1. Indexed and Relative File Processing
Chapter 15
Indexed and Relative File Processing

2. 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

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

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

5. 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

6. 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
Nothing new, studied this previously

7. 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

8. 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

9. 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

10. 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

11. SELECT Statement SELECT file-name-1 ASSIGN TO implementor-name-1
Format
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

12. SELECT Statement ORGANIZATION INDEXED ACCESS MODE SEQUENTIAL
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

13. 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

14. 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

15. WRITE … INVALID KEY WRITE record-name-1 [FROM identifier-1]
Format
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

16. Updating Index File Randomly
Not really Randomly
Master records can be updated directly without creating a new file
With index file, changes can be made in any sequence

17. 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

18. 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

19. 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

20. REWRITE … INVALID KEY REWRITE record-name-1 [FROM identifier-1]
Format
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

21. Add New Record To add new record to indexed file
Move data to master record fields
Use WRITE … INVALID KEY to create new record
Page 661, 662 for example

22. Delete Existing Record
Format
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

23. 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

24. 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

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

26. 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

27. 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 next slide

28. 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
Note here that Acct-No is a field in the structure of the indexed file and is automatically searched for

29. 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

30. ALTERNATE RECORD KEY [ALTERNATE RECORD KEY IS
Format
[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

31. 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.

32. 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

33. 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 (in this case Acct-No) to find record

34. 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'

35. START Statement [INVALID KEY imperative-statement-1] START file-name-1
Format
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]

36. START Statement Move 025 To Acct-No Start Accts-Receivable Invalid Key
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
Note that it just starts here it’s not filtering anything yet.

37. 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

38. START Statement Start Accts-Receivable Key > Acct-No Invalid Key …
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 …
...

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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]
Format

45. 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.
01 WS-Status Pic X(2).
Example

46. 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

47. FILE STATUS Values
File Status field value Meaning 00 No error occurred 21 Sequence error - keys not in correct order 22 Attempt to write record creating duplicate primary record key Complete list on page 681

48. 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

50. 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
You’ll need at least one other section name for the rest of your procedure division.

51. DECLARATIVES Format
DECLARATIVES. section-name SECTION. USE AFTER STANDARD EXCEPTION PROCEDURE ERROR ON file-name-1 … END DECLARATIVES.

52. 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.

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

54. 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

55. 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

56. 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] RANDOM RELATIVE KEY IS DYNAMIC data-name-1 [FILE STATUS IS data-name-2].

57. 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

58. FD for Relative Files RELATIVE KEY not part of record
Is placed 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
01 R-Key Pic 9(3).

59. 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.

60. 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

61. Processing Relative Files
REWRITE … INVALID KEY to update
DELETE … INVALID KEY to remove record from file

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

63. 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
Is there a problem here?

64. 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

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

66. 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

67. 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

68. 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
Why is this faster?
What is the disadvantage compared to indexed files?

69. 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
If you know you’re only doing sequential read then what?
Data-name used as RELATIVE KEY defined in WORKING-STORAGE

70. 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

71. 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

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