Why Recursion Exists and What Loops Cannot Express Cleanly in DSA C - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the function call itself recursively.

DSA C

void countdown(int n) {
    if (n <= 0) {
        printf("Blastoff!\n");
        return;
    }
    printf("%d\n", n);
    countdown([1]);
}

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An - 1

Bn + 1

Cn * 2

Dn / 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n * 2 or n / 2 does not count down properly.

The function calls itself with n - 1 to count down until it reaches zero.

2fill in blank

medium

Complete the code to calculate factorial using recursion.

DSA C

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

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An * 2

Bn + 1

Cn - 1

Dn / 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n * 2 or n / 2 does not compute factorial correctly.

Factorial of n is n times factorial of n - 1, reducing n each call.

3fill in blank

hard

Fix the error in the recursive Fibonacci function call.

DSA C

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

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An - 2

Bn - 1

Cn + 1

Dn * 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using n - 1 twice causes incorrect results.

Using n + 1 or n * 2 causes errors or infinite recursion.

Fibonacci is sum of fibonacci(n-1) and fibonacci(n-2).

4fill in blank

hard

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

DSA C

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

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A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication or division instead of addition.

Not reducing n in the recursive call.

Sum is n plus sum of numbers from n-1 down to 1.

5fill in blank

hard

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

DSA C

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

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An - 1

Bn

Creturn

Dn + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Calling with n + 1 causes infinite recursion.

Not returning after printing causes unexpected behavior.

The function calls itself with n-1, then prints n, then returns.