Compaq COBOL AAQ2G1FTK, Руководство пользователя

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Processing Files and Records
6.1 Defining Files and Records
Table 6–1 (Cont.) Compaq COBOL File Organizations—Advantages and Disadvantages
File
Organizations
Advantages Disadvantages
Provides the most flexible record
format
Allows READ/WRITE sharing
Allows data to be stored on
many types of media, in a
device-independent manner
Allows easy file extension
Line Sequential Most efficient storage format Allows sequential access only
Compatible with text editors Used for printable characters only
Open Mode I/O is not allowed
Relative Allows sequential, random, and
dynamic access Allows data to be stored on disk only
Provides random record deletion
and insertion Requires that record cells be the same size
Allows READ/WRITE sharing
Indexed Allows sequential, random, and
dynamic access Allows data to be stored on disk only
Allows random record deletion
and insertion on the basis of a
user-supplied key Requires more disk space
Allows READ/WRITE sharing Uses more memory to process records
Allows variable-length records to
change length on update Generally requires multiple disk accesses to
randomly process a record
Allows easy file extension
Sequential File Organization
Sequential input/output, in which records are written and read in sequence, is
the simplest and most common form of I/O. It can be performed on all I/O devices,
including magnetic tape, disk, terminals, and line printers.
Sequential files consist of records that are arranged in the order in which they
were written to the file. Figure 6–1 illustrates sequential file organization.
Figure 6–1 Sequential File Organization
RECORD
3
End of file
RECORD
(n−1)
...
ZK−6055−GE
RECORD
1
RECORD
2
Beginning of file
RECORD
n
Processing Files and Records 6–3