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.

Practice

1. What does recursion mean when applied to arrays or strings?

easy

A. A function calls itself to solve smaller parts of the problem

B. A loop runs through all elements once

C. The array or string is sorted automatically

D. The data is copied to a new array or string

Solution

1. Step 1: Understand recursion concept

   Recursion means a function calls itself to break down a problem into smaller parts.

2. Step 2: Apply to arrays or strings

   For arrays or strings, recursion processes one element and then calls itself on the rest.

3. Final Answer:

   A function calls itself to solve smaller parts of the problem -> Option A

4. Quick Check:

   Recursion = function calls itself [OK]

Hint: Recursion always involves a function calling itself [OK]

Common Mistakes:

• Confusing recursion with loops
• Thinking recursion sorts data automatically
• Believing recursion copies data

2. Which of the following is the correct base case for a recursive function that processes a string?

easy

A. if (length < 0) return;

B. if (length == 0) return;

C. if (string == NULL) continue;

D. if (length > 0) return;

Solution

1. Step 1: Identify base case role

   The base case stops recursion when no more characters remain, usually length 0.

2. Step 2: Check options for correct base case

   if (length == 0) return; correctly returns when length is 0, stopping recursion safely.

3. Final Answer:

   if (length == 0) return; -> Option B

4. Quick Check:

   Base case stops recursion at empty string [OK]

Hint: Base case usually checks for empty string or zero length [OK]

Common Mistakes:

• Using incorrect base case conditions
• Using continue instead of return
• Checking for negative length which is invalid

3. What is the output of this C code?

void printReverse(char *str) {
  if (*str == '\0') return;
  printReverse(str + 1);
  printf("%c", *str);
}

int main() {
  char s[] = "abc";
  printReverse(s);
  return 0;
}

medium

A. cba

B. abc

C. Syntax error

D. No output

Solution

1. Step 1: Understand recursion flow

   The function calls itself moving forward in the string until it hits '\0' (end).

2. Step 2: Print characters on return

   After reaching end, it prints characters backward as recursion unwinds: 'c', 'b', 'a'.

3. Final Answer:

   cba -> Option A

4. Quick Check:

   Recursion prints string in reverse [OK]

Hint: Recursion with print after call reverses output [OK]

Common Mistakes:

• Expecting normal order output
• Confusing base case with print order
• Thinking code has syntax errors

4. Find the error in this recursive function that counts characters in a string:

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

medium

A. Missing base case

B. Return type should be void

C. Recursive call does not move forward in string

D. Should use while loop instead

Solution

1. Step 1: Check base case correctness

   Base case correctly returns 0 when string ends.

2. Step 2: Analyze recursive call

   Recursive call uses same pointer 'str' without moving forward, causing infinite recursion.

3. Final Answer:

   Recursive call does not move forward in string -> Option C

4. Quick Check:

   Pointer must advance in recursion [OK]

Hint: Always move pointer forward in recursive calls on strings [OK]

Common Mistakes:

• Forgetting to advance pointer
• Changing return type unnecessarily
• Replacing recursion with loops without reason

5. You want to write a recursive function to check if a string is a palindrome (reads same forward and backward). Which approach is best?

hard

A. Count characters recursively and check if count is even

B. Reverse the string using recursion and compare with original

C. Use recursion to sort the string and compare with original

D. Compare first and last characters, then recurse on substring excluding them

Solution

1. Step 1: Understand palindrome check logic

   Palindrome means first and last characters must match, then check inside substring.

2. Step 2: Apply recursion on substring

   Recursively check smaller substring by removing first and last characters each time.

3. Final Answer:

   Compare first and last characters, then recurse on substring excluding them -> Option D

4. Quick Check:

   Check ends and recurse inside for palindrome [OK]

Hint: Check ends match, then recurse inside substring [OK]

Common Mistakes:

• Reversing string unnecessarily
• Counting characters instead of comparing ends
• Sorting string which changes order