Cob Basics 6

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File Types

• Sequential

• Indexed

• Relative



Files

• We use the term file, to describe a collection of one or more

occurrences (instances) of a record type (template).• We use the term r ecord, to describe a collection of fields

which record information about an object.

• We use the term field, to describe an item of information

recorded about an object (e.g. StudentName, DateOfBirth).

File

Record

Fields



Select Clause

• The SELECT clause chooses a file in the COBOL program to

be associated with an external data set.

• File name must be identified by an FD or SD entry in the Data

Division.

• A file-name must conform to the rules for a COBOL user-

defined name, must contain at least one alphabetic character,

and must be unique within this program.

• File-Control.

– Select <file name> assign to <device name>

• The ASSIGN clause associates the program's name for a file

with the external name for the actual data file.



Organization Clause

• The ORGANIZATION clause identifies the logicalstructure of the file.

• The logical structure is established at the time the file

is created and cannot subsequently be changed.

• ORGANIZATION IS SEQUENTIAL (format 1)

– A predecessor-successor relationship among the

records in the file is established by the order inwhich records are placed in the file when it iscreated or extended.



Organization Clause

• ORGANIZATION IS INDEXED (format 2)

– The position of each logical record in the file is determined

by indexes created with the file and maintained by the

system.

– The indexes are based on embedded keys within the file's

records.

• ORGANIZATION IS RELATIVE (format 3)

–

The position of each logical record in the file is determined by its relative record number.

• If you omit the ORGANIZATION clause, the compiler

assumes ORGANIZATION IS SEQUENTIAL.



Access Mode Clause

• Sequential-access mode

– Allows reading and writing records of a file in a serialmanner; the order of reference is implicitly determined bythe position of a record in the file.

• Random-access mode – Allows reading and writing records in a programmer-

specified manner; the control of successive references tothe file is expressed by specifically defined keys supplied

by the user.

• Dynamic-access mode

– Allows the specific input-output statement to determine theaccess mode. Therefore, records can be processedsequentially and/or randomly.



Access Mode Clause

• File organization is the permanent logical structure of the file.

You tell the computer how to retrieve records from the file by

specifying the access mode

• Sequential files

– Files with sequential organization can be accessed only

sequentially. The sequence in which records are accessed is

the order in which the records were originally written.

• Indexed files

– All three access modes are allowed.

• Relative files

– All three access modes are allowed.



Other Clauses

• The RECORD KEY clause specifies the data item within therecord that is the prime RECORD KEY for an indexed file.

• The values contained in the prime RECORD KEY data itemmust be unique among records in the file.

• The ALTERNATE RECORD KEY clause specifies a dataitem within the record that provides an alternative path to thedata in an indexed file.

• The RELATIVE KEY clause identifies a data-name thatspecifies the relative record number for a specific logicalrecord within a relative file.

• The FILE STATUS clause monitors the execution of eachinput-output operation for the file.

• When the FILE STATUS clause is specified, the systemmoves a value into the status key data item after each input-output operation that explicitly or implicitly refers to this file.

• The value indicates the status of execution of the statement.



Other Clauses

• The DATA RECORDS clause names of data records

associated with this file.

• The RECORD CONTAINS clause indicates the size of each

record.

RECORD CONTAINS integer-1 CHARACTERS

The BLOCK CONTAINS clause is included in the File

Description entry only for files on magnetic media

A group of logical records is included within one block tomaximize the efficient use of a disk area.

BLOCK CONTAINS integer-1 RECORDS



Close Verb

• Closes the file opened for processing

• Close <file name>

• You must ensure that, before terminating, your program closes

all the files it has opened. Failure to do so may result in some

data not being written to the file or users being prevented from

accessing the file.



Read Verb

• The READ copies a record occurrence/instance from the file

and places it in the record buffer.

• When the READ statement is executed, the associated file

must be open in INPUT or I-O mode.

• Read <file name> At end <statement>

• When the READ attempts to read a record from the file and

encounters the end of file marker, the AT END is triggered

and the StatementBlock following the AT END is executed



Read Verb

• Execution of a READ statement on sequential files retrievesthe next logical record from the file.

• Execution of a READ statement on indexed files causes thevalue of the key of reference to be compared with the value of

the corresponding key data item in the file records, until thefirst record having an equal value is found.

• The file position indicator is positioned to this record, which isthen made available.

• Execution of a READ statement on relative files sets the file position indicator pointer to the record whose relative recordnumber is contained in the RELATIVE KEY data item, andmakes that record available.



Write Verb

• The WRITE statement releases a logical record for an outputor input/output file.

• When the WRITE statement is executed:

– The associated sequential file must be open in OUTPUT or

EXTEND mode. – The associated indexed or relative file must be open in

OUTPUT, I-O, or EXTEND mode.

• To WRITE data to a file we must move the data to the record

buffer (declared in the FD entry) and then WRITE the contentsof record buffer to the file.

• Write <Record Name>



File Status Codes

• 00 Successful completion

• 10 AT END Condition

• 30 Permanent error condition

•

34 Boundary Violation

Cob Basics 6

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