Learn to Debug Your Code
Effectively and Systematically

Introduction to Debugging

Debugging is the process of identifying, analyzing, and removing bugs or errors in your program. A bug could be anything from a syntax error to an unexpected logical flaw that causes incorrect behavior or results.

Types of Bugs You Might Encounter

• Syntax Errors: Mistakes in code structure or grammar
• Runtime Errors: Errors that crash the program during execution
• Logical Errors: Program runs but returns wrong results

Step-by-Step Debugging Techniques

1. Use Print/Log Statements

One of the simplest ways to debug is by printing intermediate values or messages to track the program’s flow and data state.

function calculateTotal(price, taxRate)
    total = price + (price * taxRate)
    print("Price:", price)
    print("Tax Rate:", taxRate)
    print("Total:", total)
    return total
end function

Output:

Price: 100
Tax Rate: 0.05
Total: 105.0

Question: What should you print when a program behaves unexpectedly?

Answer: Start by printing values of inputs, outputs, and intermediate variables that influence your logic path.

2. Trace the Execution Path

If your program has conditions and loops, follow the execution flow to understand which blocks are being executed and why.

function checkEligibility(age)
    if age > 18
        print("User is eligible")
    else
        print("User is not eligible")
    end if
end function

Tip: Add a print before each decision point to confirm the path being taken.

3. Simplify the Problem

If your codebase is large, try to isolate the problematic part. Create a smaller version (minimal reproducible example) to test your assumptions.

# Full function is long and hard to debug. Let's isolate part of it.

function sumFirstN(n)
    sum = 0
    for i from 1 to n
        sum = sum + i
    end for
    return sum
end function

Question: Why is isolating code helpful?

Answer: It reduces distractions and makes it easier to pinpoint errors in a focused section.

4. Use Assertions

Assertions help verify assumptions in your code. If the condition fails, the program stops and alerts you.

function divide(a, b)
    assert(b != 0, "Divider must not be zero")
    return a / b
end function

Output:

Assertion failed: Divider must not be zero

5. Walk Through the Code Manually

Read your code line by line and simulate it on paper or a whiteboard. Note down variable values at each step.

6. Rubber Duck Debugging

Explain your code to someone else—or to a rubber duck! Speaking your logic aloud often reveals overlooked errors.

Tips for Debugging

• Always read the error message carefully—it usually tells you where the problem is.
• Work from the top of the stack trace downward.
• Check for common issues: typos, off-by-one errors, missing return statements.
• Test your code incrementally after every small change.

Summary

Debugging is a skill that improves with practice. It’s not just about fixing errors, but about understanding your code better. Use a combination of techniques—logging, tracing, assertions, and isolation—to solve problems efficiently.

In the next lesson, we’ll explore Custom Exceptions and how to create error messages that make debugging even easier.