Recursion on Arrays and Strings in DSA C - Build from Scratch

Sum of Array Elements Using Recursion

📖 Scenario: Imagine you have a list of daily expenses saved in an array. You want to find the total amount spent using a simple program.

🎯 Goal: Build a C program that uses recursion to calculate the sum of all elements in an integer array.

📋 What You'll Learn

Create an integer array with exact values

Create a variable for the array size

Write a recursive function to sum the array elements

Print the final sum

💡 Why This Matters

🌍 Real World

Recursion helps solve problems like summing expenses, searching, or processing data step-by-step in many software applications.

💼 Career

Understanding recursion is important for software developers to write clean and efficient code for tasks like data processing, algorithms, and problem-solving.

1

Create the array of expenses

Create an integer array called expenses with these exact values: 100, 200, 300, 400, 500.

DSA C

# Your code here

Hint

Use curly braces to list the values inside the array.

2

Set the size of the array

Create an integer variable called size and set it to the number of elements in the expenses array, which is 5.

DSA C

int expenses[] = {100, 200, 300, 400, 500};
# Your code here

Hint

Count the number of elements in the array and assign it to size.

3

Write the recursive function to sum the array

Write a recursive function called sumArray that takes two parameters: an integer array arr and an integer n representing the size. The function should return the sum of the first n elements. Use the base case when n is 0 to return 0. Otherwise, return the last element plus the sum of the rest.

DSA C

int expenses[] = {100, 200, 300, 400, 500};
int size = 5;

// Your code here

Hint

Use the last element arr[n - 1] and call sumArray with n - 1.

4

Print the sum of the array

Call the sumArray function with expenses and size, store the result in an integer variable called total, and print total using printf.

DSA C

int expenses[] = {100, 200, 300, 400, 500};
int size = 5;

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

// Your code here

Hint

Use printf("%d\n", total); to print the sum.

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