Structure

Unit IV : Structure and Union

CHAPTER 9 : STRUCTURE AND UNION

Takeaways

• Structure declaration, initialization, and access

• Structures and Functions

• Arrays of unions

• Enumerated data type

• Nested structures

• Self-referential structures

• Unions within structures

• Arrays of structures

• Unions

• Structure within unions

INTRODUCTION

A structure is similar to records. It stores related information about an entity. A structure is a user-defined data type that can store related information (even of different data types) together. The major difference between a structure and an array is that, an array contains related information of the same data type.

A structure is, therefore, a collection of variables under a single name. The variables within a structure are of different data types and each has a name that is used to select it from the structure.

Structure Declaration

A structure is declared using the keyword struct followed by a structure name. All the variables of the structure are declared within the structure. A structure type is generally declared by using the following syntax:

Programming Tip: Do not forget to place a semicolon after the definition of structures and unions.

struct struct-name

{

data_type var-name;

data_type var-name;

};

For example, if we have to define a structure for a student, then its related information probably would be: roll_number, name, course, and fees. This can be declared as:

struct student

{

int r_no;

char name [20];

char course [20];

float fees;

};

Now, the structure has become a user-defined data type. Each variable name declared within a structure is called a member of the structure. The structure declaration, however, does not allocate any memory or consume storage space. It just gives a template that conveys to the C compiler how the structure should be laid out in memory and gives details of the member names. Like any other data type, memory is allocated for the structure when we declare a variable of the structure. For example, we can define a variable student by writing

struct student stud1;

Here, struct student is a data type and stud1 is a variable. Look at another way of declaring variables. In the following syntax, the variable is declared at the time of structure declaration.

struct student

{

int r no;

char name [20];

char course [20];

float fees;

}stud1, stud2;

In this declaration we declare two variables stud1 and stud2 of the structure student. So if you want to declare more than one variable of the structure, then separate the variables using a comma.

When we declare variables of the structure, separate memory is allocated for each variable. This is shown in Figure 8.1.

Let us see some more structure declarations.

Example 1

Declare a structure to store information about a point in the coordinate system.

struct point

{

int x,y;

};

Example 2

Declare a structure to store customer information.

struct customer

{

int cust id;

char name [20];

char address [20];

long int phone_num;

int DOB;

};

Example 3

Declare a structure to store information of a particular date.

struct date

{

int day;

int month;

int year;

};

Example 4

Declare a structure to store information of a particular book.

Programming Tip: Use different member names for different structures for clarity.

struct BOOK

{

char title [20];

char author [20];

int pages;

float price; T

int yr_of_publication;

};

Example 5

Declare a structure to create an inventory record.

struct inventory

{

char prod_name [20];

float price;

int stock;

};

Note

Structure type and variable declaration of a structure can be either local or global depending on their placement in the code.

Last but not the least, structure member names and names of the structure follow the same rules as laid down for the names of ordinary variables. However, care should be taken to ensure that the name of structure and the name of a structure member should not be the same. Moreover, structure name and its variable name should also be different.

Typedef Declarations

Programming Tip: C does not allow declaration of variables at the time of creating a typedef definition. So variables must be declared in an independent statement.

The typedef (derived from type definition) keyword enables the programmer to create a new data type name from an existing data type. By using typedef, no new data is created, rather an alternate name is given to a known data type.

The general syntax of using the typedef keyword is given as:

typedef existing data_type new data_type

Note that typedef statement does not occupy any memory, it simply defines a new type. For example, if we write

typedef int INTEGER;

then INTEGER is the new name of data type int. To declare variables using the new data type name, precede the variable name with the data type name (new). Therefore, to define an integer variable, we may now write

INTEGER num=5;

When we precede a struct name with typedef keyword, then the struct becomes a new type. It is used to make the construct shorter with more meaningful names for types already defined by C or for types that you have declared. A typedef declaration is a synonym for the type. For example, writing

typedef struct student

{

int r_no;

char name [20];

char course [20];

float fees;

};

Now that you have preceded the structure's name with the keyword typedef, the student becomes a new data type. Therefore, now you can straightaway declare variables of this new data type as you declare variables of type int, float, char, double, etc. To declare a variable of structure student you will just write

student stud1;

Note that we have not written struct student stud1.

Initialization of Structures

A structure can be initialized in the same way as other data types are initialized. Initializing a structure means assigning some constants to the members of the structure. When the user does not explicitly initialize the structure, then C automatically does that. For int and float members, the values are initialized to zero and char and string members are initialized to '\0' by default (in the absence of any initialization done by the user).

The initializers are enclosed in braces and are separated by commas. However, care must be taken to see that the initializers match their corresponding types in the structure definition.

The general syntax to initialize a structure variable is noitasilaitini given as follows:

struct struct name

{

data_type member_name1;

data_type member_name2;

data_type member_name3;

…………………………………………..

}struct_var {constantl, constant2, constant 3,... };

OR

Programming Tip: It is an error to assign a structure of one type to a structure of

another type.

struct struct name

{

data_type member_name1;

data_type member_name2;

data_type member_name3;

……………………………………………

};

struct struct_name struct_var ={constant1, constant2, constant 3,...};

For example, we can initialize a student structure by writing

struct student

{

int r_no;

char name [20];

char course [20];

float fees;

}stud1 = {01, "Rahul", "BCA", 45000};

or by writing

struct student stud1 = {01, "Rahul", "BCA", 45000};

Figure 8.2 illustrates how the values will be assigned to the individual fields of the structure.

When all the members of a structure are not initialized, it is called partial initialization. In case of partial initialization, first few members of the structure are initialized and those that are uninitialized are assigned default values.

Accessing the Members of a Structure

Each member of a structure can be used just like a normal variable, but its name will be a bit longer. A structure member variable is generally accessed using a '.' (dot) operator. The syntax of accessing a structure or a member of a structure can be given as:

Struct_var. member_name

Programming Tip: A member of the structure cannot be accessed directly using its name. Rather you must use the structure name followed by the dot operator before specifying the member name.

The dot operator is used to select a particular member of the struc- ture. For example, to assign value to the individual data members of the structure variable stud1, we may write

studl.r_no = 01;

stud1.name = "Rahul";

stud1.course = "BCA";

stud1.fees = 45000;

To input values for data members of the structure variable stud1, we may write

scanf("%d", &stul.r_no);

scanf("%s", stud1.name);

Similarly, to print the values of structure variable stud1, we may write

printf("%s", stud1.course);

printf("%f", stud1. fees);

Memory is allocated only when we declare variables of the structure. In other words, memory is allocated only when we instantiate the structure. In the absence of any variable, structure definition is just a template that will be used to reserve memory when a variable of type struct is declared.

Once the variables of a structure are defined, we can perform a few operations on them. For example, we can use the assignment operator '=' to assign the values of one variable to another.

Note

Of all the operators ->,.,(), and [] have the highest priority. This is evident from the following statement:

stud1.fees++ will be interpreted as (stud1. fees) ++

Copying and Comparing Structures

We can assign a structure to another structure of the same type. For example, if we have two structure variables studl and stud2 of type struct student given as

Programming Tip: An error will be generated if you try to compare two structure variables.

struct student stud1 = {01, "Rahul", "BCA", 45000};

struct student stud2;

Then to assign one structure variable to another we will write,

stud2 = stud1;

This statement initializes the members of stud2 with the values of members of stud1. Therefore, now the values of stud1 and stud2 can be given as shown in Figure 8.3.

C does not permit comparison of one structure variable with another. However, individual members of one structure can be compared with individual members of another structure. When we compare one structure member with another structure's member, the comparison will behave like any other ordinary variable comparison. For example, to compare the fees of two students, we will write

if (studl.fees > stud2. fees)

Fees of studl is greater than stud2

1. Write a program using structures to read and display the information about a student.

#include <stdio.h>

#include <conio.h>

int main()

{

struct student

{

int roll_no;

char name [80];

float fees;

char DOB [80];

};

struct student stud1;

clrscr();

printf("\n Enter the roll number: ");

scanf("%d", &studl.roll_no);

printf("\n Enter the name: ");

scanf("%s", stud1.name);

printf("\n Enter the fees: ");

scanf("%f", &stud1.fees);

printf("\n Enter the DOB: ");

scanf("%s", stud1. DOB);

printf("\n *******STUDENT'S DETAILS******* " ");

printf("\n ROLL No. = %d", stud1.roll_no);

printf("\n NAME = %s", stud1.name);

printf( "\n FEES = %f", stud1. fees);

printf("\n DOB = %s", stud1. DOB);

getch();

return 0;

}

Output

Enter the roll number: 01

Enter the name: Rahul

Enter the fees: 45000

Enter the DOB: 25-09-1991

*******STUDENT'S DETAILS *******

ROLL NO. = 01

NAME = Rahul

FEES = 45000.00

DOB = 25-09-1991

2. Write a program, using structures to find the biggest of three numbers.

#include <stdio.h>

#include <conio.h>

int main()

{

struct numbers

{

int a, b, c;

int largest;

};

struct numbers num;

clrscr();

printf("\n Enter the three numbers: ");

scanf("%d %d %d", &num. a, &num. b, &num. c);

if (num.a > num.b && num.a > num. c)

num. largest = num.a;

if (num.b > num. a && num.b > num.c)

num.largest = num.b;

if (num.c > num. a && num.c > num. b)

num. largest = num.c;

printf("\n The largest number is: %d", num. largest);

getch();

return 0;

}

Output

Enter the three numbers: 7 9 1

The largest number is: 9

3. Write a program to read, display, add, and subtract two complex numbers.

#include <stdio.h>

#include <conio.h>

int main()

{

typedef struct complex

{

int real;

int imag;

} COMPLEX;

COMPLEX c1, c2, sum_c, sub_c;

int option;

clrscr();

do

{

printf("\n **** MAIN MENU ****");

printf("\n 1. Read the complex nos.");

printf("\n 2. Display the complex nos.");

 printf("\n 3. Add the complex nos.");

printf("\n 4. Subtract the complex nos.");

printf("\n 5. EXIT");

printf("\n Enter your option: ");

scanf("%d", &option);

switch (option)

{

case 1:

printf("\n Enter the real and imaginary parts of the first complex number: ");

scanf("%d %d", &c1. real, &cl.imag);

printf("\n Enter the real and imaginary parts of the second complex number: ");

scanf("%d %d", &c2. real, &c2.imag);

break;

case 2:

printf("\n The first complex number is: %d %di", c1. real, cl.imag);

printf("\n The second complex number is: %d %di", c2. real, c2.imag);

break;

case 3:

sum_c.real = c1. real + c2. real;

sum_c.imag= c1.imag + c2.imag;

printf("\n The sum of two complex numbers is: %d+%di", sum_c.real, sum_c.imag);

break;

case 4:

sub_c.real = c1.real - c2.real;

sub_c.imag = c1.imag - c2.imag;

printf("\n The difference between two complex numbers is: %d + %di", sub_c.real, sub_c.imag);

break;

}

} while (option != 5);

getch();

return 0;

}

Output

******** MAIN MENU ****

1. Read the complex numbers

2. Display the complex numbers

3. Add the complex numbers

4. Subtract the complex numbers

5. EXIT

Enter your option: 1

Enter the real and imaginary parts of the first complex number: 2 3

Enter the real and imaginary parts of the second complex number: 4 5

******** MAIN MENU *********

1. Read the complex numbers

2. Display the complex numbers

3. Add the complex numbers

4. Subtract the complex numbers

5. EXIT

Enter your option: 3

The sum of two complex numbers is: 6 + 8i

4. Write a program to enter two points and then calculate the distance between them.

#include <stdio.h>

#include <conio.h>

#include <math.h>

int main()

{

typedef struct point

int x, y;

} POINT;

POINT p1, p2;

float distance;

clrscr();

printf("\n Enter the coordinates of the first point: ");

scanf("%d %d", &pl.x, &pl.y);

printf("\n Enter the coordinates of the second point: ");

scanf("%d %d", &p2.x, &p2.y);

distance = sqrt (pow((pl.x p2.x), 2) + pow((pl.y p2.y), 2));

printf("\n The coordinates of the first point are: %dx %dy", pl.x, pl.y);

printf("\n The coordinates of the second point are: %dx %dy", p2.x, p2.y);

printf("\n Distance between p1 and p2 = %f", distance);

getch();

return 0;

}

Output

Enter the coordinates of the first point: 2 3

Enter the coordinates of the second point: 9 9

The coordinates of the first point are: 2x 3y

The coordinates of the second point are: 9x 9y

Distance between p1 and p2 = 9.219544