Recursion Concept and Call Stack Visualization in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the recursive function call itself with a smaller number.

DSA Python

def countdown(n):
    if n <= 0:
        print("Blastoff!")
    else:
        print(n)
        countdown([1])

Drag options to blanks, or click blank then click option'

An - 1

Bn + 1

Cn * 2

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n does not change the argument.

The function calls itself with n - 1 to count down towards zero.

2fill in blank

medium

Complete the code to return the factorial of n using recursion.

DSA Python

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial([1])

Drag options to blanks, or click blank then click option'

An + 1

B0

Cn - 1

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n returns the same value repeatedly.

The factorial of n is n times factorial of n minus 1.

3fill in blank

hard

Fix the error in the recursive Fibonacci function call.

DSA Python

def fibonacci(n):
    if n <= 1:
        return n
    else:
        return fibonacci([1]) + fibonacci(n - 2)

Drag options to blanks, or click blank then click option'

An - 1

Bn + 1

Cn

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n or 2 as argument is incorrect.

The Fibonacci function calls itself with n - 1 and n - 2 to sum the two previous numbers.

4fill in blank

hard

Fill both blanks to complete the recursive sum of a list.

DSA Python

def recursive_sum(lst):
    if not lst:
        return 0
    else:
        return lst[[1]] + recursive_sum(lst[[2]:])

Drag options to blanks, or click blank then click option'

A0

B1

C-1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 for the first element is wrong.

Using -1 or 2 causes wrong slicing.

The function adds the first element (index 0) to the sum of the rest of the list starting from index 1.

5fill in blank

hard

Fill all three blanks to create a recursive function that reverses a string.

DSA Python

def reverse_string(s):
    if len(s) == [1]:
        return s
    else:
        return s[[2]] + reverse_string(s[[3]:])

Drag options to blanks, or click blank then click option'

A0

B1

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as base case length causes infinite recursion.

Using 1 as index does not get last character.

The base case is when the string length is 1. The function takes the last character (index -1) and adds the reverse of the rest (from start to before last).

Practice

1. What does recursion mean in programming?
def recurse():
recurse()


easy

A. A function calling itself to solve smaller parts of a problem

B. A loop that runs forever

C. A function that never ends

D. A way to store data in a list

Solution

1. Step 1: Understand recursion definition

   Recursion is when a function calls itself to break down a problem into smaller parts.

2. Step 2: Match with options

   A function calling itself to solve smaller parts of a problem correctly describes recursion as self-calling function for smaller problems.

3. Final Answer:

   A function calling itself to solve smaller parts of a problem -> Option A

4. Quick Check:

   Recursion = self-calling function [OK]

Hint: Recursion means function calls itself to solve smaller parts [OK]

Common Mistakes:

• Confusing recursion with infinite loops
• Thinking recursion is just a loop
• Believing recursion stores data like lists

2. Which of the following is the correct syntax to define a recursive function in Python?

easy

A. def recurse(): recurse()

B. def recurse(): return recurse()

C. def recurse(): recurse() return

D. def recurse(): print('recurse')

Solution

1. Step 1: Check function syntax for recursion

   A recursive function must call itself. It does not always require a return statement unless the return value is used.

2. Step 2: Analyze options

   def recurse(): recurse() correctly defines a recursive function that calls itself.

3. Final Answer:

   def recurse(): recurse() -> Option A

4. Quick Check:

   Recursive call without return is valid syntax [OK]

Hint: Recursive function must call itself; return is optional if return value unused [OK]

Common Mistakes:

• Missing recursive call
• Incorrect use of return
• Confusing print statement with recursion

3. What is the output of this code?

def countdown(n):
    if n == 0:
        return 'Done'
    else:
        print(n)
        return countdown(n-1)

countdown(3)

medium

A. Done

B. 3 2 1 Done

C. 3 2 1

D. 1 2 3 Done

Solution

1. Step 1: Trace recursive calls and prints

   Function prints n then calls countdown(n-1) until n=0, then returns 'Done'. So prints 3, 2, 1 in order.

2. Step 2: Understand return value and print output

   Only numbers 3, 2, 1 are printed. 'Done' is returned but not printed, so output shows numbers only.

3. Final Answer:

   3 2 1 -> Option C

4. Quick Check:

   Prints countdown down to 1 then returns 'Done' [OK]

Hint: Print happens before recursive call, so numbers print descending [OK]

Common Mistakes:

• Thinking 'Done' is printed automatically
• Confusing order of printed numbers
• Ignoring base case return

4. Find the error in this recursive function:

def sum_to_n(n):
    if n == 1:
        return 1
    else:
        return n + sum_to_n(n)

print(sum_to_n(3))

medium

A. Missing base case

B. Recursive call uses n instead of n-1

C. Function does not return anything

D. Syntax error in function definition

Solution

1. Step 1: Check recursive call parameter

   Recursive call should reduce n by 1 to approach base case, but it calls sum_to_n(n) again causing infinite recursion.

2. Step 2: Identify correct fix

   Change recursive call to sum_to_n(n-1) to reduce n each time and reach base case.

3. Final Answer:

   Recursive call uses n instead of n-1 -> Option B

4. Quick Check:

   Recursive call must reduce n to avoid infinite loop [OK]

Hint: Recursive call must move toward base case by changing argument [OK]

Common Mistakes:

• Calling function with same argument causing infinite recursion
• Forgetting to reduce argument in recursion
• Confusing base case presence

5. Given this recursive function to calculate factorial:

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n-1)

What is the call stack sequence when calling factorial(3)?

hard

A. factorial(1) -> factorial(2) -> factorial(3)

B. factorial(0) -> factorial(1) -> factorial(2) -> factorial(3)

C. factorial(3) -> factorial(3) -> factorial(3)

D. factorial(3) -> factorial(2) -> factorial(1) -> factorial(0)

Solution

1. Step 1: Understand recursive calls order

   factorial(3) calls factorial(2), which calls factorial(1), which calls factorial(0) base case.

2. Step 2: Trace call stack sequence

   Calls go down from 3 to 0 before returning results back up the stack.

3. Final Answer:

   factorial(3) -> factorial(2) -> factorial(1) -> factorial(0) -> Option D

4. Quick Check:

   Recursive calls descend to base case 0 [OK]

Hint: Recursive calls go down to base case before returning [OK]

Common Mistakes:

• Thinking calls go from base case up
• Repeating same call multiple times
• Skipping base case call