COBOL Tutorial

COBOL DEBUG-SUB-2 Clause - Quick Reference

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Overview

The DEBUG-SUB-2 clause is used to define debugging data items that track the second subscript value being used in two-dimensional array or table operations during program debugging. This clause is part of COBOL's debugging facility and provides essential second subscript tracking for matrix debugging and analysis.

Purpose and Usage

Second subscript tracking - Track second subscript in 2D array operations
Matrix access monitoring - Monitor matrix element access
Column bounds checking - Identify matrix column bounds issues
Matrix pattern analysis - Analyze matrix access patterns
Development support - Aid in matrix debugging

Second Subscript Tracking Concept

Matrix Access: MATRIX[1,1] → MATRIX[1,2] → MATRIX[2,1] → ...

DEBUG-SUB-1: [Row Index]

DEBUG-SUB-2: [Column Index]

Tracks second subscript (column) value in 2D array operations

DEBUG-SUB-2 provides real-time tracking of the second subscript value in two-dimensional array operations.

Syntax

The DEBUG-SUB-2 clause follows specific syntax patterns and is typically used at the 77 level for independent debugging data items.

Basic Syntax

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* Basic DEBUG-SUB-2 clause syntax
77 debug-sub-2-item DEBUG-SUB-2.
 
* Examples
77 DEBUG-SECOND-SUB DEBUG-SUB-2.
77 DEBUG-SUB-2-VALUE DEBUG-SUB-2.
77 DEBUG-MATRIX-SUB-2 DEBUG-SUB-2.
 
* Complete example with data description
77 DEBUG-SECOND-SUB DEBUG-SUB-2 PIC 9(5).
77 DEBUG-SUB-2-VALUE DEBUG-SUB-2 PIC 9(6).
77 DEBUG-MATRIX-SUB-2 DEBUG-SUB-2 COMP.
 
* Usage in program
PROCEDURE DIVISION.
    * Display current second subscript value
    DISPLAY "Second subscript: " DEBUG-SECOND-SUB
    
    * Store subscript for later use
    MOVE DEBUG-SECOND-SUB TO SAVED-SUBSCRIPT-2
    
    * Check if at specific subscript
    IF DEBUG-SECOND-SUB = 100
        DISPLAY "Accessing column 100"
    END-IF

DEBUG-SUB-2 is typically used at the 77 level for independent debugging items.

Data Type Considerations

Data Type	Size	Use Case
PIC 9(5)	5 digits	Matrices up to 99,999 columns
PIC 9(6)	6 digits	Large matrices up to 999,999 columns
COMP	Binary	Efficient storage and processing
COMP-3	Packed decimal	Mainframe compatibility

Debugging Mode Requirements

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* Enabling debugging mode for DEBUG-SUB-2
* Compiler option
* Example: cobc -fdebug program.cob
 
* Runtime debugging activation
* Set environment variable
* Example: COBOL_DEBUG=1 ./program
 
* Program-level debugging declaration
PROGRAM-ID. DEBUG-PROGRAM.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
SPECIAL-NAMES.
    DEBUGGING MODE IS ON.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
77 DEBUG-SECOND-SUB DEBUG-SUB-2 PIC 9(5).

DEBUG-SUB-2 requires debugging mode to be enabled for second subscript tracking functionality.

Common Use Cases

DEBUG-SUB-2 is commonly used in specific debugging scenarios where second subscript tracking is needed for two-dimensional array analysis.

Matrix Access Tracking

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* Tracking matrix access patterns
77 DEBUG-SECOND-SUB DEBUG-SUB-2 PIC 9(5).
 
PROCEDURE DIVISION.
    DISPLAY "Starting matrix access at column: " DEBUG-SECOND-SUB
    
    * Process matrix elements
    PERFORM VARYING I FROM 1 BY 1 UNTIL I > 10
        PERFORM VARYING J FROM 1 BY 1 UNTIL J > 10
            * Access matrix element
            MOVE MATRIX(I, J) TO TEMP-VALUE
            DISPLAY "Accessing element at column: " DEBUG-SECOND-SUB
        END-PERFORM
    END-PERFORM
    
    DISPLAY "Matrix processing completed at column: " DEBUG-SECOND-SUB

Track which matrix columns are being accessed.

Matrix Bounds Monitoring

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* Monitoring matrix bounds
77 DEBUG-SECOND-SUB DEBUG-SUB-2 PIC 9(5).
77 MATRIX-COLUMNS PIC 9(5) VALUE 100.
 
PROCEDURE DIVISION.
    * Check for column bounds violations
    IF DEBUG-SECOND-SUB > MATRIX-COLUMNS
        DISPLAY "WARNING: Matrix column bounds violation at: " DEBUG-SECOND-SUB
        DISPLAY "Matrix columns is: " MATRIX-COLUMNS
    END-IF
    
    * Monitor specific column ranges
    IF DEBUG-SECOND-SUB >= 90 AND DEBUG-SECOND-SUB <= 100
        DISPLAY "Accessing high-index columns at: " DEBUG-SECOND-SUB
        * Add extra validation here
    END-IF
    
    * Track zero-based access
    IF DEBUG-SECOND-SUB = 0
        DISPLAY "WARNING: Zero-based matrix access detected"
    END-IF

Monitor matrix column bounds and identify potential violations.

Conditional Matrix Debugging

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* Conditional debugging based on second subscripts
77 DEBUG-SECOND-SUB DEBUG-SUB-2 PIC 9(5).
 
PROCEDURE DIVISION.
    * Only show debugging info for specific column ranges
    IF DEBUG-SECOND-SUB >= 1 AND DEBUG-SECOND-SUB <= 10
        DISPLAY "Debug info for low columns: " DEBUG-SECOND-SUB
        DISPLAY "Matrix value = " MATRIX(DEBUG-SUB-1, DEBUG-SECOND-SUB)
    END-IF
    
    * Break execution at specific column
    IF DEBUG-SECOND-SUB = 50
        DISPLAY "Breakpoint reached at column 50"
        * Add breakpoint logic here
    END-IF
    
    * Track access frequency
    IF DEBUG-SECOND-SUB = 25
        ADD 1 TO COLUMN-ACCESS-COUNT
        DISPLAY "Column 25 accessed " COLUMN-ACCESS-COUNT " times"
    END-IF

Use second subscripts to implement conditional debugging logic.

Best Practices and Tips

Following these best practices ensures effective use of the DEBUG-SUB-2 clause for matrix debugging and second subscript tracking.

DEBUG-SUB-2 Design Principles

Choose appropriate size - Select data type that accommodates your largest matrix
Use meaningful names - Choose descriptive names for debugging subscript items
Minimize overhead - Don\'t overuse subscript tracking to avoid performance impact
Document matrix dimensions - Document expected matrix bounds for validation
Consider production impact - Remember that subscript tracking can affect performance
Test debugging functionality - Verify that subscript tracking works correctly

Common Pitfalls to Avoid

Pitfall	Problem	Solution
Insufficient field size	Subscript values overflow	Choose appropriate PIC size for largest matrix
Overuse in production	Performance degradation	Use conditional compilation or separate versions
Not enabling debug mode	No subscript tracking information	Ensure debugging mode is properly enabled
Poor naming conventions	Confusing debugging output	Use clear, descriptive names
Not documenting usage	Difficult to understand subscript tracking	Document all subscript tracking items and their purpose

Performance Considerations

Subscript tracking overhead - DEBUG-SUB-2 adds runtime overhead when active
Memory usage - Subscript tracking items consume additional memory
I/O impact - Subscript tracking output may affect I/O performance
Production considerations - Disable subscript tracking in production environments
Selective tracking - Use conditional logic for selective subscript tracking
Debugging levels - Consider different levels of subscript tracking detail

When to Use DEBUG-SUB-2

Use Case	DEBUG-SUB-2 Suitability	Reasoning
Matrix debugging	Excellent	Essential for matrix second subscript tracking
Complex matrix operations	Good	Helps track complex matrix access patterns
Column bounds checking	Good	Provides matrix column bounds validation
Production monitoring	Poor	Performance impact outweighs benefits
Simple programs	Poor	Unnecessary overhead for simple logic

DEBUG-SUB-2 Clause Quick Reference

Usage	Syntax	Example
Basic second subscript tracking	77 debug-sub-2-item DEBUG-SUB-2	77 DEBUG-SUB-2 DEBUG-SUB-2
With data description	77 debug-sub-2-item DEBUG-SUB-2 PIC...	77 DEBUG-SUB-2 DEBUG-SUB-2 PIC 9(5)
Multiple subscript trackers	Multiple 77 items with DEBUG-SUB-2	77 DEBUG1 DEBUG-SUB-2 77 DEBUG2 DEBUG-SUB-2
Numeric subscript tracking	77 debug-sub-2-item DEBUG-SUB-2 PIC 9(n)	77 DEBUG-SUB-2 DEBUG-SUB-2 PIC 9(6)
Binary subscript tracking	77 debug-sub-2-item DEBUG-SUB-2 COMP	77 DEBUG-SUB-2 DEBUG-SUB-2 COMP

Test Your Knowledge

1. What is the primary purpose of the DEBUG-SUB-2 clause in COBOL?

To define data types
To track the second subscript value during debugging
To control file operations
To perform calculations

2. At what level is the DEBUG-SUB-2 clause typically used?

01 level only
88 level only
77 level only
Any level except 01

3. What information does DEBUG-SUB-2 provide during debugging?

Only variable names
Only program flow
Second subscript value in two-dimensional array operations
Only error messages

4. When is DEBUG-SUB-2 most useful?

During production runs
During program development and debugging with two-dimensional arrays
Only during compilation
Only during program termination

5. How does DEBUG-SUB-2 relate to two-dimensional array debugging?

They are completely independent
DEBUG-SUB-2 provides essential second subscript tracking for 2D arrays
DEBUG-SUB-2 overrides other debugging features
They serve the same purpose

COBOL Tutorial

COBOL DEBUG-SUB-2 Clause - Quick Reference

Overview

Purpose and Usage

Second Subscript Tracking Concept

Syntax

Basic Syntax

Data Type Considerations

Debugging Mode Requirements

Common Use Cases

Matrix Access Tracking

Matrix Bounds Monitoring

Conditional Matrix Debugging

Best Practices and Tips

DEBUG-SUB-2 Design Principles

Common Pitfalls to Avoid

Performance Considerations

When to Use DEBUG-SUB-2

DEBUG-SUB-2 Clause Quick Reference

Test Your Knowledge

Frequently Asked Questions

Related Concepts

DEBUG-SUB-1 Clause

DEBUG-SUB-3 Clause

OCCURS Clause

Table Handling

Data Types

Related Pages

COBOL Tutorial

COBOL DEBUG-SUB-2 Clause - Quick Reference

Overview

Purpose and Usage

Second Subscript Tracking Concept

Syntax

Basic Syntax

Data Type Considerations

Debugging Mode Requirements

Common Use Cases

Matrix Access Tracking

Matrix Bounds Monitoring

Conditional Matrix Debugging

Best Practices and Tips

DEBUG-SUB-2 Design Principles

Common Pitfalls to Avoid

Performance Considerations

When to Use DEBUG-SUB-2

DEBUG-SUB-2 Clause Quick Reference

Test Your Knowledge

Frequently Asked Questions

What is the difference between DEBUG-SUB-1 and DEBUG-SUB-2?

Can DEBUG-SUB-2 be used in production programs?

What types of two-dimensional array operations can DEBUG-SUB-2 track?

What data type should be used for DEBUG-SUB-2?

Are there performance implications when using DEBUG-SUB-2?

Related Concepts

DEBUG-SUB-1 Clause

DEBUG-SUB-3 Clause

OCCURS Clause

Table Handling

Data Types

Related Pages