1. 15-1 COBOL for the 21 st Century Nancy Stern Hofstra University Robert A. Stern Nassau Community College James P. Ley University of Wisconsin-Stout (Emeritus) John Wiley & Sons, Inc. 11th edition

2. 15-2 Indexed and Relative File Processing Chapter 15

3. 15-3 Chapter Objectives To familiarize you with Methods of disk file organization Random processing of disk files How to create, update, and access indexed disk files How to create, update, and access relative files Methods used for organizing relative files

4. 15-4 Chapter Contents System Considerations for Organizing Disk Files Feature of Magnetic Disks and Disk Drives Processing Indexed Disk Files –Creating, updating, accessing for reporting

5. 15-5 Chapter Contents Additional Options for Indexed File Processing –ALTERNATE RECORD KEYS –START Statement –Dynamic Access –FILE STATUS Clause –Exception Handling

6. 15-6 Chapter Contents Using Indexed Disk File As External Table Processing Relative Disk Files Converting Key Field to RELATIVE KEY

7. 15-7 Disk File Organization File is collection of records Three major ways records stored or organized on disk - Sequential File Organization - Indexed File Organization - Relative File Organization

8. 15-8 Sequential File Organization Records stored in order they are written to file Must be accessed in sequence - to access 50th record in file, must read past first 49 Typically sorted into sequence by a key field

9. 15-9 Indexed File Organization Consists of two files –Data file - records in sequence –Index file - contains value of Each key field Disk address of record with that corresponding key field For random access, look up key field in index file to find address Then access record in data file directly

10. 15-10 Relative File Organization When records created, key field used to compute a disk address where record is written To randomly access records –User enters key field –Disk address computed from key field –Record then accessed directly No index needed

11. 15-11 Magnetic Disks, Disk Drives Storage media for data Can be accessed at very high speeds Have metal oxide coating that stores data as magnetized bits Disks may be floppy disk, hard disk or disk pack

12. 15-12 Magnetic Disks, Disk Drives Data stored in concentric circles on disk called tracks Disk drives have access arm with read/write head for accessing data –Access arm can be positioned over any track on disk –Makes random access possible

13. 15-13 Addressing Disk Records Address individual records on disks by –Surface number - top or bottom side of disk –Track number –Sector (floppy disks) or cylinder (larger units) number Sector or cylinder is wedge-shaped section of tracks

14. 15-14 Creating an Indexed File Records written in sequence by key field as for sequential disk file Once index file created, records can be accessed randomly

15. 15-15 SELECT Statement SELECT file-name-1 ASSIGN TO implementor-name-1 [ORGANIZATION IS] INDEXED [ACCESS MODE IS SEQUENTIAL] RECORD KEY IS data-name-1 SELECT to create indexed file Format

16. 15-16 SELECT Statement ORGANIZATION INDEXED –Indicates index file to be created along with data file –Index file must be established to be able to randomly access file later ACCESS MODE SEQUENTIAL –Records written in sequence by key field –Optional since SEQUENTIAL is default mode

17. 15-17 SELECT Statement RECORD KEY clause –Names key field within disk record used to form index –Must be in same physical location in each record (usually first field) –Value must be unique for each record –Best to use numeric field as key

18. 15-18 WRITE … INVALID KEY INVALID KEY clause required when writing indexed records to handle I/O errors –Key field not in sequence –Key field same as one already in file If error detected with WRITE –Record not written –Statement(s) following INVALID KEY executed

19. 15-19 WRITE … INVALID KEY WRITE record-name-1 [FROM identifier-1] [INVALID KEY imperative-statement-1] [NOT INVALID KEY imperative-statement-2] [END-WRITE] Statement(s) following NOT INVALID KEY executed if WRITE is successful Format

20. 15-20 Updating Index File Randomly Master records can be updated directly without creating a new file With index file, changes can be made in any sequence

21. 15-21 Updating Index File Randomly Read transaction record (or get update data interactively) Move key field value to RECORD KEY of master file Read master record into storage (READ … INVALID KEY) Make needed changes to fields REWRITE record to master file

22. 15-22 Updating Index File Randomly In SELECT statement specify ACCESS MODE IS RANDOM Open indexed file for I-O –I (Input) to read in records –O (Output) to rewrite or update records

23. 15-23 READ … INVALID KEY To locate record with key field equal to value stored in record key Move Trans-No To Master-No Read Indexed-File Invalid Key Perform 600-Err-Rtn Not Invalid Key Perform 500-OK-Rtn End-Read

24. 15-24 REWRITE … INVALID KEY REWRITE record-name-1 [FROM identifier-1] [INVALID KEY imperative-statement-1] [NOT INVALID KEY imperative-statement-2] [END-REWRITE] To update existing indexed record INVALID KEY occurs if programmer has changed key field of record Format

25. 15-25 Add New Record To add new record to indexed file –Move data to master record fields –Use WRITE … INVALID KEY to create new record

26. 15-26 Delete Existing Record DELETE index-file-name-1 RECORD [INVALID KEY imperative-statement-1] [NOT INVALID KEY imperative-statement-2] [END-DELETE] To delete record with key field equal to value stored in record key Format

27. 15-27 Debugging Tips Must run program to create indexed file –Cannot be created using text editor To test an update program, always run index file creation program first May not be able to DISPLAY or print indexed records on your system directly –Move data to standard sequential record first

28. 15-28 Printing Indexed File Sequentially Process file in same way as a sequential file Specify ACCESS IS SEQUENTIAL SORT file before printing if report is to be in sequence by field other than key field

29. 15-29 Printing Indexed File Randomly May need to access only records about which inquiries have been made Specify ACCESS IS RANDOM

30. 15-30 Printing Indexed File Randomly Read inquiry record Move inquiry key field value to RECORD KEY of master file Read master record into storage (READ … INVALID KEY) Move desired fields to output record WRITE output record to print file

31. 15-31 Random Interactive Inquiries Inquiries often made interactively with output displayed on screen Assume indexed accounts receivable file with customer's Acct-No as key field Interactive inquiries made about balance due for a customer Code using random access for one inquiry follows

32. 15-32 Random Interactive Inquiries Display 'Enter account number' Accept Acct-No Read Accts-Receivable Invalid Key Display 'Account not on file' Not Invalid Key Display 'Balance due = ', Bal-Due End-Read

33. 15-33 ALTERNATE RECORD KEY Clause to enable file to be accessed randomly using more than one key field –May want to access accounts receivable records by account number or name Add to SELECT statement after RECORD KEY clause to establish multiple key fields for indexing

34. 15-34 ALTERNATE RECORD KEY [ALTERNATE RECORD KEY IS data-name-2 [WITH DUPLICATES] ] … Multiple ALTERNATE keys allowed Need not be unique Access records by RECORD KEY or any ALTERNATE RECORD KEYs Format

35. 15-35 SELECT Example Select Accts-Receivable Assign To 'C:\AcctRec.ndx' Organization Is Indexed Access Is Sequential Record Key Is Acct-No Alternate Record Key Is Cst-Last-Name With Duplicates.

36. 15-36 Random Access by ALTERNATE To access file by Cst-Last-Name Display 'Enter last name' Accept Cst-Last-Name Read Accts-Receivable Key is Cst-Last-Name Invalid Key Display 'No record found' Not Invalid Key Display 'Balance due = ', Bal-Due End-Read

37. 15-37 Random Access by ALTERNATE KEY clause used with READ to specify access by ALTERNATE RECORD KEY Since WITH DUPLICATES specified –May be more than one record with same last name –Record retrieved is first one placed on disk If KEY clause omitted, uses RECORD KEY field (Acct-No) to find record

38. 15-38 START Statement To begin processing indexed file sequentially starting from any record location –Print file beginning with customer record with Acct-No = 025 –Print all customers with Cst-Last-Name beginning with letter 'S'

39. 15-39 START Statement START file-name-1 IS = KEY IS > data-name-1 IS NOT < IS >= [INVALID KEY imperative-statement-1] [NOT INVALID KEY imperative-statement-2] [END-START] Format

40. 15-40 START Statement To begin processing with record whose account number equals 025 Move 025 To Acct-No Start Accts-Receivable Invalid Key Display 'Acct-No 025 not found' Not Invalid Key Perform 300-Proc-Rec End-Start

41. 15-41 START Statement START locates record with Acct-No = 025 INVALID KEY clause executed only if no such record found START locates record but does not READ it 300-Proc-Rec must include READ … AT END to bring record into storage for processing

42. 15-42 START Statement KEY clause can be omitted only if checking for value equal to RECORD KEY value To locate record with Acct-No > 100: Move 100 To Acct-No Start Accts-Receivable Key > Acct-No Invalid Key …...

43. 15-43 ACCESS IS DYNAMIC Mode used to access indexed file both randomly and sequentially in single program For example, update selected records, then print control listing of entire indexed file –Random access used for updating –Sequential access used for printing report

44. 15-44 ACCESS IS DYNAMIC Mode required for reading records in sequence by ALTERNATE RECORD KEY Also required when records accessed by both RECORD KEY and ALTERNATE RECORD KEY

45. 15-45 READ … NEXT RECORD To perform sequential read of indexed file when ACCESS MODE IS DYNAMIC To sequentially read from file by its ALTERNATE RECORD KEY To begin reading sequentially from some point other than beginning of file

46. 15-46 READ … NEXT RECORD Assume first record with Acct-No > 100 has been located using START Use READ … NEXT RECORD to read in records sequentially from this position Only word NEXT required

47. 15-47 READ … NEXT RECORD Perform Until More-Records = 'NO' Read Accts-Receivable Next Record At End Move 'NO' To More-Records Not At End Perform 300-Proc-Rec End-Read End-Perform

48. 15-48 FILE STATUS Clause To determine exact type of input or output error that occurred when accessing a file Included in SELECT statement for a file as last clause SELECT … [FILE STATUS IS data-name] Forma t

49. 15-49 FILE STATUS Clause Data-name must appear in WORKING- STORAGE as two-position alphanumeric field Select Indexed-Pay-File … File Status Is WS-Status. … Working-Storage Section. 01WS-StatusPic X(2). Exam ple

50. 15-50 FILE STATUS Clause When input or output operation performed on Indexed-Pay-File –Value placed in WS-Status –Can be tested by programmer in PROCEDURE DIVISION Several FILE STATUS field values and their meaning follow

51. 15-51 FILE STATUS Values File Status field value Meaning 00 No error occurred 21Sequence error - keys not in correct order 22Attempt to write record creating duplicate primary record key

52. 15-52 Checking FILE STATUS Write Indexed-Pay-Rec Invalid Key If WS-Status = '21' Display 'Key not in sequence' End-If … Not Invalid Key Perform 600-OK-Rtn End-Write

53. 15-53 Exception Handling Most comprehensive method for handling input/output errors is to establish separate section(s) for this Place exception handling routines in DECLARATIVES segment –Always appears first in PROCEDURE DIVISION –Must begin with section-name

54. 15-54 DECLARATIVES Format DECLARATIVES. section-name SECTION. USE AFTER STANDARD EXCEPTIONPROCEDURE ERROR ON file-name-1 … END DECLARATIVES.

55. 15-55 DECLARATIVES Example Procedure Division. Declaratives. A000-Exception-Handling Section. Use After Error Procedure On Indexed-Pay-File A100-Check-It. If WS-Status = '21' Display 'Key not in sequence' End-If … End Declaratives.

56. 15-56 DECLARATIVES Example B000-Regular-Processing Section. B100-Main-Paragraph. … Read Indexed-Pay-File … Write Indexed-Pay-File …

57. 15-57 DECLARATIVES Example Once section header used, rest of PROCEDURE DIVISION must be divided into sections Statements in paragraph A100-Check-It in DECLARATIVES test value of FILE STATUS field INVALID KEY not needed for READ or WRITE since errors handled in DECLARATIVES

58. 15-58 Relative Files File organization that converts key field to actual disk address to find location of record –No need to look up disk address in index –Convert key to disk address and access record directly Records may be accessed both sequentially and randomly

59. 15-59 SELECT for Relative Files SELECT file-name-1 ASSIGN to implementor-name-1 [ORGANIZATION IS] RELATIVE [ACCESS IS SEQUENTIAL [RELATIVE KEY IS data-name-1] RANDOMRELATIVE KEY IS DYNAMIC data-name-1 [FILE STATUS IS data-name-2].

60. 15-60 SELECT for Relative Files RELATIVE KEY clause –Optional if ACCESS is SEQUENTIAL –Otherwise, required ACCESS IS DYNAMIC allows both sequential and random access in same program FILE STATUS field used same way as with indexed files

61. 15-61 FD for Relative Files RELATIVE KEY not part of record –In separate WORKING-STORAGE entry If key is a three digit field and SELECT clause is Relative Key is R-Key Entry in WORKING-STORAGE is 01R-KeyPic 9(3).

62. 15-62 Creating Relative Files When created sequentially, either computer or user can supply keys If RELATIVE KEY clause omitted, computer supplies keys First record placed in relative record location 1 (RELATIVE KEY = 1) Second record in relative record location 2 (RELATIVE KEY = 2), etc.

63. 15-63 Processing Relative Files WRITE … INVALID KEY to write record to relative file READ … AT END to read sequentially READ … INVALID KEY to read randomly –Move key value of record to locate to RELATIVE KEY before executing READ

64. 15-64 Processing Relative Files REWRITE … INVALID KEY to update DELETE … INVALID KEY to remove record from file

65. 15-65 Relative Keys Sometimes key field not feasible to use as relative key For example, a five digit Trans-No with values from 00001 to 99999 with only 1000 actual records would be wasteful –99999 record locations would need to be allocated but only a small portion used

66. 15-66 Converting to Relative Keys Methods called hashing used to convert key field into relative record number Simple hashing method Divide Trans-No by 1009 Giving Num Remainder Rel-Key Rel-Key will be number from 0 to 1008 Add 1 to get relative record number from 1 to 1009, enough positions for 1000-record file

67. 15-67 Relative Files Hashing algorithm used when: Creating relative file - each record's key field used to calculate RELATIVE KEY for positioning record in file Accessing file randomly - convert inquiry or transaction record's key to RELATIVE KEY before reading

68. 15-68 Chapter Summary Indexed Files - SELECT clauses –ORGANIZATION IS INDEXED –ACCESS IS RANDOM For nonsequential updates, inquiries –ACCESS IS SEQUENTIAL For creating, reporting, updating sequentially

69. 15-69 Chapter Summary Indexed Files - SELECT clauses –RECORD KEY - Key field for establishing index, accessing records –FILE STATUS IS data-name for indicating success of input or output operation

70. 15-70 Chapter Summary Indexed Files - PROCEDURE DIVISION –Creating indexed file ACCESS IS SEQUENTIAL Use READ … AT END –Reading from indexed file In sequence - READ … AT END Randomly - READ … INVALID KEY

71. 15-71 Chapter Summary Relative Files –No index, record's key field converted to relative record number or RELATIVE KEY –Fast for random access but may be slow for sequential access

72. 15-72 Chapter Summary Relative Files - SELECT clauses –ORGANIZATION IS RELATIVE –RELATIVE KEY clause uses For randomly accessing file For sequential reads, writes if conversion necessary from record's key field to RELATIVE KEY Data-name used as RELATIVE KEY defined in WORKING-STORAGE

73. 15-73 Chapter Summary Relative Files - SELECT clauses –Any of three ACCESS MODEs can be used Creating a relative file –ACCESS IS SEQUENTIAL –Move or convert input record's key field to RELATIVE KEY in WORKING-STORAGE –Use WRITE … INVALID KEY

74. 15-74 Chapter Summary Accessing a relative file –ACCESS IS RANDOM –Move or convert transaction record's key field to RELATIVE KEY in WORKING- STORAGE –Use READ … INVALID KEY

75. 15-75 Chapter Summary Updating a relative file –ACCESS IS RANDOM –OPEN for I-O –Use READ, WRITE, REWRITE, or DELETE with INVALID KEY clauses

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