Introduction To Structures

Introduction To Structures

Arrays can be used to represent a group of data items that belong to same type,such as int or float.However, If we want to represent a collection of data items of different types using a single name,then we cannot use arrays.At this time,we use structures.Structure is used to represent a set of attributes,such as student_name, roll_number and marks.The individual structure elements
are referred to as members. Consider a book database consisting of book name,author,number of pages and price. We can define a structure to hold this information as follows.

        struct book_bank
        {
  char title[20];
         char author[15];
int pages;
float price;
        };

  The keyword ‘struct’ declares a structure to hold the details of four fields,namely title,author,pages and price. These fields are nothing but members of structure elements.Each member may belong to a different type of  data. book_bank is the name of the structure and is called the structure tag.This will be used to declare variables that have tag’s structure.     This declaration has not declared any variable. It simply describes a format called template to represent information as follows.......
       struct book_bank
title           array of 20 char’s
        author          array of 15 char’s
        pages           integer
        price           float

      general declaration of a structure

     struct tag_name
     {
        data_type member1;
        data_type member2;
         ------------
         ------------
     }
  we can declare structure variables using the tag name any where in the program.For example, struct book_bank, book_bank1,book_bank2, book_bank3;each one of these variables has 4 members as specified by the template.The complete declaration is like ....  struct book_bank
  {
   char title[20];
   int  author[15];
   int pages;
   float price;
  };
  struct book_bank book1,book2,book3,book3;

These members do not occupy any memory until they are associated with the structure variables such as book1.  In defining structure we may follow the syntax

 1.The template is terminated with a semicolon.
 2.While the entire declaration is considered as a  statement, each member is declared independently for its name and type in a separate statement inside the template.
 3.The tag name such as book_bank can be used to declare structure variables of its type,later in the program.Normally structure definitions appear at the beginning of the program profile,before any variables or functions are defined. They also appear before the main,along with macro definitions such as #define.We can assign values to the member of a structure in a no. of ways. The link between a member and a variable is established using the member operator.Which is also known
as dot period or  period operator.

For example,
         book1.price
    is the variable representing the price of book1 and
can be treated like anyother ordinary variable.
            Strcpy(book1.title,"BASIC");
            book1.pages=250;
  Like any other  data type,a structure variable can be
initialized. A structure must be declared as static if
it is to be initialized inside a function.

  main()
  {
    static struct;
    {
      int wt;
      float  ht;
    }
    student ={60,180.75};
  }
    we can initialize a structure by using different ways.
    main()
    {
     struct st_record;
     {
       int wt;
       float  ht;
     }
     static struct st_record student1={60,170.75};
     static struct st_record student2={63,170.65};
    }

  Another method is to initialize a structure variable outside the function like...

    struct st_record;
    {
       int wt;
       float  ht;
    }
    student1={60,170.75};
    main()
    {
     static struct st_record student2={63,170.65};
    }
   C language does not permit the initialization of individual structure members within the template. The initialization must be done only in the declaration of the actual variables.