Module 7: Structures in C Learning Objectives

Module 7: Structures in C

Learning Objectives

1. To understand the concept of structure

2. To learn Declaration of structure

3. To learn Initialization of structure

4. To learn accessing structure members

Table of Content

1. Introduction

2. Basics of structure

3. Declaring structure variables

4. Structure Initialisation

5. Accessing structure members

6. Example program for structure

7. Uses of structure

8. Summary

1. Introduction:

C constants can be divided in to two major categories: Primary Constant, Secondary

Constant. Primary type of constant holds only single value or single character. Whereas

secondary types of constant holds multiple values or characters. The secondary constants are

array, structures, pointers, enumeration and union.

Structure is a user-defined data type in C which allows you to combine different data types to

store a particular type of record. Structure helps to construct a complex data type in more

meaningful way. It is somewhat similar to an Array. The only difference is that array is used

to store collection of similar data types while structure can store collection of any type of

data. Structure is used to represent a record. Suppose if it is required to store record

of Student which consists of student name, address, roll number and age then simple array

cannot store this information because here some fields are characters as well as numbers. In

short, data types are different. In such situation, a structure can be defined to hold this

information

Keyword struct is generally used whenever lot of data needs to be grouped together. For

example, it can be used to hold records from a database or to store information about

personal contacts in an address book. In the contacts, we can store name, address, phone

number, email etc.

In previous module, we have studied arrays. The array allows defining type of variables that

can hold several data items of the same kind. Similarly structure is another user defined data

type available in C that allows combining data items of different kinds, or we can say

structure is the collection of variables of different types under a single name for better

data handling.

2. Basics of structure

Structures are basically derived data types because they are constructed using objects of other

data types. C Structure is a collection of different data types which are grouped together and

each element in a C structure is called member. Basically Structure is used to store the

complicated data. A structure is a convenient way of grouping several pieces of related

information together.

One popular example of a structure is the student‟s record. A college student is described by

a set of attributes such as name, address, roll number, class, division etc. Some of these in

turn could be structures: a name has several components, as does an address and even a roll

number.

A structure is a composition of multiple data types which may have same name. These

different data types are called as the fields or members of the structure. struct statement is

used to define a structure. The struct statement defines a new data type, with more than one

member. The format of the struct statement is as follows

Syntax of structure:

struct structure_name

{

data_type member1;

data_type member2;

data_type member n;

};

The structure declaration starts with the keyword struct followed by the name of the

structure which defines its data type. It then has a pair { } which contains the

members of the structure. For example,

struct test

{

int x,

int y;

float z;

}

In this example, test is the name of structure which consist of two int and one float

type data together. Here x and y are members of type int and z is of type float.

Points to remember:

1. Keyword struct introduces the structure definition.

2. Sturct is followed by the structure tag, which names the structure definition

3. Structure tag is used to declare variables of the structure type.

4. Variables declared within braces of structure definition are the members of

structure.

5. Members of the same structure type must have unique names.

6. Members can be variables of primitive data types like int, float, char or other

data types like arrays, pointers etc.

7. Each structure definition must end with a semicolon.

3. Declaring Structure variable

A struct is a complex data declaration that defines a physically grouped list of variables to be

placed under one name in a block of memory, allowing the different variables to be accessed via

a single pointer or the struct declared name which returns the same address. There are

different ways to declare structure variable.

(a) Immediately after Structure Template

When a structure is define, it creates a user defined type but no storage is allocated for it.

Such structure for employee, variable can be declared as:

struct employ

{

int sal;

char name[40];

int age;

} employ;

//„employ' is name of Structure variable

(b) Declare Variable using struct Keyword

Another way to declare a variable using struct keyword is shown below:

struct date

{

int date;

char month[20];

int year;

};

struct date today;

where “date” is name of structure and “today” is name of variable.

In many cases it is necessary to declare more than one structure variables. A simple method

to implement this is shown in the following example.

struct student

{

int roll no;

char name[20];

float markes;

} stud, stud2, stud3;

We can declare multiple variables separated by comma directly after closing curly bracket.

Another way to declare structure variables separately is shown below.

struct applicant

{

int srno;

char name[50];

int age;

} struct applicant a1, a2, a3;

Let us now learn methods for initializing the structure.

4. Structure Initialization:

Like any other data type, structure variable can also be initialized at compile time.

struct student

{

int roll_no;

char name[20];

float marks;

};

struct student stud1={23,”Obaid”,78.77}; //initialization

or,

/* initialization of each member separately */

struct student stud1;

Stud1.roll_no = 23;

Stud1.name = “Obaid”;

Stud1.markes = 78.77;

Note that if there are less initializers in the list than members in the structure, the remaining

members are automatically initialized to 0 or NULL.

In the next section, let us learn how to access the structure members.

5. Accessing Structure Members

Structure members can be accessed and assigned values in number of ways. Structure

member has no meaning independently. In order to assign a value to a structure member, the

member name must be linked with the structure variable using dot . operator. It is also

called period or member access operator. The structure member operator ``.'' connects the

structure name and the member name. One can also use scanf() to assign values to structure

member through keyboard. Another way of accessing structure members is structure pointer

operator (->) we will see in detail in the module related to pointers.

The syntax for accessing the member of a structure is

structure_variable_name.member_name

Suppose, it is necessary to find the age of any employee emp1 then it can be accessed as

emp1.age

Let us consider another example,

struct student

{

char name [20];

int rollno;

char addr[50];

}s1, s2;

s1.rollno=21;

//s1 is variable of student type and rollno is member of structure student.

We can also use scanf() to give values to structure members through terminal

scanf(" %s ", s1.name);

scanf(" %d ", &s1.rollno);

Array of structures:

Just like any other normal array, we can also declare an array of structure. Each array element

represents a structure variable. For example:

struct student stud[50];

This statement declares an array stud of size 50 elements. Each element of array stud is of

type students.

Just similar to int or float array, it is possible to read elements i.e. assign values to the

members, process or even print the values of an array elements.

In C language nesting of structure is also possible. For example:

struct student

{

char name[20];

int rollno;

struct addr

{

char area[20];

char city[20];

int pin;

};

6. Example program for c structure

This program is used to store and access “id, name and percentage” for one student.

We can also store and access these data for many students using array of structures. You can

check “C – Array of Structures” to know how to store and access these data for many

students.

#include <stdio.h>

#include <string.h>

struct student

{

int roll_no;

char name[20];

float percentage;

};

int main()

{

struct student record = {0};

//Initializing to null

record. roll_no =1;

strcpy(record.name, “Obaid");

record.percentage = 77.15;

printf(" Roll number is: %d n", record.roll_no);

printf(" Name is: %s n", record.name);

printf(" Percentage is: %f n", record.percentage);

return 0;

}

Output:

Roll number is: 7

Name is: Obaid

Percentage is: 77.15

Example program for another way of declaring c structure:

In this program, structure variable “record” is declared while declaring structure itself.

In above structure example program, structure variable “struct student record” is declared

inside main function which is after declaring structure.

#include <stdio.h>

#include <string.h>

struct student

{

int roll_no;

char name[20];

float percentage;

}record;

int main()

{

record. roll_no =1;

strcpy(record.name, “Obaid");

record.percentage = 77.15;

printf(" Roll number is: %d n", record.roll_no);

printf(" Name is: %s n", record.name);

printf(" Percentage is: %f n", record.percentage);

return 0;

}

Output:

Roll number is: 7

Name is: Obaid

Percentage is: 77.15

C structure declaration in separate header file:

In above structure programs, C structure is declared in main source file. Instead of

declaring C structure in main source file, we can have this structure declaration in another file

called “header file” and we can include that header file in main source file as shown below.

Header file name – structure.h

Before compiling and executing below C program, create a file named “structure.h” and

declare the below structure.

Header file where C structure is declared.

Program to Store Information of Single Variable

In this program, a structure(student) is created which contains name, roll and marks as

its data member. Then, a structure variable(s) is created. Then, data (name, roll and

marks) is taken from user and stored in data members of structure variable s. Finally,

the data entered by user is displayed.

#include <stdio.h>

struct student

{

char name[50];

int roll;

float marks;

};

int main()

{

struct student s;

printf("Enter information of students:\n\n");

printf("Enter name: ");

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

printf("Enter roll number: ");

scanf("%d",&s.roll);

printf("Enter marks: ");

scanf("%f",&s.marks);

printf("\nDisplaying Information\n");

printf("Name: %s\n",s.name);

printf("Roll: %d\n",s.roll);

printf("Marks: %.2f\n",s.marks);

return 0;

}

Output

Enter information of students:

Enter name: Adele

Enter roll number: 21

Enter marks: 334.5

Displaying Information

naume: Adele

Roll: 21

Marks: 334.50

7. Uses of c structures

1. C Structures can be used to store huge data. Structures act as a database.

2. C Structures can be used to send data to the printer.

3. C Structures can interact with keyboard and mouse to store the data.

4. C Structures can be used in drawing and floppy formatting.

5. C Structures can be used to clear output screen contents.

6. C Structures can be used to check computer‟s memory size etc.

8. Summary

 Structure is a user-defined data type in C which allows you to combine different data

types to store a particular type of record. Structure helps to construct a complex data

type in more meaningful way.

 Keyword struct is generally used whenever lot of data needs to be grouped together.

 Sturct is followed by the structure tag, which names the structure definition

 Structure tag is used to declare variables of the structure type.

 Variables declared within braces of structure definition are the members of

structure.

 Members of the same structure type must have unique names.

 Members can be variables of primitive data types like int, float, char or other

data types like arrays, pointers etc.

 Each structure definition must end with a semicolon.

 The structure member operator ``.'' connects the structure name and the member

name. One can also use scanf() to assign values to structure member through

keyboard.

 Just like any other normal array, we can also declare an array of structure. Each array

element represents a structure variable.

 Just similar to int or float array, it is possible to read elements i.e. assign values to the

members, process or even print the values of an array elements.