Course: MCA

Subject: Data and File Structure

Unit-2

String, Record and Pointer

Definition of a Record

A data type created/customisable by the

programmer

consisting of a set of fields/multiple data items

which can be of different data types

Records

Arrays can only hold data items with the same type

Records can contain different types of data

This makes them more complex to manipulate

Associated with databases

Record Example[1]

Field Type

Surname String

FirstName String

Gender Character

Address String

PostCode String

Customer Number String

A record can be considered as a two dimensional array with different data types.

Record Example[2]

Surname FirstName Sex Address 1 PostCode ‘phone

Tod Andy M 35 Brookside

Dr

TY7 8UK 225 3625

Boyd Mary F 27 The Grange OB7 RF1 335 2901

Bell Charles M 2 Larch Rd HT5 WA3 213 1157

Records[3]

In the computer’s memory, records are stored as

Record 1 * Record 2 * Record 3 * Record 4 * Record 5 * EOF

Each record is terminated by a CR, LF (indicated by *)

and EOF control codes.

Assigning values

BookInfo.Title = InputBox("Enter the title of the book")

BookInfo.Author = InputBox("Enter the author's name")

BookInfo.ISBN = InputBox("Enter the ISBN")

BookInfo.Price = InputBox("Enter the price")

Displaying record values

ListBox1.Items.Add("Title-" & BookInfo.Title)

ListBox1.Items.Add("Author-" & BookInfo.Author)

ListBox1.Items.Add("ISBN-" & BookInfo.ISBN)

ListBox1.Items.Add("Price- £" & BookInfo.Price)

Comparison of Arrays v. Records

Arrays (1D & 2D)– all data must be of same type (Integer,

string,..)

Records – fields of different types

Arrays – simple to implement

Records – more complex to implement

Arrays

An array is an indexed set of variables, such as

dancer[1], dancer[2], dancer[3],… It is like a set of boxes

that hold things.

A list is a set of items.

An array is a set of variables that each store an item.

Arrays and Lists[4]

You can see the difference between arrays and lists

when you delete items.

What is a pointer

• In a generic sense, a “pointer” is anything that tells us

where something can be found.

– Addresses in the phone book

– URLs for webpages

– Road signs

Java Reference

• In Java, the name of an object is a reference to that

object. Here ford is a reference to a Truck object. It

contains the memory address at which the Truck

object is stored.

Truck ford = new Truck( );

• The syntax for using the reference is pretty simple.

Just use the “dot” notation.

ford.start( );

ford.drive( 23 );

ford.turn (LEFT);

What is a pointer ?

• In C, a pointer variable (or just “pointer”) is similar to a

reference in Java except that

– A pointer can contain the memory address of any

variable type (Java references only refer to objects)

– A primitive (int, char, float)

– An array

– A struct or union

– Dynamically allocated memory

– Another pointer

– A function

– There’s a lot of syntax required to create and use

pointers

Why Pointers?

• They allow you to refer to large data structures in

a compact way

• They facilitate sharing between different parts of

programs

• They make it possible to get new memory

dynamically as your program is running

• They make it easy to represent relationships

among data items.

Pointer Caution

• They are a powerful low-level device.

• Undisciplined use can be confusing and thus the

source of subtle, hard-to-find bugs.

– Program crashes

– Memory leaks

– Unpredictable results

C Pointer Variables

To declare a pointer variable, we must do two things

Use the “*” (star) character to indicate that the

variable being defined is a pointer type.

Indicate the type of variable to which the pointer

will point (the pointee). This is necessary

because C provides operations on pointers (e.g.,

*, ++, etc) whose meaning depends on the type of

the pointee.

General declaration of a pointer

type *nameOfPointer;

Pointer Declaration

The declaration

int *intPtr;

defines the variable intPtr to be a pointer to a variable of

type int. intPtr will contain the memory address of some

int variable or int array. Read this declaration as

– “intPtr is a pointer to an int”, or equivalently

– “*intPtr is an int”

Caution -- Be careful when defining multiple variables on the

same line. In this definition

int *intPtr, intPtr2;

intPtr is a pointer to an int, but intPtr2 is not!

Pointer Operators

The two primary operators used with pointers are

* (star) and & (ampersand)

The * operator is used to define pointer variables

and to deference a pointer. “Dereferencing” a

pointer means to use the value of the pointee.

The & operator gives the address of a variable.

Recall the use of & in scanf( )

Pointer and Variable types

• The type of a pointer and its pointee must match

int a = 42;

int *ip;

double d = 6.34;

double *dp;

ip = &a; /* ok -- types match */

dp = &d; /* ok */

ip = &d; /* compiler error -- type mismatch */

dp = &a; /* compiler error */

More Pointer Code

• Use ampersand ( & ) to obtain the address of the pointee

• Use star ( * ) to get / change the value of the pointee

• Use %p to print the value of a pointer with printf( )

• What is the output from this code?

int a = 1, *ptr1;

/* show value and address of a ** and value of the pointer */

ptr1 = &a ;

printf("a = %d, &a = %p, ptr1 = %p, *ptr1 = %d\n",

a, &a, ptr1, *ptr1) ;

/* change the value of a by dereferencing ptr1

** then print again */

*ptr1 = 35 ;

printf(“a = %d, &a = %p, ptr1 = %p, *ptr1 = %d\n", a, &a, ptr1, *ptr1) ;

References

An introduction to Datastructure with application by jean Trembley and sorrenson

Data structures by schaums and series –seymour lipschutz

http://en.wikipedia.org/wiki/Book:Data_structures

http://www.amazon.com/Data-Structures-Algorithms

http://www.amazon.in/Data-Structures-Algorithms-Made-Easy/dp/0615459811/

http://www.amazon.in/Data-Structures-SIE-Seymour-Lipschutz/dp

LIST OF IMAGES

1. searchoracle.techtarget.com/definition/record

2. en.wikipedia.org/wiki/Array_list

3. An introduction to Datastructure with application by jean Trembley and sorrenson

4. en.wikipedia.org/wiki/Array_list