🔁 Recursion Debugging Flow

🔁 Recursion Debugging (Simple Explanation)

Recursion debugging means finding and fixing errors in a function that calls itself.

🧠 1. What happens in recursion?

Every recursive function must have:

1. ✅ Base Case → stopping condition
2. 🔁 Recursive Call → function calling itself

If something is wrong → program may:

• ❌ Run forever (infinite recursion)
• ❌ Give wrong output
• ❌ Crash (stack overflow)

🔍 2. Common Recursion Errors

❌ 1. Missing Base Case

def fact(n):
    return n * fact(n-1)

👉 Problem: No stopping condition → infinite loop

❌ 2. Wrong Base Case

def fact(n):
    if n == 0:
        return 0   # ❌ wrong
    return n * fact(n-1)

👉 Factorial of 0 should be 1, not 0

❌ 3. Wrong Recursive Call

def fact(n):
    if n == 1:
        return 1
    return n * fact(n+1)   # ❌ wrong direction

👉 It should reduce n, not increase

❌ 4. Stack Overflow (Too deep recursion)

def loop(n):
    return loop(n+1)

👉 Never stops → crash

🧠 Code Understanding

def fact(i, a):
    if i <= 5:
        a = a * i
        return fact(i + 1, a)
    return a

print(fact(1, 1))

Meaning:
i → current number
a → accumulated result (answer store panra variable)

🔁 Step-by-Step Flow
📍 Initial Call
fact(1, 1)

🔽 Call Phase (Going Down)

i=1, a=1 → a = 1*1 = 1 → fact(2, 1)
i=2, a=1 → a = 1*2 = 2 → fact(3, 2)
i=3, a=2 → a = 2*3 = 6 → fact(4, 6)
i=4, a=6 → a = 6*4 = 24 → fact(5, 24)
i=5, a=24 → a = 24*5 = 120 → fact(6, 120)

🔼 Return Phase (Coming Back)

👉 Important:
This is tail recursion, so no pending calculations

fact(6,120) → 120
fact(5,24) → 120
fact(4,6) → 120
fact(3,2) → 120
fact(2,1) → 120
fact(1,1) → 120

🎯 Final Output
120

📊 Simple Flow Diagram

embed fact(1,1)
↓
fact(2,1)
↓
fact(3,2)
↓
fact(4,6)
↓
fact(5,24)
↓
fact(6,120) → return 120