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Chapter 13

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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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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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

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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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Set of files used to store companies data in areas like payroll, inventory

Usually processed by batch processing Need updated on a regular basis

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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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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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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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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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Changes to master file made with separate procedure

Changed records stored in file called transaction file

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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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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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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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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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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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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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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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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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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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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 The most likely thing to happen

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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 (maybe a new client?)◦ May assume that T-Acct-No is invalid since no

match found in Old-Master file and treat as error condition (depends on how the accounting works)

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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 So the program keeps transferring the old

trans data to the new trans data May reach end of Old-Master file before

reaching end of Trans-File or vice versa

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

record Update or deletion transaction treated as

error Full example p. 588-590

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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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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, not actually deleting it.

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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 Issues here, you need to close then

open and records are added unsorted

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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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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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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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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

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