Field, Record, and Buffer

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Field, Record, and Buffer
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Field and Record

• Field the basic unit of data
• a single value
• the smallest logically meaningful unit of
  information
• array many copies of a single field
• record a list of different fields
• structure
• objects and their members (in memory)

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File Structures for Structured Data

• Field
• domain
• Record
• a list of fields
• type
• File
• a collection of records

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Stream File

• A single byte stream
• no additional information

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Field Structures

• Fixed-Length Field
• the position of each field can be found by simple
  arithmetic
• disadv.
• Need padding up to a fixed length
• padding character space
• Wasted space in the field having a large amount
  of variability in the length of value
• Variable-Length Field

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• Using a length indicator
• field length
• Using a field delimiters
• choice of the delimiter character
• keywordValue
• keyword - field name
• self-describing structure
• a good format for dealing with missing fields
• wastes a log of space (keyword)
• P124, Figure 4.4 (reading delimited fields)

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Record Structures

• Fixed-Length Record
• each record contains the same of bytes
• fixed of fixed-length fields
• a container to hold variable of variable-length
  fields

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• Variable-Length Record
• fixed of fields
• Using a length indicator (record length)
• most commonly used
• Using an index
• each entry of the index points to the beginning
  of a record
• Using an End-of-Record Mark

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How to represent the Record Length

• p129, Figure 4.7
• Formats for the record length
• a n-byte binary integer
• a character string (using formatted output)
• p131, Figure 4.9
• Figure 4.4, for op gtgt of istrstream

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File Dump

• UNIX (p132)
• od -xc filename
• DOS
• debug filename
• - d

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Buffer Classes

• Pack and Unpack
• for Delimited Text Fields
• see p135, App. E
• for Length-based Fields
• see p137, App. E
• for Fixed-length Fields
• see p138, App E

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Buffer Class Hierarchy
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• virtual public inheritance
• included only once in the ancestry
• Protected Member
• can be used by methods of derived classes
• (pure) virtual member
• there is no implementation in the class
• abstract class
• the class having virtual member

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Pack and Unpack
in file
write
read
in buffer
in object
unpack
char name??? char dept???? char
telno920-7525
pack
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HW 5

• Ex 3, 4, 11, 14, 15
• Programming
• Ex 16, 22, 23