A COBOL application program that merges the content of three input files, sorts them by a key, and writes the sorted data into an output file. This program demonstrates the use of COBOL's sorting capabilities and file handling.

The MERGE3FILES program reads data from three line-sequential input files (FILE1.DAT, FILE2.DAT, and FILE3.DAT), merges their content, sorts them by a specified key, and writes the sorted records into a single output file (OUTPUT.DAT). The sorting is performed using a temporary sort file (SORTFILE.TMP).

• Reads from multiple input files.
• Merges and sorts data by a key field (SORT-KEY).
• Writes the sorted data into an output file.
• Demonstrates the use of COBOL's SORT verb and file handling.

Each input file (FILE1.DAT, FILE2.DAT, FILE3.DAT) contains records with the following structure:

• KEY (10 characters)
• DATA (70 characters)

The output file (OUTPUT.DAT) contains records with the following structure:

• KEY (10 characters)
• DATA (70 characters)

The temporary sort file (SORTFILE.TMP) is used during the sort operation. It has the same structure as the input and output files.

1. Input Phase: The program reads records from the three input files. For each record:

   • The KEY and DATA fields are moved to the sort file (SORTFILE.TMP).

2. Sort Phase: Using the COBOL SORT verb, the program sorts the records in ascending order of the KEY field.

3. Output Phase: The sorted records are retrieved from the sort file and written to the output file (OUTPUT.DAT).

1. Prepare three input files (FILE1.DAT, FILE2.DAT, FILE3.DAT) with the desired records in the specified format.
2. Compile the COBOL program using a COBOL compiler (e.g., cobc for GnuCOBOL):

   cobc -x -free MERGE-SORT-FILES.CBL -o MERGE-SORT-FILES.exe

3. Execute the program:

   ./MERGE-SORT-FILES.exe

4. Check the output in OUTPUT.DAT.

FILE1.DAT

001     Data from File 1 Record 1
003     Data from File 1 Record 2

FILE2.DAT

002     Data from File 2 Record 1
004     Data from File 2 Record 2

FILE3.DAT

005     Data from File 3 Record 1

OUTPUT.DAT

001     Data from File 1 Record 1
002     Data from File 2 Record 1
003     Data from File 1 Record 2
004     Data from File 2 Record 2
005     Data from File 3 Record 1