The document discusses different file organization methods for storing data on external storage devices. It describes sequential, direct access, and indexed sequential file organization. Sequential files store records in the order they are entered, requiring searching through all preceding records to access a non-sequential record. Direct access files allow direct retrieval of any record through its logical address. Indexed sequential files store records sequentially but have an index file to allow direct access by key. The document compares advantages and disadvantages of each method.

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 The purpose of standard data organization methods
 Various file organizations and their application-specific
suitability such as sequential file, indexed sequential file, and
direct access file
 The advantages and disadvantages of file organizations

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 Records that hold information about similar items of data are
usually grouped together into ‘file’
 A file is a collection of records where each record consists of one
or more fields

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 External storage devices are mainly used for the
following:
 Overlay or backup of programs during execution
 Storage of programs for future use
 Storage of information in files
External storage device

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 The most common external storage devices in order of their
use, and study :
 magnetic tapes,
 drums, and
 disk drives
 When we organize data in ‘file’ data structure, the data is non-
volatile, which means data will reside on storage after data
processing is over

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 A tape is made up of a plastic material coated with a
ferrite substance that is easily magnetized
 The physical appearance of the tape is similar to the
tape used for sound recording.
 A limitation of magnetic tape devices is that records
must be processed in the order in which they reside
on the tape

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 A magnetic drum is a metal cylinder, from 10 to 36 inches
in diameter, which has an outside surface coated with a
magnetic recording material
 The cylindrical surface of the drum is divided into a
number of parallel bands called tracks
 The tracks are further divided into either sectors or blocks,
depending on the nature of the drum
 The sector or block is the smallest addressable unit

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 The magnetic disk is a direct access storage
device, which has become more widely used than
the magnetic drum, mainly because of its lower
cost

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 The proper arrangement of records within a
file is called as file organization

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 Different Schemes of File Organizations
 Sequential file
 Direct or random access file
 Indexed sequential file
 Multi-Indexed file

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 In sequential file, records are stored in the
sequential order of their entry
 This is the simplest kind of data organization

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 Though we search records using key, we still need to know
the address of the record to retrieve it directly
 The file organization that supports Files such access is
called as direct or random file organization

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 File—Records are stored sequentially but the index file is
prepared for accessing the record directly
 An index file contains records ordered by a record key
 The record key uniquely identifies the record and
determines the sequence in which it is accessed with
respect to other records

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 In multi-indexed file, the data file is associated with one
or more logically separated index files
 Inverted files and multilist files are examples of
multiindexed files

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 The factors that affect file organization are mainly
the following:
 Storage device
 Type of query
 Number of keys
 Mode of retrieval/update of record
Factors Affecting File
Organization

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 Speed
 Operations
 Capacity
 Size
 Security
Factors Involved in Selecting
File Organization

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 File I/O Classes
 Primitive Functions Files
 Binary and Text File
FILES USING C++

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 The I/O system of C++ contains a set of classes that
define the file handling methods
 They are ifstream, ofstream, and fstream
 These classes are included in ‘fstream.h’ header file
 ifstream—This class provides input operations
 ofstream—This class provides output operations
 fstream—This class provides both input and
output operations

File I/O Classes

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 There are several ways of reading (or writing) the
text from (or to) a file
 But all of them have a common approach as follows
 Open the file
 Read (or write) the data
 Close the file
Primitive Functions

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 Binary and Text File
 The binary file consists of binary data
 It can store text, graphics, sound data in binary format
 The binary files cannot be read directly
 Text File
 The text file contains the plain ASCII characters
 It contains text data which is marked by ‘end of line’ at th
end of each record
 This end of record marks help easily to perform
operations such as read and write
 Text file cannot store graphical data

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 The order of the records is fixed
 Within each block, the records are in sequence
 A sequential file stores records in the order they are entered
 New records always appear at the end of the file
 Primitive Operations
 Features of Sequential File Organization
 Drawbacks of Sequential File Organization

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 Open—This opens the file and sets the file pointer to
immediately before the first record
 Read-next—This returns the next record to the user. If
no record is present, then EOF condition will be set
 Close—This closes the file and terminates the access to
the file
 Write-next—File pointers are set to next of last record
and write the record to the file

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 Write-next—File pointers are set to next of last record
and write the record to the file
 EOF—If EOF condition occurs, it returns true,
otherwise it returns false
 Search—Search for the record with a given key
 Update—Current record is written at the same position
with updated values

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 Insertion and deletion of records in in-between positions huge
data movement
 Accessing any record requires a pass through all the preceding
records, which is time consuming. Therefore, searching a record
also takes more time.
 Needs reorganization of file from time to time. If too many
records are deleted logically, then the file must be reorganized
to free the space occupied by unwanted records

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 Files that have been designed to make direct record
retrieval as easy and efficiently as possible is known as
directly organized files
 Direct access files are of great use for immediate access to
large amounts of information
 They are often used in accessing large databases

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Beginning of file
Current position
End of file
0
1
2
SEEK_SET
SEEK_CUR
SEEK_END
Origin Value Macro Name

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0
Origin Value Macro Name
Beginning of file
Current position
End of file
0
1
2
SEEK_SET
SEEK_CUR
SEEK_END

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0
Origin Value Macro Name
Beginning of file
Current position
End of file
0
1
2
SEEK_SET
SEEK_CUR
SEEK_END

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 A file that is loaded in key sequence but can be accessed
directly by use of one or more indices is known as an
indexed sequential file
 A sequential data file that is indexed is called as
indexed sequential file
 A solution to improve speed of retrieving target is index
sequential file
 An indexed file contains records ordered by a record key
 Each record contains a field that contains the record key

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Advantages
 Accessing any record is more efficient than sequential file
organization
 Large amount of data can be stored using this type of file
organization
Disadvantages
 Often more than one indices are needed which occupies
large storage area

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 Primary index
 It is an index ordered in the same way as the data file,
which is sequentially ordered according to a key
 The indexing field is equal to this key
 Secondary index
 An index that is defined on a non-ordering field of the
data file
 In this case, the indexing field need not contain
unique values

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 Clustering indexes
 A data file can associate with utmost one primary
index and several secondary indexes
 The single-level indexing structure is the simplest one
where a file, whose records are pairs, contains a key
and a pointer
 The single-level indexing structure is the simplest one
where a file, whose records are pairs, contains a key
and a pointer

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Index file consists of three areas:
 A primary storage area—This includes some unused space
to allow for additions made in data
 A separate index or indexes—Each query will reference
this index first; it will redirect query to part of data file in
which the target record is saved
 An overflow area—This is optional separate overflow area

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 Records are stored sequentially but the index file is
prepared for accessing the record directly
 Records can be accessed randomly
 File has records and also the index
Magnetic tape is not suitable for index sequential storage
 Index is the address of physical storage of a record
 When randomly very few are required/accessed, then
index sequential is better
 Faster access method
Addition overhead is to maintain index
Index sequential files are popularly used in many
applications like digital library

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 In linked organization, the physical sequence of
records is different from the logical sequence of
records
 The next logical record is obtained by following a
link value from the present record
 Multilist Files
 Coral Rings
 Inverted Files
 Cellular Partitions

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 To make searching easy, several indexes are
maintained as per primary key and secondary
keys, one index per key
 The record may be present in different lists as per
key

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 Consider the following file of office staff
Staff ID Occupation Salary Record
106 Clerk 5000 A
150 Account 4000 B
360 clerk 3000 C
400 Account 3500 D
700 Clerk 2000 E

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 We can maintain index on the staff ID
 We can group staff ID with range 101–
300, 301–600, 601–900, etc
 Now all the records with staff ID in the
same range will be linked together as
shown in Figure
Sample multilist file
Staff range Link
101-300
301-600
601-900
rec A
rec C
rec E
rec B
rec D

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 In this doubly linked multilist structure is used.
Each list is circular list with headnode
 A C E Alink
Clerk Blink S Blink
Sample doubly linked list

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 ‘Alink’ field is used to link all records with same key
value
 ‘Blink’ is used for some records back pointer and
for others it is pointer to head node
 Owing to these back pointers, deletion is easy
without going to start.
 Indexes are maintained as per multilists

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 With the same key value are linked together and
links are kept in each record
 But in the inverted files, the link information is
kept in the index itself

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106 A
150 B
360 C
400 D
700 E
Accountant B,D
Clerk A,C,E
Salary Index
2000 E
4000 B,C,D
6000 A

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 The inversion process is associated with the
information of inverted list
 In inverted files, the record is accessed in two steps.
First, the indices are searched to obtain a list of
required records and then second, records are retrieved
using these lists
 The number of disk accesses required is equal to the
number of records being retrieved plus the number to
process the indices
 In inverted files, only the index structures are
important

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 To decrease file search time, the storage media may be
divided into cells
 A cell may be an entire disk or a cylinder. Lists are
localized to lie within a cell
 If a cylinder is used as a cell, then all records on the
same cylinder may be accessed without moving the
read/write heads
 We divide multilists organized on several different
cylinders into several small lists which are stored on
the same cylinder

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Position Student ID T. ID Link
1 100 A
2 200 B Null
3 300 C Null
4 400 A Null
1 500 D
2 600 B Null
3 700 A Null
4 800 D Null
1 900 E Null
2 1000 C Null
3 1100 D Null
4 1200 A Null
Primary key Secondary key

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