Structures and Unions

(1)

1

B.B.B.B. Naresh, Assistant Professor in B.V.R.I.C.E, Dept., of MCANaresh, Assistant Professor in B.V.R.I.C.E, Dept., of MCANaresh, Assistant Professor in B.V.R.I.C.E, Dept., of MCANaresh, Assistant Professor in B.V.R.I.C.E, Dept., of MCA

Structures and Unions

Structures are introduced to combine different data types into single block. We can access

entire block of data with the structure. It can store different data elements information as a single

unit.

Definition of Structure:

Structure is a user defined data type and it is a combination of different data types called as

members. All these members are referred by a single name known as structure name. Structure can

be placed before or within the main.

Syntax to Create Structure:

struct tagname {

data type member 1; …………..

…………..

data type member n; };

We can access the members of structures using structure variable. This structure variable

should be created after completion of structure definition. We can create structure variable in two

ways.

Structure variable can be created as follows:

Syntax: struct tagname(structurename) structure_variable_name;

Ex: struct student s;

We can also create the structure variable along with the structure definition as follows:

Syntax:

struct tagname {

data type member 1; …………..

…………..

data type member n; }structure variable name;

Accessing members of a Structure:

We can access members of structure by using dot (.) operator along with the structure

(2)

2

Syntax: structure_variable_name.member;

Ex: e.eno;

e.name;

e.sal;

The following program illustrates the structure with employee data:

Aim: To write a C-Program to read and print an employee data using structure.

Program:

#include<stdio.h>

#include<conio.h>

struct Employee

{

int eno;

char ename[20];

int esal;

};

void main()

{

struct Employee e;

clrscr();

printf("\nEnter Employee Number:");

(3)

3

printf("\nEnter Employee Name:");

scanf("%s",&e.ename);

printf("\nEnter Employee Salaray:");

scanf("%d",&e.esal);

printf("\n\nThe Employee Details are....\n");

printf("Employee Number:%d",e.eno);

printf("\nEmployee Name:%s",e.ename);

printf("\nEmployee Salary:%d",e.esal);

getch();

}

Out put:

Array of Structures:

Array is a collection of similar data types. In the same way we can also define array of

structures. In this structure array, every element is of structure type. The array of structure is

declared as follows

Syntax: struct tagname {

Data type member1; Data type member2; --- --- }array variable name[3]; Ex: struct emp

{

(4)

4

In the above example e[10] is an array of 10 elements containing 10 objects of type

structure. Each element of e[10] has structure type with three members that are eno,ename and

esal.

The following program illustrates the above concept.

Aim: To Write a C-Program that creates a structure of 10 students having student number

(sno), student name (sname), marks in three subjects (m1,m2,m3), total marks of the

students (tot) and average (avg) and display the names of failed students.

Program:

#include<stdio.h>

#include<conio.h>

void main()

{

struct Student

{

int sno;

char sname[20];

int m1,m2,m3;

int tot,avg;

};

int n,i;

struct Student s[10];

clrscr();

printf("\nEnter the Number of Students:");

scanf("%d",&n);

for(i=0;i<n;i++)

{

printf("\nEnter %d Student Details\n",i+1);

printf("Student Number:");

scanf("%d",&s[i].sno);

(5)

5

scanf("%s",&s[i].sname);

printf("Student Three Subjects Marks\n");

scanf("%d%d%d",&s[i].m1,&s[i].m2,&s[i].m3);

s[i].tot=s[i].m1+s[i].m2+s[i].m3;

s[i].avg=s[i].tot/3;

}

printf("\n\nThe failed students are...\n");

for(i=0;i<n;i++)

{

if(s[i].avg<35)

printf("%s\n",s[i].sname);

}

getch();

}

(6)

6

Pointer to Structures:

Pointer is a variable can store the address of another variable .The variable may be of any

data type. In the same way we can also define pointer to structure. Here, starting address of the

member variables can be accessed. Thus, such pointers are called structure pointers.

Ex: Struct book

{

char name[25];

char author[25];

int pages;

};struct book *ptr;

In the above example *ptr is pointer to structure book. The syntax for using pointer with

member is as given below

1) ptr-> name 2) ptr -> author 3) ptr -> pages

Structure and Functions:

Like variables of standard data type structure variables also can be passed to the function by

value or address. The syntax for the same is as follows

Syntax:

struct tagname

{

Data type member1;

Data type member 2;

---

}Structure_variable_name;

void main() {

<return type> <function name>( struct tagname[], argument list);//Protoype of function ---

--- --- function name( structure variable, argument list);//Function caller }

(7)

7

{

--- --- --- }

Example:

struct book {

char name[20]; char author[20]; int pages; }b1;

void main() {

void show(struct book);//Prototype ---

---

show(b1);//Caller ---

--- }

void show(struct book b2)//Callee {

--- --- }

Whenever a structure element is to be passed to any other function, it is essential to declare

the structure outside the main() function i.e., global.

The following program illustrates the above concept.

Aim: To write a C program to find the sum of two complex numbers.

Program:

#include<stdio.h>

#include<conio.h>

struct Complex

{

int real;

int img;

};

void f1(struct Complex x1,struct Complex x2)

(8)

8

struct Complex x3;

x3.real=x1.real + x2.real;

x3.img=x1.img + x2.img;

printf("\n The Complex Sum is: %d+i%d",x3.real,x3.img);

}

void main()

{

struct Complex c1,c2;

clrscr();

printf("\nEnter the real and imag parts of first Complex Number:\n");

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

printf("\nEnter the real and img parts of second Complex Number:\n");

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

f1(c1,c2);

getch();

}

Output:

Unions:

A union is a user defined data type available in C that enables us to store different data

types in the same memory location. We can define a union with many members, but only one

member can contain a value at any given time. Unions provide an efficient way of using the same

(9)

9

Defining a Union

To define a union, we must use the union statement. The format of the union statement is

as follows:

union [union tag]

{

member definition;

...

member definition;

} [one or more union variables];

Example:

union Data

{

int i;

float f;

char str[20];

} data;

Now, a variable of Data type can store an integer, a floating-point number, or a string of

characters.

The memory occupied by a union will be large enough to hold the largest member of the

union. For example, in above example Data type will occupy 20 bytes of memory space because

this is the maximum space which can be occupied by character string.

Following is the example which will display total memory size occupied by the above

union:

#include <stdio.h>

#include <string.h>

union Data

{

int i;

float f;

(10)

10

};

int main( )

{

union Data data;

printf( "Memory size occupied by data : %d\n", sizeof(data));

return 0;

}

When the above code is compiled and executed, it produces the following result:

Memory size occupied by data : 20

Accessing Union Members

To access any member of a union, we use the member access operator (.).

Following is the example to explain usage of union:

#include <stdio.h>

#include <string.h>

union Data

{

int i;

float f;

char str[20];

};

int main( )

{

union Data data;

data.i = 10;

data.f = 220.5;

strcpy( data.str, "C Programming");

printf( "data.i : %d\n", data.i);

printf( "data.f : %f\n", data.f);

printf( "data.str : %s\n", data.str);

return 0;

(11)

11

data.i : 1917853763

data.f : 4122360580327794860452759994368.000000

data.str : C Programming

Here, we can see that values of i and f members of union got corrupted because final value

assigned to the variable has occupied the memory location and this is the reason that the value of

str member is getting printed very well.

Now let's look into the same example once again where we will use one variable at a time

which is the main purpose of having union:

#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 : %d\n", data.i);

data.f = 220.5;

printf( "data.f : %f\n", data.f);

strcpy( data.str, "C Programming");

printf( "data.str : %s\n", data.str);

return 0;

}

data.i : 10

(12)

12

data.str : C Programming

Enumerated Data types in C:

Enumeration type allows programmer to define their own data type . Keyword enum is

used to defined enumerated data type.

enum type_name{ value1, value2,...,valueN };

Here, type_name is the name of enumerated data type or tag and value1, value2,....,valueN

are values of type type_name.

By default, value1 will be equal to 0, value2 will be 1 and so on but, the programmer can

change the default value as below:

enum suit{

club=0;

diamonds=10;

hearts=20;

spades=3;

};

Declaration of enumerated variable

The above code defines the type of the data but, no variable is created for the enum data

type. The following will shows the creation of a variable of type enum.

enum boolean{

false;

true;

};

enum boolean check;

Here, a variable check is declared which is of type enum boolean.

Example of enumerated type

#include <stdio.h>

enum week

{

sunday, monday, tuesday, wednesday, thursday, friday, Saturday

};

(13)

13

enum week today;

today=wednesday;

printf("%d day",today+1);

return 0;

}

Output

4 day

We can write any program in C language without the help of enumerations but,

enumerations helps in writing clear codes and simplify programming.

Explain the Scope and Visibility of Variables in C

A scope in any programming is a region of the program where a defined variable can have

its existence and beyond that the variable cannot be accessed. There are three places where

variables can be declared in C programming language:

Inside a function or a block which is called local variables,

Outside of all functions which is called global variables.

In the definition of function parameters which is called formal parameters. Let us explain what are local and global variables and formal parameters.

Local Variables

Variables that are declared inside a function or block are called local variables. They can

be used only by statements that are inside that function or block of code. Local variables are not

known to functions outside their own. Following is the example using local variables. Here all the

variables a, b and c are local to main() function.

#include <stdio.h>

int main ()

{

/* local variable declaration */

int a, b;

int c;

/* actual initialization */

a = 10;

(14)

14

c = a + b;

printf ("value of a = %d, b = %d and c = %d\n", a, b, c);

return 0;

}

Global Variables

Global variables are defined outside of a function, usually on top of the program. The

global variables will hold their value throughout the lifetime of a program and they can be

accessed inside any of the functions defined for the program.

A global variable can be accessed by any function. Following is the example using global

and local variables:

#include <stdio.h>

/* global variable declaration */

int g;

int main ()

{

int a, b;

/* actual initialization */

a = 10;

b = 20;

g = a + b;

printf ("value of a = %d, b = %d and g = %d\n", a, b, g);

return 0;

}

A program can have same name for local and global variables but value of local variable

inside a function will take preference. Following is an example:

#include <stdio.h>

int g = 20;

int main ()

(15)

15

int g = 10;

printf ("value of g = %d\n", g);

return 0;

}

value of g = 10

Formal Parameters

Function parameters, formal parameters, are treated as local variables with-in that function

and they will take preference over the global variables. Following is an example:

#include <stdio.h>

int a = 20;

int main ()

{

/* local variable declaration in main function */

int a = 10;

int b = 20;

int c = 0;

printf ("value of a in main() = %d\n", a);

c = sum( a, b);

printf ("value of c in main() = %d\n", c);

return 0;

}

/* function to add two integers */

int sum(int a, int b)

{

printf ("value of a in sum() = %d\n", a);

printf ("value of b in sum() = %d\n", b);

return a + b;

(16)

16

value of a in main() = 10

value of a in sum() = 10

value of a in sum() = 20

value of b in main() = 30

Important Questions:

Define structure and explain the process of accessing structure variables with an example.

Explain array of structures with an example.

Explain the pointers to structures with an example.

Explain how to pass structures to functions with an example.

Explain about the Unions in C with an example.

Explain about the Enumerated data types in C.