Recursion Base Case and Recursive Case in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Factorial Function

What is the output of the following C code when calling factorial(4)?

DSA C

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

int main() {
    int result = factorial(4);
    printf("%d\n", result);
    return 0;
}

A 120

B 10

C 0

D 24

💡 Hint

Remember factorial of 4 is 4 * 3 * 2 * 1.

The function calls itself reducing n by 1 until it reaches 0, then multiplies all values: 4 * 3 * 2 * 1 = 24.

❓ Predict Output

intermediate

Output of Recursive Sum Function

What is printed when the following C code runs sum(3)?

DSA C

int sum(int n) {
    if (n == 0) {
        return 0;
    } else {
        return n + sum(n - 1);
    }
}

int main() {
    int result = sum(3);
    printf("%d\n", result);
    return 0;
}

A 0

B 6

C 3

D 9

💡 Hint

Sum of numbers from 1 to 3 is 6.

The function adds n to the sum of numbers below it until it reaches 0: 3 + 2 + 1 + 0 = 6.

🔧 Debug

advanced

Identify the Error in Recursive Fibonacci Function

What error will occur when running this C code?

DSA C

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

int main() {
    int result = fibonacci(-1);
    printf("%d\n", result);
    return 0;
}

A Returns -1 as output

B Returns 0 without error

C Compilation error due to missing return

D Stack overflow due to infinite recursion

💡 Hint

Check what happens when n is negative.

The function does not handle negative inputs, so it keeps calling itself with decreasing n, never reaching base cases, causing infinite recursion and stack overflow.

🧠 Conceptual

advanced

Base Case Importance in Recursion

Why is the base case essential in a recursive function?

A It initializes variables for recursion

B It makes the function run faster

C It stops the recursion to prevent infinite calls

D It allows the function to call other functions

💡 Hint

Think about what happens if recursion never stops.

The base case tells the function when to stop calling itself, preventing infinite loops and stack overflow.

❓ Predict Output

expert

Output of Recursive String Length Function

What is the output of this C code when calling strlen_recursive("abc")?

DSA C

#include <stdio.h>

int strlen_recursive(const char *str) {
    if (*str == '\0') {
        return 0;
    } else {
        return 1 + strlen_recursive(str + 1);
    }
}

int main() {
    int length = strlen_recursive("abc");
    printf("%d\n", length);
    return 0;
}

A 3

B 4

C 0

D Segmentation fault

💡 Hint

Count characters until the null character '\0'.

The function counts each character until it finds the end '\0', so for "abc" it returns 3.