9. Structures and Unions

Arrays allow to define type of variables that can hold several data items of the same kind. Similarly structure is another user defined data type available in C that allows to combine data items of different kinds.

Introduction to Structures

A structure is a user defined data type in C. A structure creates a data type that can be used to group items of possibly different types into a single type.struct keyword is used to define structure. The struct statement defines a new data type, with more than one member.

Syntax:-

struct [structure tag]

 {

   member definition;

   member definition;

   ...

   member definition;

} [one or more structure variables];  

Declaring Structure Variables

It is possible to declare variables of a structure, either along with structure definition or after the structure is defined. Structure variable declaration is similar to the declaration of any normal variable of any other datatype. Structure variables can be declared in following two ways:

1) Declaring Structure variables separately

struct Student

{

    char name[25];

    int age;

    char branch[10];

    char gender;

};

struct Student S1, S2;      //declaring variables of struct Student

2) Declaring Structure variables with structure definition

struct Student

{

    char name[25];

    int age;

    char branch[10];

    char gender;

}S1, S2;

Accessing Structure Members

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use the keyword struct to define variables of structure type. 

Example:-
#include<stdio.h>
#include<string.h>
struct Student
{
    char name[25];
    int age;
};

int main()
{
    struct Student s1;    // s1 is a variable of Student type and 
    s1.age = 53;     //age is a member of Student
    strcpy(s1.name, "Rupesh");   //using string function to add name
    printf("Name of Student : %s\n", s1.name);   //displaying the stored values
    printf("Age of Student : %d\n", s1.age);
    return 0;
}

Output:-
                     Name of Student : Rupesh
                      Age of Student : 53

Array of Structure

We can also declare an array of structure variables. in which each element of the array will represent a structure variable. Example : struct employee emp[5];

The below program defines an array emp of size 5. Each element of the array emp is of type Employee.

Example:-
#include<stdio.h>

struct Employee
{
    char ename[10];
    int sal;
};

struct Employee emp[5];
int i, j;
void ask()
{
    for(i = 0; i < 3; i++)
    {
        printf("\nEnter %dst Employee record:\n", i+1);
        printf("\nEmployee name:\t");
        scanf("%s", emp[i].ename);
        printf("\nEnter Salary:\t");
        scanf("%d", &emp[i].sal);
    }
    printf("\nDisplaying Employee record:\n");
    for(i = 0; i < 3; i++)
    {
        printf("\nEmployee name is %s", emp[i].ename);
        printf("\nSlary is %d", emp[i].sal);
    }
}
void main()
{
    ask();
}

Typedef in C

The typedef is a keyword used in C programming to provide some meaningful names to the already existing variable in the C program. It behaves similarly as we define the alias for the commands. In short, we can say that this keyword is used to redefine the name of an already existing variable.

Syntax:-

    typedef <existing_name> <alias_name>  

'existing_name' is the name of an already existing variable while 'alias name' is another name given to the existing variable.

Example:-

#include <stdio.h>  

int main()  

{  

typedef unsigned int unit;  

unit i,j;  

i=10;  

j=20;  

printf("Value of i is :%d",i);  

printf("\nValue of j is :%d",j);  

return 0;  

}  

Output:-

              Value of i is :10 

              Value of j is :20 

Union in C

A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose.

Syntax:-

union [union tag] {

   member definition;

   member definition;

   ...

   member definition;

} [one or more union variables]; 

Example:-

#include <stdio.h>

#include <string.h>

union Data {

   int i;

   float f;

   char str[20];

};

 

int main( ) 

{

   union Data data;        

   printf( "Memory size occupied by data : %d\n", sizeof(data));

    return 0;

}

Output:-

                 Memory size occupied by data : 20

 Difference between structure and union:-