Recursion on Arrays and Strings in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Sum of Array Elements

What is the output of the following C code that recursively sums elements of an array?

DSA C

int recursiveSum(int arr[], int n) {
    if (n == 0) return 0;
    return arr[n-1] + recursiveSum(arr, n-1);
}

int main() {
    int arr[] = {1, 2, 3, 4};
    int result = recursiveSum(arr, 4);
    printf("%d", result);
    return 0;
}

A Error: infinite recursion

B 24

C 0

D 10

💡 Hint

Think about how the function adds the last element and calls itself with one less element.

The function adds the last element arr[n-1] to the sum of the first n-1 elements recursively. For arr = {1,2,3,4}, sum is 1+2+3+4 = 10.

❓ Predict Output

intermediate

Output of Recursive String Length Function

What is the output of this C code that calculates the length of a string recursively?

DSA C

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

int main() {
    char s[] = "hello";
    int len = recursiveLength(s);
    printf("%d", len);
    return 0;
}

A 5

B 6

C 0

D Segmentation fault

💡 Hint

Count characters until the null character '\0' is found.

The function counts each character until it reaches the string end '\0'. For "hello", length is 5.

🔧 Debug

advanced

Identify the Error in Recursive Array Search

What error does the following C code produce when searching for a value recursively in an array?

DSA C

int recursiveSearch(int arr[], int n, int x) {
    if (n < 0) return -1;
    if (arr[n] == x) return n;
    return recursiveSearch(arr, n-1, x);
}

int main() {
    int arr[] = {1, 3, 5, 7};
    int index = recursiveSearch(arr, 4, 5);
    printf("%d", index);
    return 0;
}

A Segmentation fault

B Prints -1

C Prints 2

D Prints 3

💡 Hint

Check the array index used in the base case and recursive calls.

The function accesses arr[n] where n starts at 4, but arr has indices 0 to 3. The initial call uses n=4 which is invalid, but the base case checks n<0. However, the function calls arr[n] without adjusting for zero-based indexing properly, causing out-of-bounds access.

❓ Predict Output

advanced

Output of Recursive Palindrome Check

What is the output of this C code that checks if a string is a palindrome recursively?

DSA C

int isPalindrome(char *str, int start, int end) {
    if (start >= end) return 1;
    if (str[start] != str[end]) return 0;
    return isPalindrome(str, start + 1, end - 1);
}

int main() {
    char s[] = "racecar";
    int result = isPalindrome(s, 0, 6);
    printf("%d", result);
    return 0;
}

A 0

B 1

C 6

D Segmentation fault

💡 Hint

Check characters from start and end moving inward.

The function returns 1 if the string is a palindrome. "racecar" reads the same forwards and backwards, so output is 1.

🧠 Conceptual

expert

Maximum Depth of Recursive Calls for String Reverse

Consider a recursive function that reverses a string by swapping characters from start to end indices. What is the maximum depth of recursive calls for a string of length n?

A n

B n-1

C n/2

D log n

💡 Hint

Each recursive call processes one pair of characters from start and end.

The function swaps characters at start and end, then calls itself with start+1 and end-1. It stops when start >= end. So the number of calls is about half the string length.