Structures in C (Part 1)

As you are aware that arrays store elements of similar type. Contrary to this, structures are objects that contain more than one item.

Definition:

• A structure contains a number of data types grouped together.
• These data types may or may not be of the same type.

In this article, you will learn following fundamental aspects of structures:

(a) declaration of a structure

(b) accessing of structure elements

(c) Initializing values to structure element(s)

(d) how structure elements are stored in memory

(a) declaration of a structure

The general form of a structure declaration statement is given below:

struct structure_name{

       datatype structure_element 1 ;

       datatype structure_element 2 ;

       datatype structure_element 3 ;

        ......

        ......

   } structure_variable(s);

Points to remember about above declaration:

• The first line contains the struct keyword, then the optional structure_name:struct structure_name {
• The structure_name can be used to create a copy of the structure. An opening brace follows the structure_name (or the struct keyword, if the structure_name is not used). This brace signals to the compiler that the next lines are member definitions. Each member definition consists of a variable type and a name. The members can be any valid variable type, including arrays, structures, and unions.
• Following the last member name is a closing brace and the optional structure_variable, as follows:} structure_ variable(s) ;
• The closing brace in the structure type declaration must be followed by a semicolon.
• A structure type declaration does not reserve any space in memory. All a structure declaration does is, it defines the ‘form’ of the structure.

When using the structure_ variable and the structure_name, you can choose any of the following:

• If a structure_ variable is not specified and a structure _name is specified, the structure is being defined but not declared.
• If a structure_ variable is specified and a structure _name is not specified, the structure is being declared but not defined.
• If a structure_ variable and a structure _name are provided, the structure is being both defined and declared.
• If neither a structure_ variable nor a structure _name is provided, a compile-time error will result. 

If you want to initialize the structure, you must have a structure_ variable because

it signals the compiler that this is a declaration. The structure_ variable is also necessary if you want to refer to the structure.

After the structure_variable are optional initializers:

{initializer_values};

Example:

struct student

{

  int rollno;

  int marks;

} stu1;

(b).Accessing Structure Elements

In arrays we can access individual elements of an array using a subscript. Structures use a different scheme. They use a dot (.) operator.

Note that before the dot there must always be a structure variable and after the dot there must always be a structure element.

Syntax:

   structure_variable.structure_element;

If the structure_element is also a structure ,the structure_element name is followed by a period and its structure_element name:

structure_variable.memberstructure. structure_element

 Example:

   stu1.rollno;

In this example, I will show you how to declare a structure, assign values and access the elements of structure.

#include<stdio.h>

struct student

{

  int rollno;

  int marks;

} stu1,stu2;

int main(){

  return 0;

}

(c).Initializing values to structure element(s)

Unlike standard variables, the syntax for initialising structure variables is different.The structure elements are accessed using the dot notation.

One can either initialize a structure by initializing the individual elements as shown below:

      stu1. rollno = 6532;

      stu1. marks = 89;

 or by simply listing the element's value inside curly braces, with each value separated by a comma.

     stu1 = {6532,89};

A structure variable can be assigned  to another providing that the structures have the same number of members and that the members are of the same data type.

  Example : stu2=stu1;

(d).How Structure Elements are Stored

Whatever be the elements of a structure, they are always stored in contiguous memory locations.

Example:

#include <stdio.h>

struct student{

    char *name;

    int rollno;

    int marks;

}stu1;

int main()

{

     /*Assigning the values of each struct element here*/

     stu1.name = "Steve";

     stu1.rollno = 1234;

     stu1.marks = 30;

     /* Displaying the values of struct element */

     printf("Student Name is: %s", stu1.name);

     printf("\nStudent roll no is: %d", stu1.rollno);

     printf("\nStudent marks is: %d", stu1.marks);

     return 0;

}

Please comment if you find anything incorrect, or you want to improve the topic discussed above.