COBOL-文件处理

COBOL is one of the oldest programming languages still in use today, and it's still a popular choice for mainframe programming. One of the most important features of COBOL is its ability to handle large datasets, which makes it ideal for file processing. In this article, we'll take a closer look at how COBOL handles file processing.

File Organization

COBOL supports several file organization methods, including sequential, indexed, and relative. Each method has its own unique features and benefits.

Sequential Files

Sequential files are the simplest form of file organization. Data is stored in the order in which it is entered into the file, and records are accessed one at a time in the order in which they were added. This method is ideal for processing large volumes of data that don't need to be accessed in a specific order.

Here's an example of how to open and read a sequential file in COBOL:

IDENTIFICATION DIVISION.
PROGRAM-ID. READSEQ.

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT MYFILE ASSIGN TO 'MYFILE.DAT'
           ORGANIZATION IS SEQUENTIAL.

DATA DIVISION.
FILE SECTION.
FD MYFILE.
01 FILE-RECORD.
   05 FIELD1     PIC X(20).
   05 FIELD2     PIC 9(5).

PROCEDURE DIVISION.
OPEN INPUT MYFILE.
READ MYFILE INTO FILE-RECORD.
DISPLAY FIELD1 FIELD2.
CLOSE MYFILE.
STOP RUN.

Indexed Files

Indexed files are similar to sequential files, but they include an index that allows records to be accessed in a specific order. This method is ideal for processing data that needs to be sorted or searched.

Here's an example of how to use an indexed file in COBOL:

IDENTIFICATION DIVISION.
PROGRAM-ID. READIDX.

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT MYFILE ASSIGN TO 'MYFILE.DAT'
           ORGANIZATION IS INDEXED.

DATA DIVISION.
FILE SECTION.
FD MYFILE.
01 FILE-RECORD.
   05 FIELD1     PIC X(20).
   05 FIELD2     PIC 9(5).

PROCEDURE DIVISION.
OPEN INPUT MYFILE.
READ MYFILE INTO FILE-RECORD KEY IS GREATER THAN 'SEARCH'.
DISPLAY FIELD1 FIELD2.
CLOSE MYFILE.
STOP RUN.

Relative Files

Relative files are similar to indexed files, but they allow records to be accessed based on their physical location in the file rather than their key value. This method is ideal for processing data that requires random access.

Here's an example of how to use a relative file in COBOL:

IDENTIFICATION DIVISION.
PROGRAM-ID. READREL.

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT MYFILE ASSIGN TO 'MYFILE.DAT'
           ORGANIZATION IS RELATIVE
           ACCESS MODE IS RANDOM
           RECORD KEY IS FILE-RECORD-KEY.

DATA DIVISION.
FILE SECTION.
FD MYFILE.
01 FILE-RECORD.
   05 FIELD1          PIC X(20).
   05 FIELD2          PIC 9(5).
FD MYKEYS.
01 KEY-RECORD.
   05 FILE-RECORD-KEY PIC 9(5).

PROCEDURE DIVISION.
OPEN I-O MYFILE.
READ MYFILE INTO FILE-RECORD KEY IS 1.
DISPLAY FIELD1 FIELD2.
CLOSE MYFILE.
STOP RUN.

File Access Modes

COBOL supports several file access modes, including input, output, and update. Each mode provides different levels of access to the file.

Input Mode

Input mode allows a program to read data from a file, but not modify it. This mode is used when a program needs to read data from an existing file without changing it.

Output Mode

Output mode allows a program to write data to a file, but not read from it. This mode is used when a program needs to create a new file or overwrite an existing file.

Update Mode

Update mode allows a program to both read and modify data in a file. This mode is used when a program needs to read data from an existing file, modify it, and write the modified data back to the file.

Conclusion

COBOL's file processing capabilities make it an ideal language for working with large datasets. Whether you're processing sequential data, searching through indexed data, or accessing random data in a relative file, COBOL provides a powerful set of tools for working with files. By understanding how to use COBOL's file processing features, you can more effectively work with data in your mainframe applications.