ch15_97

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PowerPoint Presentation:Richard H. Baum, Ph.D.DeVry Institute of Technology

9th Edition

Structured COBOLProgrammingNancy Stern

Hofstra UniversityRobert A. Stern

Nassau CommunityCollege

Copyright @ 2000 John Wiley & Sons, In. Allr ights reserved. Reproduction or translation of thiswork beyond that permitted in Section 117 of the1976 United States Copyright Act without theexpress permission of the copyright owner isunlawful. Request for further information should beaddressed to the permissions Department , JohnWily & Sons, Inc. The purchaser may make back-upcopies for his/her own use only and not fordistribution or resale. The Publisher assumes noresponsibility for errors, omissi ons, or damages,caused by the use of these programs or from th euse of the information contained herein.

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Structured COBOL Programming, Stern & Stern, 9th Edition

CHAPTER 15Indexed and RelativeFile Processing

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OBJECTIVESTo familiarize you with:

1. Methods of disk file organization.2. Random processing of disk files.3. How to create, update, and accessindexed disk files.4. How to create, update, and accessrelative files.5. Methods used for organizing relativefiles.

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CONTENTS Systems Considerations for OrganizingDisk Files

Sequential File Organization Indexed File Organization Relative File Organization

Features of Magnetic Disks and DiskDrives Processing Indexed Disk Files

Creating an Indexed File Updating an Indexed File Randomly Updating an Indexed File with MultipleTransaction Records for Each Master Record

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CONTENTS Accessing or Reading from an Indexed File

for Reporting Purposes The FILE STATUS Clause

Processing Relative Disk Files What is a Relative File? Creating Relative Files Sequential Reading of Relative Files Random Reading of Relative Files Random Updating of Relative Files

Converting a Key Field to a RELATIVEKEY

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Systems Considerations forOrganizing Disk Files

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Sequential File Organization The simplest type of disk fileorganization is sequential. Sequential files are processed in the

same way regardless of the type ofmagnetic media on which they arestored. Typically, the records to be stored in asequential file are first sorted intosequence by a key field such ascustomer number, part number, oremployee number.

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Indexed File Organization An indexed file is really two files:

the data file, which is created in sequencebut can be accessed randomly, and the index file, which contains the value ofeach key field and the disk address of therecord with that corresponding key field.

To access an indexed record randomly,the key field is looked up in the indexfile to find the disk address of therecord.

Then the record is accessed in theindexed data file directly.

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Indexed File Organization With an indexed file we can accessrecords either sequentially or randomly,depending on the user's needs. The term random access implies thatrecords are to be processed or accessedin some order other than the one in

which they were physically written onthe disk.

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Relative File Organization Relative files also permits random

access. A relative file does not use an index toaccess records randomly.

Rather, the key field of each record is usedto calculate the record's relative location inthe file. When records are created as output in arelative file, the key field is used tocalculate a disk address where the

record is written.

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Relative File Organization When records are randomly accessedfrom a relative file, the user enters thekey field, which is used to determine the

physical location of the correspondingrecord. That record is then accessed directly.

Thus there is no need for an index witha relative file.

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FEATURES OF MAGNETICDISKS AND DISK DRIVES

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GENERAL FEATURES Magnetic disk is a storage medium thatcan serve as either input to or outputfrom any computer system - frommainframes to micros.

The disk has a metal oxide coating that canstore hundreds of mill ions of characters ofdata, or more. The magnetic disk drive, which can be a

hard disk drive or a floppy disk drive onthe micro, is used both for recording andfor reading information from the disk atvery high speeds.

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ADDRESSING DISK RECORDSIndividual records on disks can typicallybe addressed in the following way:

1. Surface number.2. Track number.3. Sector number (for floppy disks) orcylinder number (for larger units).

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Processing Indexed DiskFiles

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Creating an Indexed File Indexed files are created in sequence;

that is, the indexed file is created byreading each record from an input fi le, insequence by the key field, and writing theoutput indexed disk records in the samesequence.

Note, however, that once the indexedfile is created, it can be accessedrandomly.

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CREATING AN INDEXED FILEThe full SELECT statement for an indexedfile is as follows:SELECT file-name-1 ASSIGN TOimplementor-name-1

[ORGANIZATION IS] INDEXED[ACCESS MODE IS SEQUENTIAL]

RECORD KEY IS data-name-1

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The ORGANIZATION Clause The clause ORGANIZATION IS INDEXEDindicates that the file is to be createdwith an index. Even though we are creating the filesequentially, we must indicate that thisis an indexed file. This instructs the computer to establishan index so that we can randomly accessthe file later on.

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The ACCESS Clause The ACCESS clause is used to denote if

the file is to be accessed sequentially orrandomly. If the ACCESS clause is omitted, the

compiler will assume that the file isbeing processed in SEQUENTIAL mode. Thus, the ACCESS clause is optionalwhen the file is to be accessedsequentially since ACCESS ISSEQUENTIAL is the default.

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The RECORD KEY Clause The RECORD KEY clause names the keyfield within the disk record that will beused to form the index. This field must be in the same physicallocation in each indexed record.

Usually, it is the first field. It must have a unique value for eachrecord, and it usually has a numericvalue as well.

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DEBUGGING TIP: GUIDELINESFOR RECORD KEYS1. COBOL 85 states that the RECORD KEYshould be defined with a PIC of X's.

Most compilers also allow a PIC of 9's as anenhancement. Regardless of whether the record key isdefined with X's or 9's, it is best to usea RECORD KEY that has a numeric value.

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DEBUGGING TIP: GUIDELINESFOR RECORD KEYS Fields such as ACCT- NO, SOC-SEC-NO, orPART-NO , for example, make ideal keyfields. Also, different collating sequences (EBCDICor ASCII) can cause records to be ordereddifferently in an indexed file if the recordkey values are not all numeric.

2. We recommend that key fields be thefirst fields in a record, for ease ofreference.

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The INVALID KEY Clause The INVALID KEY clause is used with aWRITE instruction to test for twopossible errors:

1. A key field that is not in sequence or2. A key field that is the same as onealready in the indexed fi le.

Format:WRITE record-name-1 [FROM identifier-1]

[INVALID KEY imperative-statement-

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Updating an Indexed FileRandomly As we have seen, one main feature ofdisk processing is that master recordscan be updated directly without having

to create a new file. That is, a disk record can be read intostorage where changes are made and the

changed record can be rewritten backonto the disk in place. This eliminates the need to create anentirely new file.

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Updating an Indexed FileRandomly: The SELECTStatementACCESSING AN INDEXED FILE RANDOMLYSELECT file-name-1 ASSIGN TO

implementor-name-1[ORGANIZATION IS] INDEXED

ACCESS MODE IS RANDOMRECORD KEY IS data-name-1

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Updating an Indexed FileRandomly Opening an IndexedFile as I-O When updating an indexed file, we openit as I-O, for input-output, because it

will be read from and written to. That is,(1) it is used as input [I] for readingor accessing disk records(2) it is also used as output [O] forrewriting or updating the records read.

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Updating an Indexed FileRandomly: The READ Statement A COBOL 85 sample READ:READ INDEXED-FILE INVALID KEY PERFORM 600-ERR-RTNNOT INVALID KEY PERFORM 500-OK-RTNEND-READ

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There is No AT END Clause WhenReading from a Disk Randomly When reading a disk file randomly, wedo not test for an AT END conditionbecause we are not reading the file insequence; instead, we include an

INVALID KEY test. If there is no record in the INDEXED-FILE with a RECORD KEY equal to T-

PART-NO, the INVALID KEY clause willbe executed. Thus, the computer executes the INVALIDKEY option only if the T-PART-NO does notmatch any of the master disk records.

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Updating an IndexedMaster File For updating an indexed file, we havethe following:

1. OPEN the indexed master file as I-O.2. READ transaction data from atransaction file or accept transactiondata from a keyboard.

Move the key field for the transaction record,which was either read in or accepted as input,to the RECORD KEY of the indexed masterfile. When a READ (master file) instruction isexecuted, the indexed master file record withthat RECORD KEY will be transmit ted to themaster record storage area in the FILE

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Updating an IndexedMaster File3. When the READ (master file)instruction is executed, thecorresponding master record that

needs to be updated will be read intomain memory.4. Make the changes to the master

record directly by moving transactiondata to the master I/O record area.5. REWRITE the master record.

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Update an Indexed MasterFile REWRITE record-name-1 [FROMidentifier-1]

[INVALID KEY imperative-statement-1][NOT INVALID KEY imperative-

statement-2]*[END-REWRITE]**These are optional with COBOL 85 only.

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Additional Features of anUpdate Procedure: TheFormat for the DELETEDELETE indexed-file-name-1 RECORD

[INVALID KEY imperative-statement-1][NOT INVALID KEY imperative-statement- 2]*

[END-DELETE]** These are optional with COBOL 85 only.

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QUESTIONS?

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SELF-TEST1. To access records in an indexed filerandomly, we move the transactionrecord's key field to the ____ .

SOLUTION: RECORD KEY of the indexedrecord

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SELF-TEST2. When a record is to be deleted from anindexed file, we use a ____ instruction.

SOLUTION: DELETE file-name

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SELF-TEST3. The INVALID KEY option can be part ofwhich statements?

SOLUTION: The READ (where ACCESS ISRANDOM is specified), WRITE, REWRITE,or DELETE

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SELF-TEST4. The INVALID KEY option tests thevalidity of the ____ KEY.

SOLUTION: RECORD

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SELF-TEST5. If READ FILE-X INVALID KEY PERFORM800-ERROR-1 is executed, 800-ERROR-1will be performed if ____ .

SOLUTION: a record with the indicatedRECORD KEY cannot be found in FILE-X

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SELF-TEST6. (T or F) Indexed files are typicallycreated in sequence by RECORD KEY.

SOLUTION: T

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SELF-TEST7. If a record is to be added to a disk file,a (WRITE, REWRITE) statement is used.

SOLUTION: WRITE

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Processing Relative DiskFiles

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What Is a Relative File? With indexed files, the key fields ofrecords to be accessed are looked up inan index to find the disk address. With relative files, the key field isconverted to an actual disk address sothat there is no need for an index or for

a search to find the location of a record.

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What Is a Relative File? FORMATSELECT file-name-1 ASSIGN TOimplementor-name-1

[ORGANIZATION IS] RELATIVE[ACCESS IS {SEQUENTIAL

[RELATIVE KEY IS data- name-1]}] {RANDOM} {DYNAMIC}

RELATIVE KEY IS data-name-1}][FILE STATUS IS data-name-2]

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Creating Relative Files Relative files are created sequentially,

and either the computer or the user cansupply the key. When a relative file's SELECT statement

includes ACCESS IS SEQUENTIAL, theRELATIVE KEY clause can be omitted. If the RELATIVE KEY clause is omitted, thecomputer writes the records with keys

designated as 1 to n. That is, the first record is placed in relativerecord location 1 (RELATIVE KEY = 1), thesecond in relative record location 2(RELATIVE KEY = 2), and so on.

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Sequential Reading ofRelative Files The records in relative files may be readsequentially, that is, in the order thatthey were created. Because a relative file is created insequence by RELATIVE KEY, a sequentialREAD reads the records in ascendingrelative key order. There is no need to specify a RELATIVEKEY for reading from a relative filesequentially.

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Random Updating of RelativeFiles When updating a relative file, you canaccess each record to be changed andREWRITE it directly. The relative file must be opened as I-O,the required record must be read,changed, and then rewritten for each

update.

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MORE QUESTIONS?

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SELF-TEST1. When creating a relative file, ACCESS IS____ . When using a relative file asinput, ACCESS IS either ____ or ____ .

SOLUTION: SEQUENTIAL; SEQUENTIAL;RANDOM (or DYNAMIC)

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SELF-TEST2. RELATIVE KEY is optional when readingor writing a relative file (sequentially,randomly).

SOLUTION: sequentially

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SELF-TEST3. (T or F) If ACCT-NO is used to calculatea disk address when writing records on arelative file, then ACCT-NO must be

moved to a WORKING-STORAGE entrydesignated as the RELATIVE KEY beforea WRITE is executed.

SOLUTION: T

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SELF-TEST4. (T or F) To read the record with CUST-

NO 125, move 125 to the record's CUST-NO key field and execute a READ.

SOLUTION: F - 125 must be moved to aWORKING-STORAGE entry specified inthe RELATIVE KEY clause of the SELECTstatement or converted to theWORKING-STORAGE RELATIVE KEY, asdescribed in the next section.

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SELF-TEST5. (T or F) Relative file organization is themost popular method for organizing adisk file that may be accessed randomly.

SOLUTION: F - Indexed file organization isthe most popular.

CONVERTING A KEY FIELD TO A

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RELATIVE KEY: CLAUSES TOUPDATE RANDOM-ACCESS FILESOPEN I-O

Used when a relative fi le is beingupdated.REWRITE

Writes back onto a relative fi le (you canonly use REWRITE when the file is openedas I-O and a record has al ready been readfrom it).INVALID KEY

Is required with relative (and indexed) fi lesfor a random READ and any WRITE,DELETE, and REWRITE unless a USE AFTER

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CONVERTING A KEY FIELD TO ARELATIVE KEY: CLAUSES TOUPDATE RANDOM-ACCESS FILESINVALID KEY

The computer will perform the statementsfollowing the INVALID KEY if the recordcannot be found or if the RELATIVE KEY isblank or not numeric. A NOT INVALID KEYclause may be used with COBOL 85.DELETE

Eliminates records from the file.

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CHAPTER SLIDES END HERECHAPTER SUMMARY COMES NEXT

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CHAPTER SUMMARYI. Indexed File ProcessingA. What is an Indexed File?1. ENVIRONMENT DIVISION - SELECTclause specifies:

ORGANIZATION IS INDEXEDACCESS IS RANDOM - Fornonsequential updates, inquiries,etc.

SEQUENTIAL - Forcreating an indexed file, reportingfrom it in sequence, and updating itsequentially.

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CHAPTER SUMMARY2. DATA DIVISIONa. LABEL RECORDS are usuallySTANDARD for all disk files.b. Records are usually blocked.3. PROCEDURE DIVISION

a. Creating an indexed file(1) Indexed files are created withan ACCESS IS SEQUENTIAL clausein the ENVIRONMENT DIVISION.(2) The WRITE statement shouldinclude the INVALID KEY clause.The statement following INVALIDKEY is executed (1) if a record

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CHAPTER SUMMARYII. Relative File ProcessingA. What is a Relative File?

1. Relative files, like indexed files, canbe accessed randomly.2. With a relative file, there is no index.Instead, a record's key field such asACCT-NO is converted to a relative

record number or RELATIVE KEY. Theconversion can be one-to-one(RELATIVE KEY = record key), or arandomizing algorithm may be used to

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CHAPTER SUMMARY3. The random accessing of a relativefile is very fast because there is noneed to look up a disk address from anindex.4. Sequential access of a relative filemay be slow because records adjacentto one another in the file do not

necessarily have key fields insequence.B. Processing Relative Files

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CHAPTER SUMMARYb. RELATIVE KEY clause uses

(1) For randomly accessing the file.(2) For sequential reads and writesif a conversion is necessary from arecord's key field to a RELATIVEKEY.(3) The data-name used as therelative record number orRELATIVE KEY is defined inWORKING-STORAGE.

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CHAPTER SUMMARY2. Processing routines.a. Creating a relative file:

(1) ACCESS IS SEQUENTIAL in theSELECT statement.(2) Move the input record's keyfield to the RELATIVE KEY, whichis in WORKING-STORAGE (orconvert the input key to aWORKING- STORAGE relative key)and WRITE ... INVALID KEY

b. Accessing a relative file randomly:

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