Back to first page...
SORTING FILES IN COBOL (RM-Cobol)
Line sequential and Sequential files are used very frequently
in data processing applications.
The records in these files are usually need to be put in ascending or descending order
for proper and easy handling. (Remember that searching records in an ordered file ( a file
sorted in respect
to the field which is the search key; you can stop reading records when you find a record which has
the key field having greater value than the value you are searching for).
Sorting a set of data which will fit into arrays (therefore fitting into the memory) is quite easy.
(Remember the Internal Sort techniques you have learned in previous courses). Sorting
large files which will not fit into arrays in memory, on the other hand,
is not easy. This requires special algorithms to be implemented and generally called
"EXTERNAL SORT TECHNIQUES".
Many COBOL compilers provide facilities to solve this "Exteral Sort" problem. RM-COBOL, for instance, has
a very powerful SORT statement which will let the user SORT and MERGE sequential files very easily.
SORTING does not need much explanation here. The MERGE process however, needs a
little bit explanation.
MERGING DATA is simply combining two or more SORTED sequential files together into a single file so that
the resulting file is also sorted. We shall not cover the MERGE facility of the COBOL SORT
statement in this course.
The RM-COBOL SORT Statement
The general syntax of the RM-COBOL SORT statement is :
SORT sort-work-file ON {ASCENDING,DESCENDING} KEY data-name1, data-name2, ...
ON {ASCENDING,DESCENDING} KEY data-name1, data-name2, ...
USING file-to-be-sorted
GIVING sorted-file
In order to use this SORT statement, you will have to declare 3 files;
- The WORK FILE for the SORTING Process
- The file to be sorted (input file)
- The file which will contain the sorted records (output file)
The WORK FILE is not an actual file; it is a special declarative file structure with which
you tell the compiler that you will perform an external sort on a file and also indicate the fields
on which you shall set the sort criteria. The concept will become clearer with an example :
Suppose you have sequential file with personnel records (PERSONEL.DAT) and you want to sort this file
on the persons' names and surnames. In order to use the SORT statement, you must declare a SORT WORK file
which will declare the structure of the records to be sorted and indicate the lenghts and positions of the
fields which contain the names and surnames:
Suppose that the structure of the PERSONEL.DAT file is :
01 PERS-REC.
02 ID-NO PIC X(8).
02 NAME PIC X(16).
02 SURNAME PIC X(16).
02 GENDER PIC X.
02 DEPT-CODE PIC X.
02 NCHILDREN PIC 99.
02 HOME-ADR1 PIC X(25).
02 HOME-ADR2 PIC X(25).
02 HOME-ADR3 PIC X(25).
02 HOME-TEL PIC X(12).
02 EMPLOYMENT-DATE.
03 R-DAY PIC 99.
03 R-MONTH PIC 99.
03 R-YEAR PIC 9999.
02 LEAVE-DATE.
03 R-DAY PIC 99.
03 R-MONTH PIC 99.
03 R-YEAR PIC 9999.
02 LEAVE-REASON PIC X.
The corresponding SORT WORK file declaration might look something like :
SD SORT-WORK-FILE.
01 PERS-REC.
02 ID-NO PIC X(8).
02 NAME PIC X(16).
02 SURNAME PIC X(16).
02 FILLER PIC X(107).
This declaration will enable the user to issue a SORT statement using the
NAME and SURNAME fields; into either ascending or
descending order.Please note that a 108 byte filler is used in the
SORT-WORK-FILE record description so that the work file's record length matches the
record length of the PERSONEL.DAT file.
| Please also note the "SD" indicator used in place of "FD" in FILE SECTION. |
You can perform sort on more than one field at time; and also you can sort into ascending order
(increasing) on one field, and into descending order (decreasing) on another. For example, while sorting
the PERSONEL.DAT file, you can declare the NAME field to be the primary sort field and the SURNAME to be
the secondary sort field; so that records with identical names will be sorted into surnames among themselves.
(Just like the entries in a telephone directory).
Referring to our PERSONEL.DAT example, typical file declarations and a SORT statement would look like :
In the ENVIRONMENT DIVISION :
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT SORT-FILE ASSIGN TO DISK "SORTWORK".
SELECT UNSORTED-PERSON ASSIGN TO DISK "PERSONEL.DAT"
ORGANIZATION IS LINE SEQUENTIAL.
SELECT SORTED-PERSON ASSIGN TO DISK "SPERSONL.DAT"
ORGANIZATION IS LINE SEQUENTIAL.
In the DATA DIVISION :
FILE SECTION.
SD SORT-FILE.
01 SORT-RECORD.
02 FILLER PIC X(8).
02 S-NAME PIC X(16).
02 S-SURNAME PIC X(16).
02 FILLER PIC X(108).
FD UNSORTED-PERSON.
01 FILLER PIC X(148).
FD SORTED-PERSON.
01 FILLER PIC X(148).
In the PROCEDURE DIVISION :
SORT SORT-FILE
ON ASCENDING KEY S-NAME S-SURNAME
ON DESCENDING S-EMP-DATE
USING UNSORTED-PERSON
GIVING SORTED-PERSON.
Notes :
- The lengths of records in the work, input and output files should match.
- You do not need to declare the details of input and output file records if you do
not need these details in your program.
- You should not OPEN or CLOSE the work, input and output files before, after
or during the SORT operation.
- You can specify more than one sort field and the fields may have different
ordering (ascending/descending).
A complete RM-COBOL program which sorts the PERSONEL.DAT file, creating the sorted
file SPERSONL.DAT might look like :
Please study the SORT statement and its relevant file declarations carefully!
IDENTIFICATION DIVISION.
PROGRAM-ID. "SORT DEMO".
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
SOURCE-COMPUTER. RMCOBOL-85.
OBJECT-COMPUTER. RMCOBOL-85.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT SORT-FILE ASSIGN TO DISK "SORTWORK".
SELECT UNSORTED-PERSON ASSIGN TO DISK "PERSONEL.DAT"
ORGANIZATION IS LINE SEQUENTIAL.
SELECT SORTED-PERSON ASSIGN TO DISK "SPERSONL.DAT"
ORGANIZATION IS LINE SEQUENTIAL.
*
DATA DIVISION.
FILE SECTION.
SD SORT-FILE.
01 SORT-RECORD.
02 S-ID-NO PIC X(8).
02 S-NAME PIC X(16).
02 S-SURNAME PIC X(16).
02 S-GENDER PIC X.
02 S-DEPT-CODE PIC 9.
02 FILLER PIC X(89).
02 S-EMP-DATE.
03 S-EMP-DATE-DD PIC 99.
03 S-EMP-DATE-MM PIC 99.
03 S-EMP-DATE-YY PIC 9999.
02 S-LEAVE-DATE.
03 S-LEAVE-DATE-DD PIC 99.
03 S-LEAVE-DATE-MM PIC 99.
03 S-LEAVE-DATE-YY PIC 9999.
02 S-LEAVE-REASON PIC X.
FD UNSORTED-PERSON.
01 FILLER PIC X(148).
FD SORTED-PERSON.
01 FILLER PIC X(148).
*
PROCEDURE DIVISION.
MAIN-PGM.
SORT SORT-FILE
ON ASCENDING KEY S-NAME S-SURNAME
ON DESCENDING S-EMP-DATE
USING UNSORTED-PERSON
GIVING SORTED-PERSON.
STOP RUN.
Back to first page...