BIS 1753 Introduction to Computer Programming 1. Computer Program A set of instructions that enables...

BIS 1753

• Introduction to Computer Programming

1

Computer Program

• A set of instructions that enables a computer to process data

• Also called software

• Two types of computer software– Systems software - controls overall

operations of computer– Application software - perform specific

tasks

2

Application Software

• Written by application programmers• May provide quick solution to one-time

problem– Generate ad hoc report, summarize data

• Or may be run on regularly scheduled basis– Payroll system, Billing system

3

Customized Application Software

• Written for specific users like retail establishments, manufacturing firms, banks, and more

• Necessary if user has special requirements

4

Programming Languages

Symbolic language (like COBOL)• English-like languages used to write

programs• Easier than writing programs in machine

language• Must be translated or compiled into

machine language to run on computer

5


Machine language• Only language computer understands• All programs executed on computer

must be in machine language• Machine language programs difficult to

write

6


Compiler• Translates source code (such as

COBOL) into object code (roughly, executable code)

• Checks for syntax errors in the source code

7

Nature of COBOL

• Business-oriented language

• Standard language

• English-like language

• Relatively easy to understand

8

History of COBOL

• Developed in 1959 as standard language to meet needs of business

• Committee to develop language convened by Department of Defense

• Included representatives from academia, business, computer manufacturers

9

Standard Versions of COBOL• 1960s

• 1968

• 1974

• 1985

wide variations in COBOL compilers

first COBOL standard set by American National Standards Institute (ANSI)

second ANSI standard to make COBOL more efficient, standardized

this ANSI standard incorporated structured programming techniques

10

Current and Future Standards

• 1985 currently the most widely used

• 2008 is next standard – Approval expected in 2008 or so– Information on 2008 COBOL standard at

http://www.ansi.org

11

Future of COBOL

Likely to remain important language

• Older COBOL programs need to be updated

• Still used by many for new program development

12

Use of COBOL

• About 200 billion lines of COBOL source code in use

• 5 billion new lines added each year

• Used by 42.7% of application programmers in medium to large U.S. companies

13

Overview of the Four Divisions

• Every COBOL program contains up to four separate divisions in the following order:

IDENTIFICATION DIVISION

ENVIRONMENT DIVISION

DATA DIVISION

PROCEDURE DIVISION

14


• IDENTIFICATION DIVISION– Identifies program to operating system– Provides documentation about program

• ENVIRONMENT DIVISION– Defines file-names– Describes devices used to store them– Not included in fully interactive programs

15


• DATA DIVISION– Describes input and output format of data

in files– Defines any constants and work areas

• PROCEDURE DIVISION– Contains instructions to read input, process

it and create output

16

BIS 1753

• Introduction to Structured Program Design in COBOL

19

Basic COBOL Program Structure

• Originally, each COBOL instruction coded on single line of 80 characters

• Positions on line reserved for special purposes

• Rigid column rules dropped in 2008

• OpenCOBOL supports free-format COBOL

20

Coding Rules (fixed format)

• Columns 1-6 and 73-80 optional and rarely used today

• Column 7 for continuation, comment, starting new page

• Columns 8-72 for COBOL program statements

21

Coding Rules (fixed format)

Column 7

* (asterisk) designates entire line as comment

/ (slash) forces page break when printing source listing

- (dash) to indicate continuation of nonnumeric literal

22

Coding Rules (free format)

*> at beginning of line indicates comment

23

Margin Rules

• Columns 8-72 divided into two areas– Area A - columns 8, 9, 10, 11– Area B - columns 12-72

• Division, section and paragraph-names must all begin in Area A– First letter of name must begin in column 8,

9, 10 or 11– Entry may extend into Area B

24

Margin Rules

• All other statements, clauses, and sentences begin anywhere in Area B (column 12, 13, 14, etc.)– Select entries in ENVIRONMENT

DIVISION– Data description entries in DATA DIVISION– All PROCEDURE DIVISION instructions

25

Rules for User-Defined Words

1. 1 to 30 characters

2. Letters, digits, hyphens (-) only

3. No embedded blanks

4. At least one alphabetic character

5. May not begin or end with hyphen

6. May not be COBOL reserved word

26

Coding Guidelines

1. Separate divisions by blank comment line, page eject symbol or blank line

2. Code a single statement per line

3. Code paragraph-names on line by themselves

4. Be liberal in use of comments. Box lengthy comments using asterisks.

27

Coding Guidelines

5. Code SELECT statements in logical order (input files first, then output files) although order not required

6. Use separate lines for SELECT, ASSIGN, ORGANIZATION clauses for readability

7. Avoid use of device-specific file-names

28

COBOL 2008 Changes

• Coding rules for Areas A and B will be recommended not required.

• PROGRAM-ID will be only paragraph in IDENTIFICATION DIVISION. All others can be specified as comments.

• Length of user-defined words will be increased from 30 to 60 characters.

29

BIS 1753

• IDENTIFICATION and ENVIRONMENT Divisions

31


• Provides identifying information about program

• Divided into paragraphs

• PROGRAM-ID only required paragraph

• Other paragraphs optional

32


Syntax

IDENTIFICATION DIVISION.

PROGRAM-ID. program-name.

[AUTHOR. [comment-entry] …]

[other optional paragraphs]

33

Syntax Symbology• Uppercase words are COBOL reserved

words• Lowercase words are user-defined entries



– DIVISION is reserved word– program-name is user-defined data-name

Example

34

Syntax Symbology

• Underlined words are required• Punctuation if specified is required

IDENTIFICATION DIVISION. PROGRAM-ID. program-name.

– IDENTIFICATION, DIVISION required– PROGRAM-ID is required paragraph– Periods required after division header, paragraph

name and program-name

Example

35

Syntax Symbology

• Brackets [ ] mean item is optional, braces { } mean one of enclosed items required

• Ellipses (. . .) mean entry may be repeated



[AUTHOR. [comment-entry] …]

– AUTHOR paragraph optional– If included it may have any number of comment

entries

Example

36

ENVIRONMENT DIVISION

• Describes files and computer devices used to process them

• Required by programs that process files

• This division is machine-dependent since devices differ from computer to computer

• Only division that may change if program run on different computer

37

Sections of Environment Division

• CONFIGURATION SECTION– Describes computer used to

compile/execute program– Optional and recommended that you omit it

• INPUT-OUTPUT SECTION– Describes input and output files– Required for all programs using files

38

INPUT-OUTPUT SECTION

Format

INPUT-OUTPUT SECTION.FILE-CONTROL.

SELECT internal-file-name-1 ASSIGN TO externale-file-name-1 [ORGANIZATION IS LINE SEQUENTIAL].1

1Use this clause for all PC files so each line treated as separate record.

39

SELECT Statement file-names

• File-names are user-defined words– Words chosen by programmer to represent

some element of program

40

BIS 1753

• DATA Division

43

Data-Name Guidelines

1.Use meaningful data-names that describe contents of field• Amount-Due-In instead of A1

2.Use prefixes or suffixes in data-names when appropriate• -IN and -OUT for fields (Emp-Salary-IN and

Emp-Salary-OUT)• -FILE and -RECORD for file and record

names (Emp-File and Emp-Record)

44

DATA DIVISION

• Defines, describes storage for all data• Two main sections

– FILE SECTION • Defines all input and output files, records, fields• Required for any program that uses files,

typically batch programs

– WORKING-STORAGE SECTION • Defines constants, end-of-file indicators and

work areas • Defines fields not part of input or output files

45

WORKING-STORAGE SECTION

• Follows FILE SECTION

• Begins with heading on line by itself

• Starts in Area A, ends with period

• All items must be defined at 01 level or in entries subordinate to 01 level entry, or 77 level (independent data item)

46

WORKING-STORAGE SECTION

• Rules for user-defined data-names apply

• Elementary items:– Must include PICTURE clause– May be assigned initial value with VALUE

clause

47

Uses of WORKING-STORAGE

To define fields used for

• Keyed input and displayed output

• Intermediate arithmetic results

• Counters and totals

• End-Of-File Indicators

48

PICTURE (PIC) clauses

• Specify type of data stored in field• Indicate size of field

49

Types of data fields• Alphanumeric (x- prefix)

• Any character - letters, digits, special characters• For an address like 123 N. Main St.

• Numeric (n- prefix)• Only digits• Used for input and calculations

• Numeric edited (ne-prefix)• Digits and special characters • Used for output

50

Data Types in PICTURE clause

• X for alphanumeric• 9 V for numeric• Z , $ . and more for numeric edited

51

Size of Data Fields

Denote size of field by:

• Number of formatting characters used in PICTURE

01 Cust-Rec-In.05 Cust-ID-In Picture XXXX.

05 Amt-In Picture 99999.

52

Size of Data Fields

My also denote size of field by:

• A, X or 9 followed by number in parentheses

01 Cust-Rec-In.05 Cust-ID-In Picture X(4).

05 Amt-In Picture 9(5).

53

Implied Decimal Point

• For fields used in arithmetic operations• Symbol V used in PIC clause to denote

location of implied decimal point• Decimal point itself not stored as part of

number• To store value 26.79 in field AMT-IN,

code entry as

05 Amt-In Pic 99V99.54

VALUE clause

• To define initial value for field• If omitted, field’s value undefined when

program begins execution• May be used only in WORKING-

STORAGE SECTION

55

VALUE vs MOVE

• VALUE clause used in DATA DIVISION– Gives initial value before execution begins

• MOVE used in PROCEDURE DIVISION– Assigns value to field after program begins

• MOVE may change initial value of field

56

VALUE Clause

• Contains literal or figurative constant• Data type must match PICTURE

• Numeric literals or ZEROS used with PIC 9 fields 01 WS-Tax-Rate Pic V99 Value .06. 01 WS-Total Pic 999 Value Zeros.

57

VALUE Clause

• Nonnumeric literals, ZEROS or SPACES used with PIC X fields 01 WS-EOF Pic X(3) Value ‘YES’. 01 WS-Descrip Pic X(8) Value Spaces.

58

Rules for Numeric Literals

1. 1 to 18 digits.2. + or - sign may be included to left of

first digit.3. Decimal point permitted within literal.

May not follow last digit.

Valid numeric literals

23 +2359.4 .125 -68734

59

Rules for Nonnumeric Literals

• Must be enclosed in quotation marks• From 1 to 160 characters, including space• Any character in COBOL character set except

quotation mark

Valid Nonnumeric Literals

'123 Main St.' '$14.99' '12,342' 'Enter a value from 1 to 10'

60

Nonnumeric Literals

• Only characters within quotes are moved or displayed

• May contain all numbers ('125') but not same as numeric literal (125)– Cannot be used in arithmetic operations– Cannot be moved to field with PIC of 9’s

• Are not data-names– 'Amount-In' not same as field defined in

DATA DIVISION called Amount-In

61

Figurative Constants - ZERO

• ZERO, ZEROS or ZEROES means all zeros

Move Zeros To Total-Out

• Fills each position in Total-Out with a zero

• May be used with both numeric and alphanumeric fields

Example

62

Figurative Constants - SPACE

• SPACE or SPACES means all spaces or blanks

Move Spaces To Code-Out

• Fills each position in Code-Out with a space or blank

• Use only with alphanumeric fields since blank is invalid numeric character

Example

63

BIS 1753

• PROCEDURE Division (Interactive I/O)

64

PROCEDURE DIVISION

Contains instructions for

• initialization activities

• processing activities

• termination activities

65

PROCEDURE DIVISION

• Interactive processing instructions

– Accept input from keyboard

– Display output on screen

66

Paragraphs

• PROCEDURE DIVISION divided into

paragraphs

• Each is independent module or routine

• Made up of series of instructions to

perform specific set of operations

67

Rules for Paragraph-Names

• Coded in Area A, followed by period

• Follow rules for forming data-names

except may be all digits

– 1010, 1020, 1030, etc. are valid paragraph

names

• Must be unique

68

Procedure Division Statements

• All statements coded in Area B

• Statement begins with verb (Accept,

Display)

• Last statement in paragraph ends with

period

• Sentence - series of statements ending

with period69

Interactive Program Statements

• DISPLAY to prompt for input

• ACCEPT to store input in WORKING-STORAGE areas

• Various statements to process input

• DISPLAY to show output

70

ACCEPT statement

71

DISPLAY statement

72

COMPUTE statement

73

STOP RUN statement

• Terminates the program

• Usually last instruction in main paragraph

• Execution continues with next paragraph if STOP RUN is omitted

74

Comments in COBOL• Start with asterisk (*) in column 7 (fixed

format)

• Start with *> in column 1 (free format)

• Use as reminders and explanations of processing performed by program

• Use to describe program in IDENTIFICATION DIVISION

• Use to describe each paragraph in PROCEDURE DIVISION

75

BIS 1753

• Move statement

76

Move statement

•

MOVE identifier-1 TO identifier-2 …

literal-1

Move 0 To Total-1, Total-2, Total-3

• Move may have multiple receiving fields

Full Format

77

Numeric MOVE Rules

• Decimal alignment always maintained

• Digits in integer part moved right to left starting at decimal point

• Digits in decimal part moved left to right starting at decimal point

78

Numeric MOVE Rules

• If receiving field has more integer positions than sending field– Unfilled high-order (leftmost) integer

positions filled with zeros

• If receiving field has more decimal positions than sending field– Unfilled low-order (rightmost) decimal

positions filled with zeros

79

Numeric MOVE Rules

• If receiving field has fewer integer positions than sending field– High-order (leftmost) digits truncated

• If receiving field has fewer decimal positions than sending field– Low-order (rightmost) digits truncated

80

Rules for Nonnumeric MOVE

• Characters moved from left to right

• If receiving field longer, low-order (rightmost) positions replaced with spaces

• If receiving field shorter, low-order characters in sending field truncated

81

BIS 1753

• Full-screen interactive I/O

82

Displaying Output Interactively

• No COBOL standard for interactive processing

• Two techniques used by compilers to add interactivity– ACCEPT, DISPLAY enhancements with

options to define appearance of data on screen

– SCREEN SECTION added in DATA DIVISION to define screen’s format

83

ACCEPT, DISPLAY Enhancements

Display "Enter date of birth"At Line 13 Column 1

With Blank ScreenForeground-Color 1Background-Color 7

• Displays prompt at specific position on screen after clearing screen, using one color for characters and another for background

EXAMPLE

84

SCREEN SECTION

• Specify formats, options for screen

• Follows WORKING-STORAGE

• Define each screen with 01 level entry followed by subordinate entries

• DISPLAY screen-name displays all prompts described in entries for screen

• ACCEPT screen-name captures all data typed by user for that screen

85

SCREEN SECTION

01 Screen-1.05 Blank Screen.05 Line 3 Column 1 Value 'City'.05 Column 17 Pic X(13) To City-In.

• Display Screen-1 blanks screen and displays City in first column of line 3

• Accept Screen-1 moves 13 characters user enters starting in line 3, column 17 to City-In

EXAMPLE

86

BIS 1753

• File Input/Output (I/O)

88

COBOL Data Organization

• File - group of related records– Customer file made up of customer records

• Record - group of related fields– All fields related to one customer

• Field - group of characters forming a meaningful unit or basic fact– Characters in a name or digits in an

amount

89

DATA DIVISION

• Defines, describes storage for all data• Two main sections

– FILE SECTION • Defines all input and output files, records, fields• Required for any program that uses files,

typically batch programs

– WORKING-STORAGE SECTION • Defines constants, end-of-file indicators and

work areas • Defines fields not part of input or output files

90

Types of Files to Define

• Input files• Master files• Transaction files

• Output files• New Master files• Report files

91

File Description Entries

• Each file must be described with an FD (File Descriptor) entry

• One FD entry for each SELECT statement in ENVIRONMENT DIVISION

• FD followed by• File-name• Optional clauses to describe file and format

of its records

92

File Description Format

FD file-name RECORD IS OMITTED LABEL RECORDS ARE

STANDARD

RECORD CONTAINS integer-1 CHARACTERS

BLOCK CONTAINS integer-2 RECORDS .

93

BIS 1753

• PROCEDURE Division (File I/O)

94

PROCEDURE DIVISION

• Interactive processing instructions

– Accept input from keyboard

– Display output on screen

• Batch processing instructions

– Access files and read them

– Write output to files

95

Batch Processing File I/O

Statements to:

• Initialization activities: open files,

generate headings

• Processing activities: read records,

process, generate detail output

• Termination activities: generate footers,

close files96

Batch Program Instructions

• OPEN - Open files to be processed

• PERFORM UNTIL

– Loop to repeatedly READ and process

input records, and WRITE results to output

file

• CLOSE – Close files when done processing

• STOP RUN – Terminate program execution

97

Environment/Data Divisions

For each input or output file:

• Select statement: binds internal file name to external file name

• fd entry (file description entry)

• associated record

98

OPEN Statement

• Accesses and makes files available for processing

• Identifies whether files will be used for input or output

99

OPEN Statement

INPUT file-name-1 … OPEN OUTPUT file-name-2 …

• File-names used must appear in SELECT statement

• File must be accessed with OPEN before reading from or writing to it

FORMAT

100

PERFORM statement

PERFORM paragraph-name

• To execute instructions in separate paragraph one time

• Control returns to statement following PERFORM

FORMAT

101

PERFORM/UNTIL Statement

PERFORM paragraph-name UNTIL condition

• Step 1: Evaluate condition

• Step 2: If condition evaluates to false, perform paragraph one time

FORMAT

102

In-Line PERFORM

PERFORM UNTIL condition statement(s) END-PERFORM

• Step 1: Evaluate condition• Step 2: If condition evaluates to false,

execute in-line statements one time

FORMAT

103

READ Statement

• Reads record from file opened for input

• Makes one record available at a time, not entire file (the record is the unit of access)

• Makes data available in input file’s associated record

104

READ Statement

• READ file-name-1 AT END statement-1 … [NOT AT END statement-2 …] [END-READ]

• File-name appears in SELECT statement, FD entry and OPEN

• AT END tests if there are more records

FORMAT

105

READ Statement

• If no more records– Executes statement(s) after AT END– Typically statement(s) to cause loop

containing READ to end

• If more records– Reads in next record– Executes statement(s) after NOT AT END– Typically statement(s) to process record

just read

106

WRITE statement

• WRITE record-name-1

• Transmits data from associated record to output file

• record-name-1 must be an associated record of a file opened for output

FORMAT

107

CLOSE statement

• CLOSE file-name-1 ...

• Close specified file(s)

• Indicates files no longer needed for processing

• Releases files and deactivates devices

FORMAT

108

STOP RUN

• Terminates the program

• Usually last instruction in main paragraph

• Execution “falls through” to next paragraph if STOP RUN is omitted

109

BIS 1753

• Report Design Guidelines

112

Report Design Guidelines

1. Include heading to identify report

2. Include date, page number

3. Include column headings to identify fields printed

4. Place most significant fields where they are most visible

5. Edit numeric fields for readability

113

Report Design Guidelines

6. Include totals at end of page or report

7. Include page footings at end of each page, report footings at end of report

114

WRITE Statement

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

AFTER integer-1 LINE BEFORE identifier-2 LINES

– integer-1 or identifier-2 must be non-negative integer value

– AFTER ADVANCING prints line after paper is spaced

– BEFORE ADVANCING prints line before spacing occurs

FORMAT

ADVANCING

115

Defining Print Records

• Define each type of output line as separate 01-level record in WORKING-STORAGE

• May include 01-level records for heading, detail, total, footing lines, etc.

• Establishes separate storage area for each record

• All constants and blanks may be preassigned with VALUE clauses

116

WRITE … FROM Statement

• To transfer data from storage to print area and then print

• Replaces MOVE and WRITE

Write Print-Rec From Heading-1

instead of

Move Heading-1 to Print-RecWrite Print-Rec

117

BIS 1753

• Tables

118

Defining a Table with the OCCURS clause

01 xTempTable.

05 nTemp occurs 24 times pic S9(3).

- Indicates repeated occurrence an element with same format- Defines series of related elements with same format as a table

119

Accessing Elements in Table

• Identifier nTemp is table name• Use nTemp along with a subscript to

access fields or elements within array• Subscript indicates which of the 24

elements to access

Statement OutputDisplay nTemp (2) 2 AM valueDisplay nTemp (23) 11 PM value

120

Valid Subscripts• Integer literal, data item, or expression

• Valid values are 1 to number of elements in table

121

Processing Elements in Table

• PERFORM VARYING perfectly suited to process the elements in a table

perform 210-processvarying nSubscript from 1 by 1until nSubscript > 24.

210-process.add nTemp(nSubscript) to nTotalTemp.

122

In-line PERFORM VARYING

perform varying nSubscript

from 1 by 1 until nSubscript > 24

add nTemp (nSubscript) To nTotalTemp

end-perform.

123

Elementary Items with OCCURS

• Occurs clause may be used with elementary data item

01 xTotalsTable.

05 nTotal occurs 12 times pic 9(5)V99.

• Defines xTotalstable as 84-character array (12 x 7) of 12 elementary items

124

Group Items with OCCURS

• OCCURS may be used with group item

01 xTaxTable.

05 xGroup occurs 20 times.

10 xCity pic X(6).

10 nTaxRate pic V999.

• xCity and nTaxRate each occur 20 times in group item xGroup.

125

Initializing Elements

• Two ways to use VALUE clause to initialize all elements to zero

01 xTable-1.

05 nTotal occurs 50 times pic 9(5) value zero.

01 xTable-2 value zero.

05 nTotal occurs 50 times pic 9(5).

126

Initializing Elements

• Can also initialize each element to different value

01 xDayNames

value 'SUNMONTUEWEDTHUFRISAT'.

05 xDay occurs 7 times pic x(3).

• Defines table with 7 three-character elements. xDay(1) = SUN, xDay(2) = MON, etc.

127

SEARCH Statement

SEARCH identifier-1[AT END imperative-statement-1]

WHEN condition-1 imperative-

statement-2 ...

CONTINUE[END-SEARCH]

• Use in place of PERFORM VARYING to search table

Format

128

SEARCH Statement

• Identifier used after SEARCH is table name specified in OCCURS entry

• Condition compares search argument to table argument

• WHEN clause indicates action to take when condition is met

• AT END clause specifies action to take if table searched but no match found

129

INDEXED BY clause

• Special field called index must be used with SEARCH

• Similar to subscript but defined along with table as part of OCCURS

05 xTableEntries occurs 1000 timesindexed By nIndex.

• Compiler automatically supplies appropriate pic clause for nIndex

130

Index with SEARCH

• Must initialize index before SEARCH• SEARCH performs table look-up,

automatically incrementing index• Internally, computer can use faster

method to access table entries with an index than with a subscript, even when SEARCH not used

• Both can have values from 1 to number of table elements

131

Modifying Index

• PERFORM … VARYING can modify subscript or index

• SET is only other statement that can modify index

TOSET index-name-1 UP BY integer-1

DOWN BY

Format

132

Sequential Search

Each entry (usually starting with first) checked in order until

• Condition is met

• Table completely searched

133

Sequential Search

Best used when

• Entries not in order by table argument value (not in numerical or alphabetical order)

• Entries can be organized so first values are ones searched for most frequently, minimizing search time

134

Binary Search

• Most efficient type of look-up when table entries in sequence by some table field

• On average, takes fewer comparisons to find match than serial search

• Called binary search because each comparison eliminates half of entries under consideration

135

Binary Search Statement

SEARCH ALL identifier-1[AT END imperative-statement-1]WHEN data-name-1 = identifier-2

literal-1 condition-1 arithmetic-

expression-1imperative-statement-2CONTINUE

[END-SEARCH]

Format (partial)

136

SEARCH ALL Limitations

• Condition in WHEN can test only for equality between table and search argument

• Condition following WHEN may be compound– Only ANDs permitted, not Ors– Each relational test can test only for

equality

137

SEARCH ALL Limitations

• Only one WHEN clause can be used• VARYING option may not be used• Table argument and its index must

appear to left of equal sign– Valid:

When nCustomerNo (X1) = nCustNoIn– Invalid:

When nCustNoIn = nCustomerNo (X1)

138

Key Field

• Must include clause to indicate which table entry serves as key field

• Must specify whether KEY is– ASCENDING KEY - entries in sequence,

increasing in value– DESCENDING KEY - entries in sequence,

decreasing in value

139

BIS 1753

• Random access files

142

Disk File Organization

• File is collection of records

• Three major ways records stored or organized on disk

- Sequential File Organization

- Relative File Organization

- Indexed File Organization

143

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

144

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

145

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 directly146

Relative Files

• Use the value of the unique identifying key value as the record number

• Records may be accessed either randomly or sequentially

147

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

148

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

01 R-Key Pic 9(3).

149

Creating Relative Files

• May not use a standard text editor

• Program must be written to create the relative file

• Relative files are platform-dependent; must create file on the platform on which it will be accessed

150

Reading Relative Files

• (Random access) Move desired key value to RELATIVE KEY data item, then execute READ statement

• (Random access) May get invalid key condition

• (Sequential access) READ

• (Sequential access) May get at end condition

151

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

152

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

153

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

mode154

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

155

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

executed156

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

157

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-Read158

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

159

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

160

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

161

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'

162

START Statement

START file-name-1 IS =

KEY IS > data-name-1IS NOT <IS >=

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

imperative-statement-2][END-START]

Format

163

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

164

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

165

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

166

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

167

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

168

BIS 1753 Introduction to Computer Programming 1. Computer Program A set of instructions that enables...

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