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Sequential File Processing

Chapter 13

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Chapter Contents

• Overview of Sequential File Processing

• Sequential File Updating - Creating a New Master File

• Validity Checking in Update Procedures

• Update Procedures with Multiple Transaction Records

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Chapter Contents

• Balanced Line Algorithm for Sequential File Updating

• Sequential File Updating - Rewriting Records on Disk

• Matching Files for Checking Purposes

• Interactive Updating of Sequential File

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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

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Sequential Files

• Records always read in sequence

• Read first record, process it, then read second record, process it and so on– For example payroll records in order by

employee number may be processed in sequence for updating or printing reports

– Example used in book

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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.

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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

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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

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Designing a Master File

• Choose field size large enough to accommodate data that may be stored in it

• Use coded fields where possible to save space (not a big problem anymore)

• Be sure all date fields include four-digit year

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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

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Creating a Transaction File

• Changes to master file made with separate procedure

• Changed records stored in file called transaction file

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Updating a Master File

• Create a new blank file (New Master)

• Updating is process of making master file current

• Update master file by incorporating changes from transaction records

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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

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Creating Master, Transaction Files

• Data may be read in from another file or entered interactively from keyboard

• If data entered is valid, move it to master or transaction record fields

• WRITE new record to file

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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

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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

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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

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Sequential Update Example

• For accounts receivable file, account number may be selected as key field

• Records in both input files in sequence by account number– Old-Master key field is M-Acct-No– Trans-file key field is T-Acct-No

• New-Master file also created in account number sequence (Acct-No-Out)

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Sequential Update Procedure

• One transaction record for each master record to be updated (p. 570)– Contains total amount to be added to

amount field in corresponding master record

• 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

• Write record to New-Master file

• Read next record from both Old-Master and Trans-File

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2) T-Acct-No > 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

• Basically do nothing but copy

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3) T-Acct-No < 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

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Use of HIGH-VALUES

• COBOL reserved word HIGH-VALUES

• Move it into a variable (p. 573)

• 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

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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 < M-Acct-No – Processed as new accounts and added to

New-Master– This allows the transaction file to keep

going when you are out of files from the old master file

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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 > M-Acct-No – Old-Master records added to New-Master

without changes– Old keeps going

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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

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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

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Multiple Transaction Records

• To process more than one change for each master record requires different update procedure

• In accounts receivable file example, transaction record may be created for each purchase or credit by customer

• Amounts in all transaction records for one customer need to be added to amount in Old-Master file record

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Multiple Transaction Records

• Add loop to regular update routine to read and process multiple transaction records for same Old-Master record– Continue to read records from Trans-File

until T-Acct-No Not = M-Acct-No

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Balanced Line Algorithm

• Technique to update master file – With any number of transactions– With checks for error conditions

• Viewed as most efficient and effective sequential file updating method

• Two new fields used to control updating– WS-Control-Key– WS-Allocated-Switch

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WS-Control-Key

• Determines type of processing

• Called control or active key field

• Set to smaller of M-Acct-No or T-Acct-No

• Then comparison of M-Acct-No or T-Acct-No to WS-Control-Key used to control further processing

• Kind of like the control break processing

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WS-Allocated-Switch

• Used for error control– To ensure records processed correctly– To detect erroneous codes in transactions

• Set to 'YES' when record should be written to New-Master file

• Set to 'NO' if record should not be written to New-Master file

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

• WS-Control-Key set to M-Acct-No

• WS-Allocated-Switch set to 'YES' because WS-Control-Key and M-Acct-No are same

• Since M-Acct-No and T-Acct-No not equal, Old-Master has no transactions– Old-Master record written to New-Master

record as is

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2) M-Acct-No = T-Acct-No

• WS-Control-Key set to M-Acct-No

• WS-Allocated-Switch set to 'YES' because WS-Control-Key and M-Acct-No are same

• Transactions with T-Acct-No = M-Acct-No processed

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2) M-Acct-No = T-Acct-No

• Processing transactions with T-Acct-No = M-Acct-No– Updates posted to New-Master– Additions displayed as errors because WS-

Allocated-Switch set to 'YES' (master record with this M-Acct-No already exists)

– Deletions move 'NO' to WS-Allocated-Switch to ensure this Old-Master record is not written to New-Master file

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3) M-Acct-No > T-Acct-No

• WS-Control-Key set to T-Acct-No

• WS-Allocated-Switch set to 'NO' because WS-Control-Key not equal to M-Acct-No– No master record exists for this transaction

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3) M-Acct-No > T-Acct-No

• Only additions acceptable– Reset WS-ALLOCATED-SWITCH to 'YES'– New-Master record created from

transaction record

• Update or deletion transaction treated as error

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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

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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– Probably want this code anyway for various

bookkeeping reasons

• All master records initially active (1 in Code-X)

• Master record deactivated by changing activity code to 2

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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 modeOPEN EXTEND file-name

• Disk automatically positioned at end of file, immediately after last record

• Sort file after records added to arrange them in sequence

• Issues here, you need to close then open and records are added unsorted

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Chapter Summary

• Sequential updating by creating new master uses three files– Incoming master file– Transaction file with change records– New output master file that will incorporate

all changes

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Chapter Summary

• Sequential updating technique– All three files must be in sequence by

same key field– Record read from each file– Routine performed depends on whether

key fields match

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Chapter Summary

• Sequential updating technique – Transaction record may include coded field

to designate update, deletion, addition– Use HIGH-VALUES to ensure all records in

both old master and transaction files are processed

• Balanced line algorithm used for sequential updates

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Chapter Summary

• Can also update records in sequential file by rewriting them in place– Must open file as I-O

• Records can be added to end of sequential file– Open in EXTEND mode