Objectives:

1
Chapter 4 Fundamental File Structure Concepts

• Objectives
• To get familiar with
• Alternatives in field and record organizations
• Object-oriented approach to buffered I/O

2
Outline

• Field organization
• Record organization
• A record structure that uses a length indicator
• Using classes to manage buffers
• A class hierarchy for record buffers

3
Field

• A field is the smallest logically meaningful unit
  of information in a file.
• A record is a collection of fields which belong
  to an object.
• A field does not need to exist in any physical
  sense, but keeping the identity of the fields is
  important. A stream file that loses the track of
  fields makes the information unusable.
• For example
• Mary Ames 123 Maple Stillwater, OK 74075
• Alan Mason 90 Eastgate Ada, OK 74820
• A stream of bytes
• AmesMary123 MapleStillwater, OK 74075MasonAlan90
  EastgateAda, OK 74820
• Problem There is no way to get the information
  back in the organized record format.

4
Field Organization

• Ways to maintain the identity of fields
• Force the field into a predictable length
• fixed-length
• Begin each field with a length indicator
• Place a delimiter at the end of each field to
  separate it from the next field
• Use a keyword value expression to identify
  each field and its content
• Choose an organization for fields
• Space utilization
• Data characteristics fixed-length or variable
  length
• Delimiter must not appear in file
• Addressing is it easy to determine the start
  position a field?

5
Examples of Field Organization

• Fixed Length
• Mary Ames 123 Maple
  Stillwater OK 74075
• Alan Mason 90 Eastgate Ada
  OK 74820
• Length Indicator
• 04Mary04Ames09123 Maple10Stillwater02OK0574075
• 04Alan05Mason1190 Eastgate03Ada02OK0574820
• Delimiters
• MaryAmes123 MapleStillwaterOK74075
• AlanMason90 EastgateAdaOK74820
• Keywords (with delimiters)
• firstMarylastAmesaddress123
  MaplecityStillwaterstateOKzip74075
• firstAlanlastMasonaddress90
  EastgatecityAdastateOKzip74820

6
Reading Delimited Fields

• istream operator gtgt (istream stream, Person
  p)
• // read fields from file
• char delim
• stream.getline(p.LastName, 30,'')
• if (strlen(p.LastName)0) return stream
• stream.getline(p.FirstName,30,'')
• stream.getline(p.Address,30,'')
• stream.getline(p.City, 30,'')
• stream.getline(p.State,15,'')
• stream.getline(p.ZipCode,10,'')
• return stream
• This time, we do preserve the notion of fields,
  but something is missing Rather than a stream of
  fields, these should be two records

• Last Name Ames
• First Name Mary
• Address 123 Maple
• City Stillwater
• State OK
• Zip Code 74075
• Last Name Mason
• First Name Alan
• Address 90 Eastgate
• City Ada
• State OK
• Zip Code 74820

7
Record Organization

• A record can be defined as a set of fields that
  belong together when the file is viewed in terms
  of a higher level of organization.
• Like the notion of a field, a record is another
  conceptual tool which needs not exist in the file
  in any physical sense.
• Yet, they are an important logical notion
  included in the files structure.
• Methods for organizing the records of a file
  include
• Requiring that the records be a predictable
  number of bytes in length.
• fixed-length records
• Requiring that the records be a predictable
  number of fields in length.
• Beginning each record with a length indicator
  consisting of a count of the number of bytes (or
  number of fields) that the record contains.
• Using a second file to keep track of the
  beginning byte address for each record.
• Placing a delimiter at the end of each record to
  separate it from the next record.

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10
A Record Structure that Uses a Length Indicator

• Writing the variable-length records to the file
• length indicator at the beginning of a record --gt
  know the sum of field length before writing
  record to file
• Use a buffer
• Representing the record length
• Binary
• Text
• Be careful of byte order
• Reading the variable-length record from the file

11
Using Classes to Manage Buffers

• Goal encapsulate the pack, unpack, read, write
  operations of buffers
• Usage
• Output start with an empty buffer object, pack
  field values into the object, then write buffer
  to output stream.
• Input initialize a buffer object by reading a
  record from input stream, then unpack field
  values one by one.
• Constraints
• No updates on packed data
• No mixing of pack and unpack operations
• Design approach look at concrete classes first,
  then abstract out a base class.

12
A Class Hierarchy for Record Buffers

• IOBuffer
• char array for buffer value
• VariableLengthBuffer FixedLengthBuffer
• read and write operations read and write
  operations
• for variable length records for fixed length
  records
• DelimitedFieldBuffer LengthFieldBuffer FixedFieldB
  uffer
• pack and unpack pack and unpack pack and
  unpack
• operations for operations for operations for
• delimited fields length-based fields fixed sized
  fields