Recursion Base Case and Recursive Case in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the base case for the factorial function.

DSA C

int factorial(int n) {
    if (n == [1]) {
        return 1;
    }
    return n * factorial(n - 1);
}

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

A0

B1

C-1

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n == 1 as base case instead of n == 0

Not defining a base case causing infinite recursion

The base case for factorial is when n is 0, factorial(0) = 1.

2fill in blank

medium

Complete the code to correctly call the recursive case in the factorial function.

DSA C

int factorial(int n) {
    if (n == 0) {
        return 1;
    }
    return n [1] factorial(n - 1);
}

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

A/

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication

Using division or subtraction incorrectly

The recursive case multiplies n by factorial of (n-1).

3fill in blank

hard

Fix the error in the recursive Fibonacci function base case.

DSA C

int fibonacci(int n) {
    if (n == [1] || n == 1) {
        return n;
    }
    return fibonacci(n - 1) + fibonacci(n - 2);
}

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

A1

B2

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using n == 2 as base case

Missing the base case for n == 0

The base case for Fibonacci includes n == 0 and n == 1.

4fill in blank

hard

Fill both blanks to complete the recursive sum function that sums numbers from n down to 1.

DSA C

int sum(int n) {
    if (n [1] 0) {
        return 0;
    }
    return n [2] sum(n - 1);
}

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

A<=

B>

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operator in base case

Using subtraction instead of addition in recursive case

The base case checks if n is less than or equal to 0, then returns 0. The recursive case adds n to sum(n-1).

5fill in blank

hard

Fill all three blanks to complete the recursive function that counts down from n to 1 and prints each number.

DSA C

void countdown(int n) {
    if (n [1] 0) {
        return;
    }
    printf("%d\n", n);
    countdown(n [2] 1);
    printf("[3]\n");
}

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

A<=

B-

C+

DDone

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operator in base case

Using addition instead of subtraction in recursive call

Not printing the completion message

The base case checks if n is less than or equal to 0. The recursive call decreases n by 1. After recursion, it prints "Done".

Practice

1. What is the purpose of the base case in a recursive function?

easy

A. To call the function again with a smaller input

B. To stop the recursion and prevent infinite calls

C. To increase the input size for the next call

D. To print the result of the recursion

Solution

1. Step 1: Understand what recursion does

   Recursion calls the same function repeatedly with smaller inputs.

2. Step 2: Identify the role of the base case

   The base case stops the recursion by providing a condition where no further calls happen.

3. Final Answer:

   To stop the recursion and prevent infinite calls -> Option B

4. Quick Check:

   Base case = stop recursion [OK]

Hint: Base case stops recursion to avoid infinite loops [OK]

Common Mistakes:

• Confusing base case with recursive case
• Thinking base case calls function again
• Ignoring the need for a stopping condition

2. Which of the following is the correct syntax for a base case in a recursive function in C?

easy

A. if (n == 0) return 1;

B. while (n > 0) callFunction(n-1);

C. for (int i=0; i

D. return callFunction(n+1);

Solution

1. Step 1: Identify base case syntax

   The base case uses an if condition to check when to stop recursion.

2. Step 2: Check each option

   if (n == 0) return 1; uses if (n == 0) return 1; which correctly stops recursion. Others use loops or increase n, which are incorrect.

3. Final Answer:

   if (n == 0) return 1; -> Option A

4. Quick Check:

   Base case uses if condition [OK]

Hint: Base case uses if condition to stop recursion [OK]

Common Mistakes:

• Using loops instead of base case
• Increasing input instead of decreasing
• Missing return statement in base case

3. What is the output of the following C code?

int factorial(int n) {
  if (n == 0) return 1;
  else return n * factorial(n - 1);
}

int main() {
  printf("%d", factorial(3));
  return 0;
}

medium

A. 6

B. 9

C. 3

D. 0

Solution

1. Step 1: Trace factorial(3) calls

   factorial(3) calls factorial(2), factorial(2) calls factorial(1), factorial(1) calls factorial(0).

2. Step 2: Calculate return values

   factorial(0) = 1; factorial(1) = 1*1=1; factorial(2) = 2*1=2; factorial(3) = 3*2=6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   3! = 6 [OK]

Hint: Factorial n! = n*(n-1)! with base case 0! = 1 [OK]

Common Mistakes:

• Not returning 1 for factorial(0)
• Multiplying incorrectly
• Confusing output with input

4. Identify the error in this recursive function that calculates the sum of numbers from 1 to n:

int sum(int n) {
  if (n == 1) return 1;
  else return n + sum(n);
}

medium

A. Base case condition is incorrect

B. Missing return statement

C. Function should not be recursive

D. Recursive call does not reduce input

Solution

1. Step 1: Check base case

   Base case is if (n == 1) return 1; which is correct.

2. Step 2: Analyze recursive call

   Recursive call is sum(n) which does not reduce n, causing infinite recursion.

3. Final Answer:

   Recursive call does not reduce input -> Option D

4. Quick Check:

   Recursive input must decrease [OK]

Hint: Recursive calls must reduce input to reach base case [OK]

Common Mistakes:

• Calling sum(n) instead of sum(n-1)
• Wrong base case value
• Forgetting to return recursive call

5. You want to write a recursive function to reverse a string in C. Which of the following correctly shows the base case and recursive case?

hard

A. Base case: always call reverse with length-1; Recursive case: print string

B. Base case: if string length > 1, call reverse again; Recursive case: return string as is

C. Base case: if string is empty or length 1, return string; Recursive case: return last char + reverse(rest of string)

D. Base case: if string is empty, print string; Recursive case: add first char to end

Solution

1. Step 1: Understand base case for string reversal

   Base case stops recursion when string is empty or length 1, returning string as is.

2. Step 2: Understand recursive case

   Recursive case returns last character plus reversed substring excluding last character.

3. Final Answer:

   Base case: if string is empty or length 1, return string; Recursive case: return last char + reverse(rest of string) -> Option C

4. Quick Check:

   Base case stops; recursive case reduces string [OK]

Hint: Base case returns small string; recursive case reduces string size [OK]

Common Mistakes:

• Wrong base case condition
• Recursive case not reducing string length
• Confusing base and recursive cases