Debugging

1. What is Debugging?
Debugging is the process of identifying, analyzing, and fixing errors (bugs) in a program.

2. Why Debugging is Important:
Debugging helps to:

• Fix runtime errors
• Identify logical mistakes
• Prevent infinite loops
• Solve recursion errors
• Improve program performance

Without debugging, even small mistakes can cause the entire application to fail.

3. Debugging Rule (Recursion):

Always check:

• Is there a base condition?
• Is the recursive call correct?
• Is the value changing in each call?

If these three are missing, recursion will fail. ⚠️

4. Debugging Recursion Example:

Recursion is a function calling itself until a condition is met. If the stopping condition is missing, the program will run infinitely and crash.

Incorrect Program:

public class DebugExample {

    static int fact(int n) {
        int factorial = 0; **// mistake**

        if (n == 1)
            return factorial;

        factorial = n * fact(n - 1);
        return factorial;
    }

    public static void main(String[] args) {
        int number = 5;
        int result = fact(number);

        System.out.println(result);
    }
}

1. Check the Output

Program output = 0

2. Compare with Expected Result
Expected = 120
Actual = 0 → Output is wrong ❌

3. Find the Mistake

int factorial = 0; // wrong

In factorial calculation, multiplication with 0 always gives 0.

Example:

5 × 0 = 0
4 × 0 = 0
3 × 0 = 0

Also in base condition:

if (n == 1)
return factorial;

When n == 1, the function returns 0, and the entire recursion becomes 0.

Correct Program:

public class DebugExample {

    static int fact(int n) {
        if (n == 1)
            return 1;

        return n * fact(n - 1);
    }

    public static void main(String[] args) {
        int number = 5;
        int result = fact(number);

        System.out.println(result);
    }
}

Debugging Process (Correct Version):
1. Check the Input
number = 5

Program calls:

fact(5)

2. Check Base Condition
if (n == 1)
return 1;

This is the Base Condition
When n = 1, recursion stops.

Base condition exists ✅

3. Check Recursive Call
return n * fact(n - 1);

Function calling itself:

fact(n - 1)

Recursive call correct ✅

4. Check Value Changing
n - 1

Value decreases every time:

5 → 4 → 3 → 2 → 1

Value changing correctly ✅

5. Step-by-Step Execution

Program Flow:

fact(5)
= 5 * fact(4)

fact(4)
= 4 * fact(3)

fact(3)
= 3 * fact(2)

fact(2)
= 2 * fact(1)

fact(1)
= 1 (Base condition reached)

6. Backtracking (Return Values)

Now values return back:

fact(2) = 2 * 1 = 2
fact(3) = 3 * 2 = 6
fact(4) = 4 * 6 = 24
fact(5) = 5 * 24 = 120

7. Final Output
120