Recursion vs Iteration When Each Wins in DSA C++ - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to implement a recursive factorial function.

DSA C++

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

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' causes infinite recursion.

Using '*' or '/' in the recursive call is incorrect.

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

2fill in blank

medium

Complete the code to implement an iterative factorial function.

DSA C++

int factorial(int n) {
    int result = 1;
    for (int i = 1; i [1] n; ++i) {
        result *= i;
    }
    return result;
}

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

A<=

B<

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' excludes n from the multiplication.

Using '>' or '>=' causes the loop not to run correctly.

The loop runs from 1 up to and including n, multiplying result by each i.

3fill in blank

hard

Fix the error in the recursive Fibonacci function to avoid infinite recursion.

DSA C++

int fibonacci(int n) {
    if (n == 0) return 0;
    if (n == 1) return 1;
    return fibonacci(n [1] 1) + fibonacci(n - 2);
}

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

A-

B*

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' causes infinite recursion.

Using '*' or '/' is incorrect in this context.

The recursive call must reduce n by 1 to approach the base case and avoid infinite recursion.

4fill in blank

hard

Fill both blanks to complete the iterative Fibonacci function.

DSA C++

int fibonacci(int n) {
    if (n <= 1) return n;
    int a = 0, b = 1, c;
    for (int i = 2; i [1] n; ++i) {
        c = a + b;
        a = [2];
        b = c;
    }
    return b;
}

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

A<=

Bb

Ca

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' excludes the last Fibonacci number.

Assigning 'a = a' incorrectly updates variables.

The loop runs while i is less than or equal to n. Variable 'a' is updated to the old 'b' value.

5fill in blank

hard

Fill all three blanks to create a recursive function that sums numbers from n down to 1.

DSA C++

int sumToOne(int n) {
    if (n == 1) return 1;
    return n [1] sumToOne(n [2] 1) [3] 0;
}

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' or '/' instead of '+' causes wrong results.

Using '+' instead of '-' in the recursive call causes infinite recursion.

The function adds n to the sum of numbers from n-1 down to 1, with + and - operators.

Practice

1. Which scenario is best suited for using recursion instead of iteration?

easy

A. When the problem can be divided into similar smaller subproblems

B. When you need to process a large list without stack overflow risk

C. When you want to avoid function call overhead

D. When the problem requires simple repeated loops

Solution

1. Step 1: Understand recursion use case

   Recursion works well when a problem can be broken down into smaller versions of itself, like tree traversals or factorial calculation.

2. Step 2: Compare with iteration scenarios

   Iteration is better for simple repeated tasks without nested subproblems, so recursion is preferred for divide-and-conquer problems.

3. Final Answer:

   When the problem can be divided into similar smaller subproblems -> Option A

4. Quick Check:

   Recursion fits divide-and-conquer problems [OK]

Hint: Recursion fits problems breaking into smaller similar parts [OK]

Common Mistakes:

• Thinking iteration is always better
• Ignoring stack overflow risk in recursion
• Confusing recursion with simple loops

2. Which of the following is the correct syntax to write a recursive function in C++?

easy

A. void func() { if(condition) func(); }

B. void func() { func(); }

C. void func() { while(true) func(); }

D. void func() { for(int i=0; i<10; i++) func(); }

Solution

1. Step 1: Identify base case in recursion

   Recursive functions must have a condition to stop calling themselves, usually an if statement.

2. Step 2: Check options for base case usage

   void func() { if(condition) func(); } uses if(condition) to control recursion, preventing infinite calls. Others cause infinite recursion or loops.

3. Final Answer:

   void func() { if(condition) func(); } -> Option A

4. Quick Check:

   Recursion needs a stopping condition [OK]

Hint: Recursive functions must have a stopping condition [OK]

Common Mistakes:

• Writing recursion without base case
• Using loops inside recursion incorrectly
• Confusing infinite loops with recursion

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

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

int main() {
  std::cout << factorial(4);
  return 0;
}

medium

A. Error: infinite recursion

B. 10

C. 120

D. 24

Solution

1. Step 1: Trace factorial calls

   factorial(4) calls factorial(3), factorial(2), factorial(1), factorial(0). factorial(0) returns 1.

2. Step 2: Calculate return values

   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 D

4. Quick Check:

   4! = 24 [OK]

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

Common Mistakes:

• Confusing factorial with sum
• Missing base case causing infinite recursion
• Incorrect multiplication order

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

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

medium

A. Missing base case

B. Recursive call does not reduce n, causing infinite recursion

C. Should use a loop instead of recursion

D. Function should return void

Solution

1. Step 1: Check base case correctness

   The base case is correct: if n == 1, return 1.

2. Step 2: Analyze recursive call argument

   The recursive call is sum(n), which does not reduce n, so it calls itself infinitely without progress.

3. Final Answer:

   Recursive call does not reduce n, causing infinite recursion -> Option B

4. Quick Check:

   Recursive calls must reduce problem size [OK]

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

Common Mistakes:

• Not reducing argument in recursion
• Confusing base case with recursive step
• Assuming recursion always works without checks

5. You want to process a large dataset with millions of elements. Which approach is better and why?

A recursive function that processes one element and calls itself for the rest,
or an iterative loop that processes elements one by one?

hard

A. Recursive approach, because it uses less memory

B. Recursive approach, because it is easier to write

C. Iterative approach, because recursion risks stack overflow with large data

D. Both are equally good for large datasets

Solution

1. Step 1: Understand recursion limits

   Recursion uses the call stack for each call, which is limited and can overflow with millions of calls.

2. Step 2: Compare iteration benefits

   Iteration uses a single loop and constant stack space, making it safer and more efficient for large datasets.

3. Final Answer:

   Iterative approach, because recursion risks stack overflow with large data -> Option C

4. Quick Check:

   Iteration avoids stack overflow in large data [OK]

Hint: Use iteration for large data to avoid stack overflow [OK]

Common Mistakes:

• Assuming recursion is always simpler
• Ignoring stack size limits
• Thinking recursion uses less memory