Structures in C Language

                                                           C Structures
C arrays allow you to define type of variables that can hold several data items of the same
kind  but  structure is  another  user  defined  data  type  available  in  C  programming,  which
allows you to combine data items of different kinds.
Structures are used to represent a record, suppose you want to keep track of your books in
a library. You might want to track the following attributes about each book:

•  Title
• Author
• Subject
• Book ID

Defining a Structure
To define a structure, you must use the struct statement. The struct statement defines a
new  data  type,  with  more  than  one  member  for  your  program.  The  format  of  the  struct
statement is this:
struct [structure tag]
{
member definition;
member definition;
...
member definition;
} [one or more structure variables];

The structure  tag is  optional  and  each  member  definition  is  a  normal  variable  definition,
such as int i; or float f; or any other valid variable definition. At the end of the structure's
definition, before the final semicolon, you can specify one or more structure variables but it
is optional. Here is the way you would declare the Book structure:
struct Books
{
char  title[50];
char  author[50];
char  subject[100];
int  book_id;
} book;

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 struct keyword  to  define
variables of structure type. Following is the example to explain usage of structure:
#include <stdio.h>
#include <string.h>
struct Books
{
char  title[50];
char  author[50];
char  subject[100];
int  book_id;
};
int main( )
{
struct Books Book1;  /* Declare Book1 of type Book */
struct Books Book2;  /* Declare Book2 of type Book */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
/* book 2 specification */
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;
/* print Book1 info */
printf( "Book 1 title : %s\n", Book1.title);
printf( "Book 1 author : %s\n", Book1.author);
printf( "Book 1 subject : %s\n", Book1.subject);
printf( "Book 1 book_id : %d\n", Book1.book_id);
/* print Book2 info */
printf( "Book 2 title : %s\n", Book2.title);
printf( "Book 2 author : %s\n", Book2.author);
printf( "Book 2 subject : %s\n", Book2.subject);
printf( "Book 2 book_id : %d\n", Book2.book_id);
return 0;
}

When the above code is compiled and executed, it produces the following result:
Book 1 title : C Programming
Book 1 author : Nuha Ali
Book 1 subject : C Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Zara Ali
Book 2 subject : Telecom Billing Tutorial
Book 2 book_id : 6495700

Structures as Function Arguments
You can pass a structure as a function argument in very similar way as you pass any other
variable  or  pointer.  You  would  access  structure  variables  in  the  similar  way  as  you  have
accessed in the above example:
#include <stdio.h>
#include <string.h>
struct Books
{
char  title[50];
char  author[50];
char  subject[100];
int  book_id;
};
/* function declaration */
void printBook( struct Books book );
int main( )
{
struct Books Book1;  /* Declare Book1 of type Book */
struct Books Book2;  /* Declare Book2 of type Book */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
/* book 2 specification */
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;
/* print Book1 info */
printBook( Book1 );
/* Print Book2 info */
printBook( Book2 );
return 0;
}
void printBook( struct Books book )
{
printf( "Book title : %s\n", book.title);
printf( "Book author : %s\n", book.author);
printf( "Book subject : %s\n", book.subject);
printf( "Book book_id : %d\n", book.book_id);
}
When the above code is compiled and executed, it produces the following result:
Book title : C Programming
Book author : Nuha Ali
Book subject : C Programming Tutorial
Book book_id : 6495407
Book title : Telecom Billing
Book author : Zara Ali
Book subject : Telecom Billing Tutorial
Book book_id : 6495700

Pointers to Structures
You can define pointers to structures in very similar way as you define pointer to any other
variable as follows:
struct Books *struct_pointer;
Now,  you  can  store  the  address  of  a  structure  variable  in  the  above  defined  pointer
variable.  To  find  the  address  of  a  structure  variable,  place  the  &  operator  before  the
structure's name as follows:
struct_pointer = &Book1;
To access the members of a structure using a pointer to that structure, you must use the
-> operator as follows:
struct_pointer->title;
Let  us  re-write  above  example  using  structure  pointer,  hope  this  will  be  easy  for  you  to
understand the concept:
#include <stdio.h>
#include <string.h>
struct Books
{
char  title[50];
char  author[50];
char  subject[100];
int  book_id;
};
/* function declaration */
void printBook( struct Books *book );
int main( )
{
struct Books Book1;  /* Declare Book1 of type Book */
struct Books Book2;  /* Declare Book2 of type Book */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
/* book 2 specification */
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
Book2.book_id = 6495700;
/* print Book1 info by passing address of Book1 */
printBook( &Book1 );
/* print Book2 info by passing address of Book2 */
printBook( &Book2 );
return 0;
}
void printBook( struct Books *book )
{
printf( "Book title : %s\n", book->title);
printf( "Book author : %s\n", book->author);
printf( "Book subject : %s\n", book->subject);
printf( "Book book_id : %d\n", book->book_id);
}
When the above code is compiled and executed, it produces the following result:
Book title : C Programming
Book author : Nuha Ali
Book subject : C Programming Tutorial
Book book_id : 6495407
Book title : Telecom Billing
Book author : Zara Ali
Book subject : Telecom Billing Tutorial
Book book_id : 6495700