Fundamental file structure concepts

1
Chapter 4

• Fundamental file structure concepts

2
Field and record organization

• Basic unit of data is a field
• A list of different fields is a record

3
Stream file

• writstrm.cpp in appendix D
• Mary Ames 123 Maple Stillwater, OK 74075
• After writing it may look like
• AmesMary123 MapleStillwaterOK74075
• How can we read it back?

4
Method 1

• Fix the length of the fields
• Wastes space or
• Data does not fit
• Can be used if the data is fixed length

5
Method 2

• Begin each field with a length indicator
• If the fields are not longer than 255 you can use
  one byte to store the length

6
Method 3

• Separate the fields with delimiters
• Use a delimiter that is not in the data
• White space is often in the data
• This is the method we will use in the examples

7
Method 4

• Use keywordvalue expression
• Self-describing
• Good if fields are missing
• Still need delimiter in some cases
• Wastes space

8
Reading delimited files

• Overload operatorgtgt
• Use istream method getline
• Reads up to the first occurrence of the delimiter
  or the end-of-line
• See readdel.cpp in appndx D

9
Record structures

• A set of fields that belong together
• Saving the state of an object stored in memory
• How to organize records of a file

10
Method 1

• Fixed length records
• Most commonly used
• Fixed length records does not imply fixed length
  fields

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Method 2

• Fix the number of fields
• For example, 6 fields as in the Person

12
Method 3

• Begin each record with a length indicator
• Common for variable length records

13
Method 4

• Use an index
• Byte offset for each record in the file

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Method 5

• Delimiter at the end of each record
• Same as for fields
• Common is end-of-line

15
Example record structure

• Use length indicator at the beginning of a record
• Accumulate the record in a buffer
• Use strcat to put the strings in the buffer
• Use strlen to calculate the length
• Write the buffer length and the buffer

16
Representing the record length

• 2-byte binary integer or
• Convert to char string

17
project

• Write a program that prompts for the three data
  items and writes them to a file. Use variable
  length records with the length stored in binary
  at the start of each record. Delimit fields with
  
• FirstNameLastnametelno