Structures and Unions in C

Advanced Programming in C Unit 2

Unit 2 Structures and Unions
Structure
2.0 Introduction
2.1 Basics of structures
Self Assessment Questions
2.2 Structures and functions
Self Assessment Questions
2.3 Arrays of structures
2.4 Pointers to structures
Self Assessment Questions
2.5 Selfreferential
structures
2.6 Unions
Self Assessment Questions
2.7 Summary
2.8 Terminal Questions
2.9 Answers to Self Assessment Questions
2.10 Answers to Terminal Questions
2.11 Exercises
2.0 Introduction
As we know an array is a data structure whose elements are all of the same
data type. We now turn our attention to the structure, which is a data
structure whose individual elements can differ in type. Thus, a single
structure might contain integer elements, floatingpoint
elements and
character elements. Pointers, arrays and other structures can also be
included as elements within a structure.
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This chapter is concerned with the use of structures within a C program. We
will see how structures are defined, and how their individual members are
accessed and processed within a program.
Closely associated with the structure is the union, which also contains
multiple members. Unlike a structure, however, the members of a union
share the same storage area, even though the individual members may
differ in type.
Objectives
At the end of this unit, you will be able to:
· Handle a group of logically related data items known as structures.
· Declare an array of structures, each element of the array representing a
structure variable.
· Pass Structure as an argument to functions and return structure from
functions.
· The ability to refer to (ie, point to) an incomplete type, including itself.
· Handle a group of logically related data items in terms of unions.
2.1 Basics of Structures
C supports a constructed data type known as structure, which is a method
for packing data of different types. A structure is a convenient tool for
handling a group of logically related data items. Structures help to organize
complex data in a more meaningful way. It is a powerful concept that we
may often need to use in our program design.
Structure Definition: A Structure definition creates a format that may be
used to declare structure variables. For e.g., Consider a book database
consisting of book name, author, number of pages and price.
struct book_bank
{
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char title[20];
char author[15];
int pages;
float price;
};
The keyword struct declares a structure to hold the details of four fields,
namely title, author, pages and price. These fields are called structure
elements or members. Each member may belong to a different type of data.
book_bank is the name of the structure and is called the structure tag. The
tag name may be used subsequently to declare variables that have the tag’s
structure. Note that the above declaration has not declared any variables. It
simply describes a format called template to represent information as shown
below:
struct book_bank
title
author
pages
price
We can declare structure variables using the tag name anywhere in the
program. e.g, the statement:
struct book_bank book1, book2, book3;
declares book1, book2 and book3 as variables of type book_bank.
array of 15 characters
integer
float
array of 20 characters
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Each one of these variables has four members as specified by the template.
The complete declaration might look like this :
struct book_bank
{
char title[20];
char author[15];
int pages;
float price;
};
struct book_bank book1, book2, book3;
It is also allowed to combine both the template declaration and variables
declaration in one statement.
struct book_bank
{
char title[20];
char author[15];
int pages;
float price;
} book1, book2, book3;
General format of a Structure Definition :
The general format of a structure definition is as follows:
struct tag_name
{
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data_type member1;
data_type member2;
}
;
In defining a structure you may note the following syntax:
1. The template is terminated with a semicolon.
2. While the entire declaration is considered as a statement, each member
is declared independently for its name and type in a separate statement
inside the template.
3. The tag name such as tag_name can be used to declare structure
variables of its type, later in the program.
Giving values to Members :
Structure members need to be linked to the structure variables in order to
make them meaningful members. The link between a member and a
variable is established using the member operator ‘.’ which is also known as
‘dot operator’ or ‘period operator’.
Here is how we would assign values to the members of book1.
strcpy(book1.title,”BASIC”);
strcpy(book1.author,”Balagurusamy”);
book1.pages = 250;
book1.price = 28.50;
We can also give the values through the keyboard.
gets(book1.title);
gets(book1.author);
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printf(“%d”,book1.pages);
printf(“%f”,book1.price);
Structure Initialization :
void main( )
{
struct st_record
{
char name[20];
int weight;
float height;
};
static struct st_record student1 = {“Suresh”, 60, 180.75};
static struct st_record student2 = {“Umesh”, 53, 170.60};
}
Program 2.1 To print the date of joining of a person
#include<conio.h>
#include<stdio.h>
struct personal
{
char name[30];
int day;
int month;
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int year;
float salary;
};
void main()
{
struct personal p;
printf(“Enter the name:\n)";
gets(p.name);
printf(“Enter the day of joining:\n)";
scanf(“%d”,&p.day);
printf(“Enter the month of joining:\n");
scanf(“%d”,&p.month);
printf(“Enter the year of joining:\n)";
scanf(“%d”,&p.year);
printf(“Enter the salary:\n)";
scanf(“%f”, & p.salary);
printf(“\nName:",p.name);
printf("\nDate of joining:%d %d %d",p.day,p.month,p.year);
printf(“Salary:",p.salary);
getch();
}
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Comparison of structure variables
Two variables of same structure type can be compared in the same way as
ordinary variables. If person1 and person2 belong to the same structure ,
then the following operations are valid:
Operation Meaning
person1 = person2 Assign person2 to person1
person1 == person2 Compare all members of person1 and
person2 and return 1 if they are
equal, 0 otherwise.
person1 != person2 Return 1 if all the members are not
equal, 0 otherwise
Program 2.2 To Compare structure variables
#include <stdio.h>
#include<conio.h>
struct stclass{
int number;
char name[20];
float marks;
};
void main()
{
int x;
static struct stclass student1 = {111,"Rao",72.50};
static struct stclass student2 = {222,"Reddy",67.00};
struct stclass student3;
student3 = student2;
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x=((student3.number == student2.number) && (student3.marks ==
student2.marks))? 1:0;
if(x==1)
{
printf("\nStudent2 and Student3 are same ");
printf(“ %d\t %s\t %f\t“,student3.number,student3.name,student3.marks);
}
else
{
printf("\nStudent2 and student3 are different)";
}
getch();
}
Self Assessment Questions
i) A___________ is a convenient tool for handling a group of logically
related data items.
ii) State true or false:
We can declare structure variables using the tag name anywhere in
the program.
iii) State true or false
Array is a method for packing data of different types.
iv) State true or false
If person1 and person2 are variables of the same type structure then
the expression person1>person2 is valid.
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2.2 Structures and Functions
We can write programs with structures by using modular programming. We
can write a function that returns the structure. While writing the function, you
should indicate the type of structure that is returned by the function. The
return statement should return the structure using a variable. It is possible
to pass a structure as an argument. We can modify a member of the
structure by passing the structure as an argument. The changes in the
member made by the function are retained in the called module. This is not
against the principle of call by value because we are not modifying the
structure variable, but are instead modifying the members of the structure.
Program 2.3 To Illustrate the concept of structures and functions
struct student
{
name char[30];
marks float;
};
main ( )
{
struct student student1;
student1 = read_student ( );
print_student( student1);
read_student_p(student1);
print_student (student1);
}
struct student read_student( )
{
struct student student2;
gets(student2.name);
scanf("%d",&student2.marks);
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return (student2);
}
void print_student (struct student student2)
{
printf( "name is %s\n", student2.name);
printf( "marks are%d\n", student2.marks);
}
void read_student_p(struct student student2)
{
gets(student2.name);
scanf("%d",&student2.marks);
}
Explanation
1. The function read_student reads values in structures and returns the
structure.
2. The function print_student takes the structure variable as input and
prints the content in the structure.
3. The function read_student_p reads the data in the structure similarly to
read_student. It takes the structure student as an argument and puts
the data in the structure. Since the data of a member of the structure is
modified, you need not pass the structure as a pointer even though
structure members are modified. Here you are not modifying the
structure, but you are modifying the structure members through the
structure.
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Self Assessment Questions
i) State true or false:
We cannot write a function that returns the structure.
ii) State true or false:
We can modify a member of the structure by passing the structure as an
argument.
2.3 Arrays of Structures
We can use structures to describe the format of a number of related
variables. For example, in analyzing the marks obtained by a class of
students, we may use a template to describe student name and marks
obtained in various subjects and then declare all the students as structure
variables. In such cases, we may declare an array of structures, each
element of the array representing a structure variable. e.g, struct stclass
student[100]; defines an array called student, that consists of 100 elements.
Each element is defined to be of the type struct stclass. Consider the
following declaration :
struct marks
{
int subject1;
int subject2;
int subject3;
};
main( )
{
static struct marks student[3]={{45,68,81},{75,53,69},{57,36,71}};
}
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This declares the student as an array of three elements student[0],
student[1] and student[2] and initializes their members as follows:
student[0].subject1 = 45;
student[0].subject2 = 68;
……..
student[2].subject3 = 71;
Program 2.4 To process employee details using structures
#include<conio.h>
#include<stdio.h>
struct employee
{
int empno;
char name[30];
int basic;
int hra;
};
void main()
{
int i,j,n,net[50];
float avg;
employee e[50];
printf("\nEnter the number of employees:");
scanf(“%d”, &n);
printf(“\nEnter Empno.\tName\tBasic\tHra of each employee:\n");
for(i=0;i<n;i++)
{
scanf(“%d”,&e[i].empno);
gets(e[i].name);
scanf(“%d”,&e[i].basic);
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scanf(%d”,&e[i].hra);
net[i]= e[i].basic+e[i].hra;
avg=avg+net[i];
}
avg=avg/n;
printf("\nEmpno.\tName\tNetpay\n");
for(i=0;i<n;i++)
{
if(net[i]>avg)
{
printf(e[i].empno\t)";
printf(e[i].name\t)";
printf(net[i]\n");
}
}
getch();
}
Program 2.5 To process student details using structures
#include<conio.h>
#include<stdio.h>
struct student
{
int rollno;
char name[30];
int marks1;
int marks2;
int marks3;
};
void main()
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{
int i,j,n,tot[50],t;
student s[50],temp;
printf("\nEnter the number of students:");
scanf(“%d”,&n);
printf("\nEnter Rollno.\tName\tMarks1\tMarks2\tMarks3 of each student:\n");
for(i=0;i<n;i++)
{
scanf(“%d”,&s[i].rollno);
gets(s[i].name);
scanf(“%d”,&s[i].marks1);
scanf(“%d”,&s[i].marks2);
scanf(“%d”,&s[i].marks3);
tot[i]= s[i].marks1+s[i].marks2+s[i].marks3;
}
for(i=0;i<n1;i++)
{
for(j=i+1;j<n;j++)
{
if(tot[i]<tot[j])
{
temp=s[i];
s[i]=s[j];
s[j]=temp;
t=tot[i];
tot[i]=tot[j];
tot[j]=t;
}
}
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}
printf("\nRollno.\tName\tTotal marks in decreasing order of total marks
is:\n");
for(i=0;i<n;i++)
{
printf(“%d\t”,s[i].rollno);
printf(“%s\t”,s[i].name);
printf(“%d\t”,s[i].tot);
}
getch();
}
2.4 Pointers to Structures
Pass by value may be very inefficient if the structure is large (i.e., has many
members). They have identical declaration syntax and member access, but
they serve a very different purpose. Defining pointer types is the same as for
variables of primitive types.
Example:
struct Point {
int x;
int y;
};
struct Rectangle {
struct Point topleft;
struct Point bottomright;
};
struct Point pt = { 50, 50 };
struct Point *pp;
pp = &pt;
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(*pp).x = 100; /* pt.x is now 100. */
Notice the parentheses around the de referenced pointer.
(*pp).x = 100;
This is necessary to enforce correct precedence.
An alternative notation permits simpler pointer access to structure members.
(*pp).x = 100;
pp>
x = 100; /* equivalent */
Another example,
struct Rectangle rect, *pr = &rect;
rect.topleft.x = 50; /* equivalent operations */
(*pr).topleft.x = 50;
pr>
topleft.x = 50;
Self Assessment Questions
i) The parentheses around the de referenced pointer is necessary to
enforce the correct_________.
ii) An alternative notation other than dot, permits simpler pointer access
to structure members is ____________.
2.5 Selfreferential
Structures
The ability to refer to (ie, point to) an incomplete type, including itself, is an
important property for constructing a variety of datastructures.
For example:
linkedlists,
binary trees, graphs, hash tables, and more.
Linked lists come in two basic varieties: singly linked and doubly linked.
We describe here a simple version of a singly linked list.
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List consists of a set of nodes, where each node contains an item and a
pointer to another list node.
struct List {
int item;
struct List *next;
};
More about linked lists, you will study in Unit 3.
2.6 Unions
Unions look similar to structures. They have identical declaration syntax and
member access, but they serve a very different purpose.
union Utype {
int ival;
float fval;
char *sval;
};
union Utype x, y, z;
Accessing members of a union is via “.” member operator or, for pointers to
unions, the >
operator.
A union holds the value of onevariable
at a time. The compiler allocates
storage for the biggest member of the union. The type retrieved from the
union must be the type most recently stored. Otherwise, the result is
implementation dependent.
union Utype x;
x.fval = 56.4; /* x holds type float. */
printf("%f\n", x.fval); /* OK. */
printf("%d\n", x.ival); /* Implementation dependent. */
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Unions are used to store one of a set of different types. These are
commonly used to implement a “variant” array. (This is a form of generic
programming.) There are other uses also, but they are quite advanced (e.g.,
concern the alignment properties of unions).
Self Assessment Questions
i) A __________ holds the value of onevariable
at a time.
ii) State true or false:
The compiler allocates storage for the smallest member of the union.
2.7 Summary
A structure is a convenient tool for handling a group of logically related data
items. Structure members need to be linked to the structure variables in
order to make them meaningful members. We can write programs with
structures by using modular programming. We can use structures to
describe the format of a number of related variables. Passing a pointer to a
structure is generally much more efficient than making a copy of the
structure itself. The ability to refer to (ie, point to) an incomplete type,
including itself, is an important property for constructing a variety of datastructures.
Unions have identical declaration syntax and member access, but they
serve a very different purpose. A union holds the value of onevariable
at a
time. The compiler allocates storage for the biggest member of the union.
2.8 Terminal Questions
1. State whether true or false
Structure is a method for packing data of different types.
2. The link between a member and a variable is established using the
member operator ______________.
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3. Write the output that will be generated by the following C program:
typedef struct
{
char *a;
char *b;
char *c;
} colors;
void main()
{
void fun( colors sample);
static colors sample = {“red”, “green”, “blue”};
printf( (“%s %s %s\n”, sample.a, sample.b,
sample.c);
fun(sample);
printf( (“%s %s %s\n”, sample.a, sample.b,
sample.c);
}
void fun (colors sample)
{
strcpy (sample.a=”cyon”);
strcpy (sample.b=”magenta”);
strcpy (sample.c=”yellow”);
printf(“%s %s %s\n”, sample.a, sample.b,
sample.c);
return;
}
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4. Describe the output generated by the following program. Distinguish
between meaningful and meaningless output.
#include <stdio.h>
main()
{
union {
int i;
float f;
double d;
} u;
printf(“%d\n”, sizeof(u));
u.i= 100;
printf(“%d %f %f\n”, u.i, u.f, u.d);
u.f=0.5;
printf(“%d %f %f\n”, u.i, u.f, u.d);
u.d = 0.0166667;
printf(“%d %f %f\n”, u.i, u.f, u.d);
}
2.9 Answers to Self Assessment Questions
2.1 i) structure
ii) true
iii) false
iv) false
2.2 i) false
ii) true
2.4 i) precedence
ii) >
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2.6 i) union
ii) false
2.10 Answers to Terminal Questions
1. true
2. dot (.)
3. red green blue
cyan magenta yellow
red blue green
4. 8
100 0.000000 0.000000
0 0.500000 0.000000
25098
391364288.000000 0.016667
The first line displays the size of the union (8 bytes, to accommodate double
data). In the second line , only the first value(100) is meaningful. In the third
line , only the second value(0.500000) is meaningful. In the last line, only
the last value(0.016667) is meaningful..
2.11 Exercises
1. What is a structure? How does a structure differ from an array?
2. What is a member? What is the relationship between a member and a
structure?
3. Describe what is wrong in the following structure declaration:
struct
{
int number;
float price;
}
main()
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{
…………….
………………
}
4. Describe Array of structures with an example program.
5. Define a structure called cricket that will describe the following
information:
(i) player name (ii) team name (iii) batting average
Using cricket , declare an array player with 50 elements and write a
program to read the information about all the 50 players and print a
teamwise
list containing names of players and print a teamwise
list
containing names of players with their batting average.
6. How is a structure type pointer variable declared?