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
Bit Fields
Bit Fields allow the packing of data in
a structure. This is especially useful when memory or data storage is at a
premium. Typical examples:
Packing several objects into a machine
word. e.g. 1 bit flags can be compacted.
Reading external file formats --
non-standard file formats could be read in. E.g. 9 bit integers.
C allows us do this in a structure definition by
putting :bit length after the variable. For example:
struct packed_struct { unsigned int
f1:1; unsigned int f2:1; unsigned int f3:1; unsigned int f4:1; unsigned int
type:4; unsigned int my_int:9;
} pack;
Here, the packed_struct contains 6 members: Four 1
bit flags f1..f3, a 4 bit type and a 9 bit my_int.
C automatically packs
the above bit fields as compactly as possible, provided that the maximum length
of the field is less than or equal to the integer word length of the computer.
If this is not the case then some compilers may allow memory overlap for the
fields whilst other would store the next field in the next word.
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