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Structure and Unions

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STRUCTURE AND UNIONS

          What is structure?
·      Structure is a user defined data type which is a collection of data of different data type.
·      Syntax:
Struct struct_name
{
         Data_type variable_name;
         ------
         ------
};
Struct  struct_name object1, object2;
·      Example:
Struct books
{
         Char title[20];
         Int pages;
         Float price;
};
struct books book1,book2;
·      The keyword struct declare a structure to hold the details of data field like, title, pages and price.
·      Title, pages and price are called structure element or member.
·      Books is the name of the structure and is called structure.
·      Book1 and book2as variable of type struct books.
·      Now we can access the structure member using structure object and with dot operator.

·      Example:
Book1.pages=250;
Book2.price=12.50;
Strcpy (book1.title,” COMPUTER”);

Advantages of structure
·       It is a mechanism for packing data of different types.
·      It is a convenient tool for managing a group of logically related data items
·      Structure help in organizing more complex data in a simple manner.
·      Each member may or may not belong to a different type of data.
·      Structure is a user-defined data type in C that allows you to combine different data types to store a specific type of record.
·      Structure is used to represent a record. Suppose we want to store record of Student which consists of student name, age and roll number for this data we can define a structure to hold this information.

  Array within a structure
arrays may be the member within structure, this is know as arrays within structure.
Example 
 #include<stdio.h>
     void main()
    {
      struct state
      {
         char name[10];
         int dis;
         long int pop;
        }state;
    clrscr();
     printf("\n Enter State name : ");  gets(state.name);
     printf("\n Enter No. of Districts : ");   scanf("%d",&state.dis);
     printf("\n Enter Population : "); scanf("%ld", &state.pop);
     printf("\n\n\t State Name >> %s", state.name);
     printf("\n\n\t No. of districts >> %d", state.dis);
     printf("\n\n\t Population >> %ld", state.pop);
     getch();
  }

 Array of structure.
Example 
#include<stdio.h>
void main()
{
struct book
{
         char title[20],author[20],pub[20];
         int year,pages,price;
}book[3];
int i;
clrscr();
for(i=0;i<=2;i++)
{
flushall();
printf("\n Enter Title :");    gets(book[i].title);
printf("\n Enter Author :");         gets(book[i].author);
printf("\n Enter Publisher :");     gets(book[i].pub);
printf("\n Enter Year :");    scanf("%d",&book[i].year);
printf("\n Enter Pages :");  scanf("%d",&book[i].pages);
printf("\n Enter Price :");   scanf("%d",&book[i].price);
clrscr();
}

for(i=0;i<=2;i++)
{
printf("\n\n\n\n Title is >> %s",book[i].title);
printf("\n\n Author is >> %s",book[i].author);
printf("\n\n Publisher is >> %s",book[i].pub);
printf("\n\n Year is >> %d",book[i].year);
printf("\n\n Pages is >> %d",book[i].pages);
printf("\n\n Price is >> %d",book[i].price);
}
getch();
}

Explain Copy of structure variable.
Copy of structure variable means copy of one variable value to the other variable. Both variable are if the same structure.
Example
#include<stdio.h>
#include<conio.h>
struct class1
{
         int number;
         char name[20];
         float marks;
};
void main()
{
         int x;
         struct class1 student1={111,"rao",72.50};
         struct class1 student2={222,"reddy",67.00};
         struct class1 student3;
         student3=student2;
         clrscr();
         x=((student3.number==student2.number)&&
            (student3.marks==student2.marks))? 1:0;
            if(x==1)
            {
                 printf("\nstudent2 and student3 are same");
                 printf("\n\n%d %s %f",student3.number,student3.name,student3.marks);
            }
            else
            printf("\nstudent3 and student2 are different");

            getch();
}

   Explain comparing of structure variable.
It means compare one structure variable value with other structure variable. Both variables are of the same structure.

Example
struct class
{
int no;
char name [20];
float marks;
}
main ( )
{
int x;
struct class stu1 = { 111, “Rao”, 72.50};
struct class stu2 = {222, “Reddy”, 67.80};
struct class stu3;
stu3 = stu2;
x = ( ( stu3.no= = stu2.no) && ( stu3.marks = = stu2.marks))?1:0;
if ( x==1)
{
printf (“ \n student 2 & student 3 are same \n”);
printf (“%d\t%s\t%f  “ stu3.no, stu3.name, stu3.marks);
}
else
printf ( “\n student 2 and student 3 are different “);
}

Out Put:
Student 2 and student 3 are same
222 reddy 67.0000

   Explain nested structure with example.
Structure within structure is called nested structure.

 Example
#include<stdio.h>
main()
{
struct pop
{
         int men,women,female;
         struct state
         {
              int tot;
         }state;
}pop;

clrscr();
printf("\n Enter No of Men :");
scanf("%d",&pop.men);
printf("\n Enter No of Female :");
scanf("%d",&pop.female);

pop.state.tot=pop.men+pop.female;

printf("\n TOTAL POPULATION : %d",pop.state.tot);

getch();
}

What is union?
·      Union are a concept borrowed from structure and there are follow the same syntax as structure.
·      The major different difference between them in term of storage.
·      In structure each member has its own storage location where’s all the member of a union uses the same location.
·      A union may contain many numbers of different types.
·      Syntax
union union_name
{
         Data type variable_name;
         ……….
         ……….
};
·                          Example:
union  item
{
         Int m;
float x;
char c;
}code;
·                          A variable code of type union item.
·                          A union contains three members each with a different data type.
·                          However we can use only one of them at a time.
·                          To access a union member like,
code.m;
code.x;
code.c;
code.m=600;
code.x=62.25;
·                          Union may be use in all places where a structure is allowed.
·                          Union may be initialized when the variable is declared.

                         Difference between structure and union.
Structure
Union
Struct keyword is used to define a structure
Union keyword is used to define a union.     
When a member is connected with a structure, the compiler   allocates the memory for each variable.
In case of Union when a member is connected with a union, the  compiler allocates the  memory by considering the size of the   largest variable.
The size of structure is more than   or equal to the sum of  sizes of its   member of structure. 
The size of union is equal to the size of largest member of union
Each member within a structure get unique storage area of   location 
Memory allocation is shared by individual members of union.   
In Structure, the address of each variable will be in ascending order This   indicates that memory for each member will start at different offset values
in unions the address is same for all the variable of a union. 
In Structure, altering the value of a variable will not affect other members of the   structure 
in union altering the value of any of the variable will alter other member   values.     
In structure Several members can be initialize at once 
In unions only the first variable can be initialized.