1. A structure allows grouping of related data as a single unit to organize complex data in a meaningful way. It defines a new user-defined data type. 2. Structures can contain nested structures to group related data. For example, a salary structure may contain an allowance sub-structure. 3. Bit fields allow defining the size of structure members in bits to efficiently utilize memory when member values are small (0 or 1). This reduces the memory used compared to normal integer variables.

1. Mrs. G. Nagalakshmi, M.Sc., M.Phil.,
Ms. N. Malathi, M.Sc.,
Programming in C (18UAMA41)
II B.Sc. Mathematics (SF)

3. A structure is a convenient tool for handling a group of
logically related data items. Structure help to organize complex
data is a more meaningful way. It is powerful concept that we may
after need to use in our program Design.

4. Example
Lets take the example of a book, if we cant to declare a book
we will be thinking about the name, title, authors and publisher of the
book and publishing year. So to declare a book we need to have some
complex data type which can deal with more than one data types.
This can be done with the help of Structure.

5. Syntax

6. We can define a structure by using the keyword struct followed by
list of member variables contained in curly braces
struct Book
{
char Name[100];
char Author[100];
char Publisher[80];
int Year;
};
The keyword struct defines a book, and each line within the braces
defines the elements of the Book. Now whenever we create an
instance of Book it will have all the elements of the structure i.e.
Name, Author, Publisher and Year.

7. We can declare the structure by using the following syntax.
Once the Structure_name is declared as new data type, then
variables of that type can be declared as:

8. We can declare the structure variable CProgrammingBook as
follows:
struct Book CProgrammingBook;
Declaration can also be done as follows:
struct Book
{
char Name[100];
char Author[100];
char Publisher[80];
int Year;
} CProgrammingBook;

9. Lets initialize the structure variable CProgrammingBook.
Book CProgrammingBook =
{
“Programming in ANSI C",
“E. Balagurusamy",
“Tata Mc Graw Hills",
2008
};
C

10. We can access all the members of a structure by adding a
period after the name of the structure variable name and then the
name of the field we want to access.
For example we want to change the Publish year from 2008 to 2012,
we will do it as
CProgrammingBook.Year = 2012;

11. If we want to store data of 50 C Programming books, we
would be required to use 50 different structure variables, from book1
to book50. To have 50 separate variable will be inconvenient.
This can be done easily as follows:
struct Book
{
char Name[100];
char Author[100];
char Publisher[80];
int Year;
} CProgrammingBook[50];

12. Structure with in a structure means nesting of structures.
Let us consider the following structure defined to store information
about the salary of employees.
struct salary
{
char name[20];
char department[10];
int basic_pay;
int dearness_allowance;
int city_allowance;
}employee;

13. This structure defines name, department, basic pay and 3 kinds of
allowance. we can group all the items related to allowance together
and declare them under a substructure are shown below:
struct salary
{
char name [20];
char department[10];
str uct
{
int dearness;
int hous_rent;
int city;
} allowance;
}employee;

14. The salary structure contains a member named allowance which
itself is a structure with 3 members. The members contained in the
inner, structure namely dearness, house_rent, and city can be
referred to as :
employee.allowance. dearness
employee. allowance. house_rent
employee. allowance. city

15. An inner-most member in a nested structure can be accessed
by chaining all the concerned. Structure variables (from outer-most
to inner-most) with the member using dot operator.
The following are invalid.
employee. allowance (actual member is missing)
employee. house_rent (inner structure variable is missing)

16. •A union in C programming is a user defined data type which may
hold members of different sizes and type.
•Union uses a single memory location to hold more than one
variables. However, only one of its members can be accessed at a
time and all other members will contain garbage values.
•The memory required to store a union variable is the memory
required for the largest element of the union.

17. •Both are user-defined data types used to store data of different
types as a single unit.
•Their members can be objects of any type, including other
structures and unions or arrays. A member can also consist of a bit
field.
•Both structures and unions support only assignment = and sizeof
operators. The two structures or unions in the assignment must
have the same members and member types.
•A structure or a union can be passed by value to functions and
returned by value by functions. The argument must have the same
type as the function parameter. A structure or union is passed by
value just like a scalar variable as a corresponding parameter.
•‘.’ operator is used for accessing members.

19. A bit field is a data structure used in computer programming.
It consists of a number of adjacent computer memory locations
which have been allocated to hold a sequence of bits, stored so that
any single bit or group of bits within the set can be addressed.

20. In C, we can state the size of our structure (struct) or union
members in the form of bits. This concept is to because of efficiently
utilizing the memory when you know that your amount of a field or
collection of fields is not going to exceed a specific limit or is in-
between the desired range.

21. Let us take a situation of C program, which contains a
number having TRUE/FALSE variables clustered together in
structure form name as decided:
struct
{
int haveTime;
int haveSpace;
} decided;
The above structure involves 8-bytes of memory space, but we
need to hold either 0 or 1 for each of the above variables.

22. When implementing such variables within a structure, you can
implicitly characterize the width of a variable that tells the compiler
to exercise only that specific number of bytes. The above program
can be written associating bit field as:
struct
{
int haveTime: 1;
int haveSpace: 1;
} decided;

23. The above program involves 4 bytes of memory space for
the status variable, but only 2 bits will be used for storing the
values.
So in our above example, the structure has occupied 4
bytes of memory for a decided variable, but only 2 bits will be
implemented for storing the values.

24. Variables that are defined using a width or size are called bit
fields. The format and syntax of bit-field declaration inside a
structure is as follows:
struct {
data - type[nameofmember]: width_of_Bit - field;
};