Structures and Pointers

ECE & EEE | I YEAR | II SEMESTER B1360 | COMPUTER PROGRAMMING | UNIT 4
1 |Prepared By : Mr. PRABU.U/AP |Dept. of Computer Science and Engineering | SKCET |
UNIT IV
Structures: Arrays and Structures - Nested Structures - Passing Structures to Pointers -
User Defined Datatyes - Union
Pointers: Pointers and Arrays - Pointers and Functions - Pointer and Strings - Pointer
and Structures
STRUCTURES
4.1 STRUCTURES
C supports a constructed data type known as structures, a mechanism for
packing data of different types. “A structure is a convenient tool for handling a group of
logically related data items”.
 Structure is a type of data structure in which each individual elements differ in
type.
 The elements of a structure are called members.
 The structure elements contain integer, floating point numbers, character arrays,
pointers as their members.
 Structure act as a tool for handling logically related data items.
 Fields of structure are called structure elements or members. Each member may
be of different type.
Syntax:
struct structure_name
{
data_type member1;
data_type member2;
.
.
data_type memebern;
} [one or more structure variables];
Example:
struct Books
{
char title[50];
char author[50];
char subject[100];
int book_id;
} book;

Structure Initialization
1. When we declare a structure, memory is not allocated for un-initialized variable.
2. We can initialize structure variable in different ways.
i) Declare and Initialize
struct student
{
char name[20];
int roll;
float marks;
}std1 = { "Pritesh",67,78.3 };
ii) Declaring and Initializing Multiple Variables
struct student
{
char name[20];
int roll;
float marks;
}
std1 = {"Pritesh",67,78.3};
std2 = {"Don",62,71.3};
iii) Initializing Single Member
struct student
{
int mark1;
int mark2;
int mark3;
} sub1={67};
iv) Initializing Inside Main
struct student
{
int mark1;
int mark2;
int mark3;
};
void main()
{
struct student s1 = {89,54,65};
- - - - --
- - - - --
}

Uses of Structures
1. Used to store huge data. Structures act as a database.
2. Used to send data to the printer.
3. Structures can interact with keyboard and mouse to store the data.
4. Used in drawing and floppy formatting.
5. Used to clear output screen contents.
6. Used to check computer’s memory size etc.
Accessing structure members:
 Members in a structure are accessed using the Period Operator ‘.’.
 Period Operator establishes a link between member and variable name.
 Structure members are processed by using a variable name with period ‘.’ and
member name.
 Period Operator is also known as member operator or dot operator.
 The syntax for accessing structure members is
structure_variable.member_name
Rules of Initializing Structures
1. We cannot initialize individual members inside the structure template.
2. The order of values enclosed in braces must match the order of members in the
structure definition.
3. It is permitted to have a partial initialization. We can initialize only the first few
members and leave the remaining bank. The uninitialized members should be only
at the end of the list.
4. The uninitialized members will be assigned default values as follows
 Zero for integer and floating point number.
 ‘/0’ for characters and strings.
Structure Operations
The period operator ‘.’ has high precedence over unary operator, arithmetic
operator, relational operator, logical operator and assignment operator.
EXPRESSION MEANING
++(variable.member) preincrement
(variable.member)++ post increment
&variable Beginning address of the variable.
&variable.member Access address of variable.member
variable.member.submember Refers member of structure within a
structure.
variable.member[expression] Refer individual element of array in a
structure
Example Program for Structures
#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 */
/* book 1 specification */
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
Book1.book_id = 6495407;
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
/* print Book1 info */
printf( "Book 1 title : %sn", Book1.title);
printf( "Book 1 author : %sn", Book1.author);
printf( "Book 1 subject : %sn", Book1.subject);
printf( "Book 1 book_id : %dn", Book1.book_id);
/* print Book2 info */
printf( "Book 2 title : %sn", Book2.title);
printf( "Book 2 author : %sn", Book2.author);
printf( "Book 2 subject : %sn", Book2.subject);
printf( "Book 2 book_id : %dn", Book2.book_id);
return 0;
}
Output:
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

4.2 ARRAYS AND STRUCTURES
C Structure is collection of different datatypes ( variables ) which are grouped
together. Whereas, array of structures is nothing but collection of structures. This is also
called as structure array in C.
Example Program for Array of Structures
#include <stdio.h>
#include <string.h>
struct student
{
int id;
char name[30];
float percentage;
};
int main()
{
int i;
struct student record[2];
// 1st student's record
record[0].id=1;
strcpy(record[0].name, "Raju");
record[0].percentage = 86.5;
// 2nd student's record
record[1].id=2;
strcpy(record[1].name, "Surendren");
// 3rd student's record
record[2].id=3;
strcpy(record[2].name, "Thiyagu");
//Printing Student's record
for(i=0; i<3; i++)
{
printf(" Records of STUDENT : %d n", i+1);
printf(" Id is: %d n", record[i].id);
printf(" Name is: %s n", record[i].name);
printf(" Percentage is: %fnn",record[i].percentage);
}
return 0;
}

OUTPUT
Records of STUDENT : 1
Id is: 1
Name is: Raju
Percentage is: 86.500000
Id is: 2
Name is: Surendren
Id is: 3
Name is: Thiyagu
4.3 NESTED STRUCTURES
Nested structure in C is nothing but structure within structure. One structure can
be declared inside other structure as we declare structure members inside a structure.
The structure variables can be a normal structure variable or a pointer variable to
access the data.
Example Program for Nested Structures
#include <stdio.h>
#include <string.h>
struct student_college_detail
{
int college_id;
char college_name[50];
};
struct student_detail
{
int id;
char name[20];
float percentage;
// structure within structure
struct student_college_detail clg_data;
}stu_data;
int main()
{
struct student_detail stu_data = {1, "Raju", 90.5, 71145,"Anna University"};
printf(" Id is: %d n", stu_data.id);
printf(" Name is: %s n", stu_data.name);
printf(" Percentage is: %f nn", stu_data.percentage);
printf(" College Id is: %d n", stu_data.clg_data.college_id);
printf(" College Name is: %s n", stu_data.clg_data.college_name);
return 0;
}

OUTPUT
Id is: 1
Name is: Raju
College Id is: 71145
College Name is: Anna University
4.4 PASSING STRUCTURES TO FUNCTIONS
 A structure can be passed to any function from main function or from any sub
function.
 Structure definition will be available within the function only.
 It won’t be available to other functions unless it is passed to those functions by
value or by address(reference).
 Else, we have to declare structure variable as global variable. That means,
structure variable should be declared outside the main function. So, this
structure will be visible to all the functions in a C program.
Passing structure to function can be done in three ways
1. Passing structure to a function by value
2. Passing structure to a function by address(reference)
3. No need to pass a structure – Declare structure variable as global
1. Example Program - Passing structure to a function by value
In this program, the whole structure is passed to another function by value. It
means the whole structure is passed to another function with all members and their
values. So, this structure can be accessed from called function.
#include <stdio.h>
#include <string.h>
struct student
{
int id;
char name[20];
float percentage;
};
void func(struct student record);
int main()
{
struct student record;
record.id=1;
strcpy(record.name, "Raju");
record.percentage = 86.5;
func(record);
return 0;
}
void func(struct student record)
{
printf(" Id is: %d n", record.id);

printf(" Name is: %s n", record.name);
printf(" Percentage is: %f n", record.percentage);
}
2. Example Program - Passing structure to a function by address
In this program, the whole structure is passed to another function by address. It
means only the address of the structure is passed to another function. The whole
structure is not passed to another function with all members and their values. So, this
structure can be accessed from called function by its address.
#include <stdio.h>
#include <string.h>
struct student
{
int id;
char name[20];
float percentage;
};
void func(struct student *record);
int main()
{
struct student record;
record.id=1;
func(&record);
return 0;
}
void func(struct student *record)
{
printf(" Id is: %d n", record->id);
printf(" Name is: %s n", record->name);
printf(" Percentage is: %f n", record->percentage);
}
3. Example Program - To declare structure variable as global
Structure variables also can be declared as global variables as we declare other
variables in C. So, When a structure variable is declared as global, then it is visible to all
the functions in a program. In this scenario, we don’t need to pass the structure to any
function separately.

#include <stdio.h>
#include <string.h>
struct student
{
int id;
char name[20];
float percentage;
};
struct student record; // Global declaration of structure
void structure_demo();
int main()
{
record.id=1;
structure_demo();
return 0;
}
void structure_demo()
{
}
4.5 USER DEFINED DATATYPES
1. Typedef
Typedef is a keyword that is used to give a new symbolic name for the existing
name in a C program. This is same like defining alias for the commands.
The C programming language provides a keyword called typedef, which you can
use to give a type, a new name. Following is an example to define a term BYTE for one-
byte numbers −
typedef unsigned char BYTE;
After this type definition, the identifier BYTE can be used as an abbreviation for
the type unsigned char, for example.
BYTE b1, b2;
Example Program for User Defined Datatypes - Typedef
#include <stdio.h>
#include <string.h>
typedef struct student
{

int id;
char name[20];
float percentage;
} status;
int main()
{
status record;
record.id=1;
return 0;
}
 When we use “typedef” keyword before struct <tag_name> like above, after that
we can simply use type definition “status” in the C program to declare structure
variable.
 Now, structure variable declaration will be, “status record”.
 This is equal to “struct student record”. Type definition for “struct student” is
status. i.e. status = “struct student”
2. Enumerated Datatypes
 Enumerated Datatypes allow users to define new types.
 They are similar to structures.
 Members are constants and written as identifiers.
 Members have signed integer values that can be assigned to corresponding
enumeration values.
The general format for defining an enumerated datatype is as follows:
enum tag{member1, member2, ..., ,membern};
Where
enum: Required keyword
tag: An identifier that names the enumeration type
member1, member 2: Identifiers called enumeration constants or enumerators
Advantage
The main advantage of enum is that if you don't initialize your constants, each
one would have a unique value. The first would be zero and the rest would then count
upwards
enum colors {RED, YELLOW, GREEN, BLUE};
Here, colors is the name given to the set of constants - the name is optional

Example Program for User Defined Datatypes - Enumerated Datatypes
#include <stdio.h>
int main()
{
enum {RED=5, YELLOW, GREEN=4, BLUE};
printf("RED = %dn", RED);
printf("YELLOW = %dn", YELLOW);
printf("GREEN = %dn", GREEN);
printf("BLUE = %dn", BLUE);
return 0;
}
OUTPUT
RED = 5
YELLOW = 6
GREEN = 4
BLUE = 5
4.6 UNION
A union is a special data type available in C that allows to store different data
types in the same memory location. Unions provide an efficient way of using the same
memory location for multiple-purpose.
Concept/Advantages of Union
 The compiler allocates sufficient space to hold the largest data item in the union
and not for all members.
 They are used to conserve memory.
Defining Union
union [union tag]
{
member definition;
member definition;
...
member definition;
} [one or more union variables];
Example
union Data
{
int i;
float f;
char str[20];
} data;

Example Program to Find Total Memory Occupied by Union
#include <stdio.h>
#include <string.h>
union Data
{
int i;
float f;
char str[20];
};
int main( )
{
union Data data;
printf( "Memory size occupied by data : %dn", sizeof(data));
return 0;
}
OUTPUT:
Memory size occupied by data : 20
In the above example, Data type will occupy 20 bytes of memory space because
this is the maximum space which can be occupied by a character string.
Accessing Union Members
To access any member of a union, we use the member access operator (.) . The
member access operator is coded as a period between the union variable name and the
union member that we wish to access. The keyword union is used to define variables of
union type. The following example shows how to use unions in a program −
Example Program to Access Union Members
#include <stdio.h>
#include <string.h>
union Data
{
int i;
float f;
char str[20];
};
int main( )
{
union Data data;
data.i = 10;
printf( "data.i : %dn", data.i);
data.f = 220.5;
printf( "data.f : %fn", data.f);

strcpy( data.str, "C Programming");
printf( "data.str : %sn", data.str);
return 0;
}
OUTPUT
data.i : 10
data.f : 220.500000
data.str : C Programming
Difference between Union and Structure
A union allocates the memory equal to the maximum memory required by the
member of the union. However, a structure allocates the memory equal to the total
memory required by the members.
In a union, one block is used by all the member of the union. However, in case of
a structure, each member has its own memory space.
POINTERS
4.7 POINTERS
A pointer is a variable which contains the address in memory of another
variable. We can have a pointer to any variable type. The unary or monadic operator &
gives the ``address of a variable''. The indirection or dereference operator * gives the
``contents of an object pointed to by a pointer''.
General form: datatype *variablename;
Example: int *ptr;
Uses of a Pointer
 Allows to keep track of address of memory locations.
 Allows to easily manipulate data in different memory locations.
 Allows dynamic allocation of memory
 Allows accessing array elements, passing arrays and strings to functions,
creating data structures such as linked lists, trees, graphs and so on.
Example of a Pointer
Consider the following declaration
int num=5;
The compiler will automatically assign memory for this data item. The data item can be
accessed if we know the location(that is the address) of the first memory cell.

Example Program for Usage of Pointers
#include <stdio.h>
int main ()
{
int var = 20; /* actual variable declaration */
int *ip; /* pointer variable declaration */
ip = &var; /* store address of var in pointer variable*/
printf("Address of var variable: %xn", &var );
/* address stored in pointer variable */
printf("Address stored in ip variable: %xn", ip );
/* access the value using the pointer */
printf("Value of *ip variable: %dn", *ip );
return 0;
}
OUTPUT
Address of var variable : bffd8b3c
Address stored in ip variable : bffd8b3c
Value of *ip variable : 20
NULL Pointer
It is always a good practice to assign a NULL value to a pointer variable in case
we do not have an exact address to be assigned. This is done at the time of variable
declaration. A pointer that is assigned NULL is called a null pointer.
The NULL pointer is a constant with a value of zero defined in several standard
libraries. Consider the following program −
#include <stdio.h>
int main ()
{
int *ptr = NULL;
printf("The value of ptr is : %xn", ptr );
return 0;
}
OUTPUT
The value of ptr is : 0
4.8 POINTERS AND ARRAYS
Array is a collection of similar data type elements stored under common name.
When we declare an array the consecutive memory location are located to the array of
elements. The elements of an array can be efficiently accessed by using pointers.
 Array elements are always stored in consecutive memory location according to
the size of the array

 The size of the data type with the pointer variables refers to, depends on the data
type pointed by the pointer.
 A pointer when incremented, always points to a location after skipped the
number of bytes required for the data type pointed to by it.
Example Program for Pointer to an Array
#include <stdio.h>
int main ()
{
/* an array with 5 elements */
double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};
double *p;
int i;
p = balance;
/* output each array element's value */
printf( "Array values using pointern");
for ( i = 0; i < 5; i++ )
{
printf("*(p + %d) : %fn", i, *(p + i) );
}
printf( "Array values using balance as addressn");
for ( i = 0; i < 5; i++ )
{
printf("*(balance + %d) : %fn", i, *(balance + i) );
}
return 0;
}
OUTPUT
Array values using pointer
*(p + 0) : 1000.000000
*(p + 1) : 2.000000
*(p + 2) : 3.400000
*(p + 3) : 17.000000
*(p + 4) : 50.000000
Array values using balance as address
*(balance + 0) : 1000.000000
*(balance + 1) : 2.000000
*(balance + 2) : 3.400000
*(balance + 3) : 17.000000
*(balance + 4) : 50.000000

4.9 POINTERS AND FUNCTIONS
 A function like a variable has a type and an address location in the memory. It is
therefore possible to declare a pointer to a function, which can be used as an
argument in another function.
 Pointer to a function contains the address of function in memory. Pointers are
passed to a function as arguments.
Example Program for Pointer to Functions
#include <stdio.h>
/* function declaration */
double getAverage(int *arr, int size);
int main ()
{
/* an int array with 5 elements */
int balance[5] = {1000, 2, 3, 17, 50};
double avg;
/* pass pointer to the array as an argument */
avg = getAverage( balance, 5 ) ;
/* output the returned value */
printf("Average value is: %fn", avg );
return 0;
}
double getAverage(int *arr, int size)
{
int i, sum = 0;
double avg;
for (i = 0; i < size; ++i)
{
sum += arr[i];
}
avg = (double)sum / size;
return avg;
}
OUTPUT
Average value is: 214.40000
4.10 POINTERS AND STRINGS
Strings are treated like character arrays and therefore, they are declared and initialized
as follows.
char str[5]=”good”;
The compiler automatically inserts the null character’0’ at the end of the
string. C supports an alternative method to create strings using pointer variables of
type char. For example

char *str=”good”;
This creates a string for the literal and then stores its address in the pointer
variable str. The pointer str now points to the first character of the string “good” as:
g o o d 0
str
 We can also us the run-time assignment for giving values to a string pointer. For
example
char * string1;
string1=”good”;
Note that the assignment
string1=”good”; is not a string copy, because the variable string1
is a pointer, not a string.
 C does not support copying one string to another through assignment operation.
 We can print the content of the string string1 using either printf or puts function
as follows
printf(“%s”, string1);
puts(string1);
Example Program to determine the length of a character string using pointers
#include<stdio.h>
#include<conio.h>
void main()
{
char *name;
int length;
char *cptr=name;
name=”India”;
printf(“%sn”,name);
while(*cptr !=’0’)
{
printf(“%c is stored at address %un”, *cptr, cptr);
cptr++;
}
length =cptr-name;
printf(“n Length of the string=%dn”, length);
}
OUTPUT
INDIA
I is stored at address : 24
N is stored at address : 25
D is stored at address : 26
I is stored at address : 27
A is stored at address : 28

4.11 POINTERS AND STRUCTURES
We can define pointers to structures in the same way as you define pointer to any other
variable −
struct Books *struct_pointer;
Now, we 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;
Example Program for Pointer to Structures
#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( )
{
strcpy( Book1.title, "C Programming");
strcpy( Book1.author, "Nuha Ali");
strcpy( Book1.subject, "C Programming Tutorial");
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Zara Ali");
strcpy( Book2.subject, "Telecom Billing Tutorial");
/* 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 : %sn", book->title);
printf( "Book author : %sn", book->author);
printf( "Book subject : %sn", book->subject);
printf( "Book book_id : %dn", book->book_id);
}
OUTPUT
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

Structures and Pointers

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