Factorial Using Recursion in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return 1 when n is 0 in 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'

A1

B0

C-1

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 as the base case condition.

Not having a base case causing infinite recursion.

The factorial of 0 is defined as 1, so the base case checks if n is 0.

2fill in blank

medium

Complete the recursive call to calculate factorial of n-1.

DSA C

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

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

An - 1

Bn + 1

Cn

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causing infinite recursion.

Using n causing no progress towards base case.

The factorial function calls itself with n-1 to reduce the problem size step by step.

3fill in blank

hard

Fix the error in the main function to correctly call factorial and print the result.

DSA C

#include <stdio.h>

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

int main() {
    int num = 5;
    int result = factorial([1]);
    printf("Factorial of %d is %d\n", num, result);
    return 0;
}

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

Anum

Bresult

C5

Dfactorial

Attempts:

3 left

💡 Hint

Common Mistakes

Passing result instead of num to factorial.

Passing the function name factorial instead of a number.

The factorial function should be called with the variable num to calculate factorial of 5.

4fill in blank

hard

Fill both blanks to complete the factorial function with proper base case and recursive call.

DSA C

int factorial(int n) {
    if (n == [1]) {
        return [2];
    }
    return n * factorial(n - 1);
}

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

A0

B1

Cn

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning n instead of 1 in base case.

Checking for n == 1 instead of n == 0.

The base case checks if n is 0 and returns 1, which is the factorial of 0.

5fill in blank

hard

Fill all three blanks to create a recursive factorial function with base case and recursive step.

DSA C

int factorial(int [1]) {
    if ([2] == 0) {
        return [3];
    }
    return [2] * factorial([2] - 1);
}

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

An

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Returning 0 instead of 1 in base case.

The function parameter is n, the base case checks if n is 0, and returns 1 as factorial of 0.

Practice

1. What does the factorial of a number represent in mathematics?

easy

A. The number divided by 2

B. The sum of all positive integers up to that number

C. The number multiplied by 2

D. The product of all positive integers up to that number

Solution

1. Step 1: Understand factorial definition

   Factorial of a number n (written as n!) is the product of all positive integers from 1 to n.

2. Step 2: Compare options with definition

   Only The product of all positive integers up to that number correctly describes factorial as a product, not sum or division.

3. Final Answer:

   The product of all positive integers up to that number -> Option D

4. Quick Check:

   Factorial = product of integers [OK]

Hint: Factorial means multiply all numbers down to 1 [OK]

Common Mistakes:

• Confusing factorial with sum
• Thinking factorial divides the number
• Mixing multiplication with addition

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

easy

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

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

C. if (n == 0) return 0;

D. if (n == 1) return n + 1;

Solution

1. Step 1: Identify base case for factorial

   Factorial of 0 is defined as 1, so base case must return 1 when n is 0.

2. Step 2: Check options for correct base case

   if (n == 0) return 1; returns 1 when n is 0, which is correct. Others return wrong values or conditions.

3. Final Answer:

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

4. Quick Check:

   Base case n=0 returns 1 [OK]

Hint: Base case for factorial is n=0 returns 1 [OK]

Common Mistakes:

• Returning 0 for base case
• Using wrong condition like n==1
• Returning n+1 incorrectly

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(4));
    return 0;
}

medium

A. 24

B. 10

C. 16

D. 120

Solution

1. Step 1: Trace factorial(4) calls

   factorial(4) calls factorial(3), 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
   factorial(4) = 4 * 6 = 24

3. Final Answer:

   24 -> Option A

4. Quick Check:

   4! = 24 [OK]

Hint: Multiply down from n to 1 recursively [OK]

Common Mistakes:

• Stopping recursion too early
• Adding instead of multiplying
• Wrong base case causing infinite recursion

4. Identify the error in this recursive factorial function and select the fix:

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

medium

A. Change base case to if (n == 0) return 0;

B. Change recursive call to factorial(n - 1);

C. Remove base case entirely;

D. Change return to n + factorial(n - 1);

Solution

1. Step 1: Analyze recursive call

   The function calls factorial(n) inside itself without reducing n, causing infinite recursion.

2. Step 2: Correct recursive call to reduce n

   Changing call to factorial(n - 1) ensures n decreases each call, reaching base case.

3. Final Answer:

   Change recursive call to factorial(n - 1); -> Option B

4. Quick Check:

   Recursive call must reduce n [OK]

Hint: Recursive calls must move toward base case [OK]

Common Mistakes:

• Calling factorial(n) again causing infinite loop
• Wrong base case value
• Using addition instead of multiplication

5. How would you modify a recursive factorial function to handle negative inputs by returning -1 for invalid input?
Choose the correct code snippet.

hard

A. if (n <= 0) return 1; else return n * factorial(n - 1);

B. if (n == 0) return -1; else return n * factorial(n - 1);

C. if (n < 0) return -1; else if (n == 0) return 1; else return n * factorial(n - 1);

D. if (n < 0) return 0; else return n * factorial(n - 1);

Solution

1. Step 1: Add check for negative input

   Factorial is not defined for negative numbers, so return -1 if n < 0.

2. Step 2: Keep base case and recursive call correct

   Return 1 if n == 0, else multiply n by factorial(n - 1).

3. Final Answer:

   if (n < 0) return -1; else if (n == 0) return 1; else return n * factorial(n - 1); -> Option C

4. Quick Check:

   Negative input returns -1 [OK]

Hint: Check negative input first, then proceed [OK]

Common Mistakes:

• Not handling negative input
• Returning wrong value for negatives
• Incorrect base case for zero