Sequential%20Files

1
Sequential Files

• Chapter 13

2
Master Files

• Set of files used to store companies data in
  areas like payroll, inventory
• Usually processed by batch processing
• Typically stored on magnetic disk
• Disks can store billions of characters
• Disk drives read, write data quickly
• Disk records can be any size

3
Sequential Files

• Records always read in sequence
• Read first record, process it, then read second
  record, process it and so on
• Payroll records in order by employee number may
  be processed in sequence for updating or printing
  reports

4
Sequential Files

• May be sorted into any sequence using any field
  in record
• To distribute checks more easily, records may be
  sorted by department
• Sequential processing then used to print checks,
  reading first record, then second, etc.

5
Master File Procedures

• Typical procedures for sequential processing in
  batch mode are
• Designing Master File
• Creating Master File
• Creating Transaction File
• Updating Master File
• Reporting from Master File

6
Designing a Master File

• Place key fields that uniquely identify record at
  beginning of record
• If possible, choose numeric key fields
• Secondary fields after primary key fields
• Remaining fields appear in order of importance

7
Designing a Master File

• Choose field size large enough to accommodate
  data stored in it
• Use coded fields where possible to save space
• Be sure all date fields include four-digit year

8
Creating a Master File

• Original master file data entered interactively,
  stored on disk file
• Ensure data integrity by using data validation
  techniques to minimize risk or errors
• Control listing or audit trail produced to show
  data stored in new master file and control totals

9
Creating a Transaction File

• Changes to master file made with separate
  procedure
• Change records stored in file called transaction
  file

10
Updating a Master File

• Updating is process of making master file current
• Update master file by incorporating changes from
  transaction records

11
Reporting from a Master File

• Scheduled reports prepared on regular basis from
  data stored in master file
• Sales reports, customer bills, checks, etc.
• Use detail, exception and group printing
  techniques
• On demand reports produced as need arises
• May be in report form or displayed on screen

12
Sequential File Updating

• Two input files
• Input Master File (Old-Master)
• Current through previous updating period
• Does not contain changes since previous update
• Input Transaction File (Trans-File)
• Contains changes since previous update to be
  applied to Old-Master

13
Sequential File Updating

• Two output files
• Output Master File (New-Master)
• Integrates data from Old-Master with all of
  changes from Trans-File
• Will become Old-Master for next update
• Control Listing or Audit Trail
• Print file showing changes made to master file,
  errors during processing and control totals

14
Sequential File Update
Transaction File
Master File
Sequential Master File Update
Updated Master File
Update Activity Report
15
Ordering of Records

• Records in Old-Master and Trans-File must be in
  order by same key field
• Compare key fields to determine if given master
  record is to be updated

16

• Master File
• 1223 ..
• 1277 ..
• 1456 ..
• 2835 ..
• 2910 ..
• 3006 ..
• 3100 ..
• 3143 ..
• 4372 ..
• z

• Transaction File
• 1235 ..A
• 1273 ..C
• 1456 ..C
• 1543 ..D
• 1678 ..B
• 2910 ..C
• 3006 ..D
• 3098 ..C
• 3099 ..D
• z

New Master File
1223 1235 1277
1456 2835 2910
3100 3143 4372
z
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Sequential Update Procedure

• Initially, record read from both Old-Master and
  Trans-File
• Comparison of M-Acct-No and T-Acct-No determines
  next step

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1) T-Acct-No M-Acct-No

• Means transaction record exists with same account
  number as master record
• Perform regular update, adding Trans-File amount
  to Old-Master amount, storing result in
  New-Master field
• Read next record from Trans-File

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2) T-Acct-No gt M-Acct-No

• Means master record exists for which there is no
  corresponding transaction record
• Write record from Old-Master to New-Master as is,
  since no changes need to be made to it
• Read next record from Old-Master

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3) T-Acct-No lt M-Acct-No

• Means transaction record exists for which there
  is no corresponding master record
• Two ways to process this transaction
• Create record in New-Master file for this
  transaction
• May assume that T-Acct-No is invalid since no
  match found in Old-Master file and treat as error
  condition

21
Use of HIGH-VALUES

• Refers to largest value in computer's collating
  sequence
• Used so all records from both Old-Master and
  Trans-File are processed
• May reach end of Old-Master file before reaching
  end of Trans-File or vice versa

22
Use of HIGH-VALUES

• When last record read from Old-Master, M-Acct-No
  set to HIGH-VALUES
• For all remaining transaction records T-Acct-No
  lt M-Acct-No
• Processed as new accounts and added to New-Master

23
Use of HIGH-VALUES

• When last record read from Trans-File, T-Acct-No
  set to HIGH-VALUES
• For all remaining Old-Master records T-Acct-No
  gt M-Acct-No
• Old-Master records added to New-Master without
  changes

24
Use of HIGH-VALUES

• Loop to process records ends only when both
  account numbers equal HIGH-VALUES
• May be used only with fields defined as
  alphanumeric
• Define account number fields with PIC of Xs even
  though fields contain numbers

25
Validity Checking in Updating

• Code field may be added to each transaction
  record to indicate whether transaction is
• New account to be added
• Update to existing account
• Deletion of existing account
• Comparison of keys and transaction code type
  enable program to detect other update errors

26
Updating Master Disk in Place

• May read a master disk record, make changes
  directly to same record, and rewrite it or update
  it in place
• Only two files needed
• Master-File OPEN as I-O
• Trans-File OPEN as INPUT
• Use REWRITE statement to replace master disk
  record, currently in storage, that was accessed
  by preceding READ

27
Updating Master Disk in Place

• To delete a record, establish each record with an
  activity code field
• For example, Code-X 1 if record is active, or 2
  if record is inactive
• All master records initially active (1 in Code-X)
• Master record deactivated by changing activity
  code to 2

28
Updating Master Disk in Place

• New records cannot be added in sequence when file
  open for I-O
• Can add records at end of file by opening file in
  EXTEND mode
• OPEN EXTEND file-name
• Disk automatically positioned at end of file,
  immediately after last record
• Sort file after records added to arrange them in
  sequence