory is allocated. To allocate memory of a given structure type and work with it, we need to create variables.

Here's how we create structure variables:

struct Person {
  // code
};

int main() {
  struct Person person1, person2, p[20];
  return 0;
}

Another way of creating a struct variable is:

struct Person {
   // code
} person1, person2, p[20];

In both cases,

• person1 and person2 are struct Person variables
• p[] is a struct Person array of size 20.

Access Members of a Structure

There are two types of operators used for accessing members of a structure.

1. . - Member operator
2. -> - Structure pointer operator (will be discussed in the next tutorial)

Suppose, you want to access the salary of person2. Here's how you can do it.

person2.salary

Example 1: C structs

#include <stdio.h>
#include <string.h>

// create struct with person1 variable
struct Person {
  char name[50];
  int citNo;
  float salary;
} person1;

int main() {

  // assign value to name of person1
  strcpy(person1.name, "George Orwell");

  // assign values to other person1 variables
  person1.citNo = 1984;
  person1. salary = 2500;

  // print struct variables
  printf("Name: %s\n", person1.name);
  printf("Citizenship No.: %d\n", person1.citNo);
  printf("Salary: %.2f", person1.salary);

  return 0;
}

Output

Name: George Orwell
Citizenship No.: 1984
Salary: 2500.00

In this program, we have created a struct named Person. We have also created a variable of Person named person1.

In main(), we have assigned values to the variables defined in Person for the person1 object.

strcpy(person1.name, "George Orwell");
person1.citNo = 1984;
person1. salary = 2500;

Notice that we have used strcpy() function to assign the value to person1.name.

This is because name is a char array (C-string) and we cannot use the assignment operator = with it after we have declared the string.

Finally, we printed the data of person1.

Keyword typedef

We use the typedef keyword to create an alias name for data types. It is commonly used with structures to simplify the syntax of declaring variables.

For example, let us look at the following code:

struct Distance{
  int feet;
  float inch;
};

int main() {
  struct Distance d1, d2;
}

We can use typedef to write an equivalent code with a simplified syntax:

typedef struct Distance {
  int feet;
  float inch;
} distances;

int main() {
  distances d1, d2;
}

Example 2: C typedef

#include <stdio.h>
#include <string.h>

// struct with typedef person
typedef struct Person {
  char name[50];
  int citNo;
  float salary;
} person;

int main() {

  // create  Person variable
  person p1;

  // assign value to name of p1
  strcpy(p1.name, "George Orwell");

  // assign values to other p1 variables
  p1.citNo = 1984;
  p1. salary = 2500;

  // print struct variables
  printf("Name: %s\n", p1.name);
  printf("Citizenship No.: %d\n", p1.citNo);
  printf("Salary: %.2f", p1.salary);

  return 0;
}

Output

Name: George Orwell
Citizenship No.: 1984
Salary: 2500.00

Here, we have used typedef with the Person structure to create an alias person.

// struct with typedef person
typedef struct Person {
  // code
} person;

Now, we can simply declare a Person variable using the person alias:

// equivalent to struct Person p1
person p1;

Nested Structures

You can create structures within a structure in C programming. For example,

struct complex {
  int imag;
  float real;
};

struct number {
  struct complex comp;
  int integers;
} num1, num2;

Suppose, you want to set imag of num2 variable to 11. Here's how you can do it:

num2.comp.imag = 11;

Example 3: C Nested Structures

#include <stdio.h>

struct complex {
  int imag;
  float real;
};

struct number {
  struct complex comp;
  int integer;
} num1;

int main() {

  // initialize complex variables
  num1.comp.imag = 11;
  num1.comp.real = 5.25;

  // initialize number variable
  num1.integer = 6;
	
  // print struct variables
  printf("Imaginary Part: %d\n", num1.comp.imag);
  printf("Real Part: %.2f\n", num1.comp.real);
  printf("Integer: %d", num1.integer);

  return 0;
}

Output

Imaginary Part: 11
Real Part: 5.25
Integer: 6

Why structs in C?

Suppose you want to store information about a person: his/her name, citizenship number, and salary. You can create different variables name, citNo and salary to store this information.

What if you need to store information of more than one person? Now, you need to create different variables for each information per person: name1, citNo1, salary1, name2, citNo2, salary2, etc.

A better approach would be to have a collection of all related information under a single name Person structure and use it for every person.

More on struct

• Structures and pointers
• Passing structures to a function