# 1. Introduction

Recursion is a powerful concept in programming where a function calls itself to solve a problem. One of the classic problems solved using recursion is finding the sum of the first 'n' natural numbers. This post will guide you through writing a C program to achieve this using recursion.

# 2. Program Overview

The C program will:

1. Prompt the user to input a number 'n'.

2. Calculate the sum of the first 'n' natural numbers using recursion.

3. Display the result.

# 3. Code Program

``````#include <stdio.h>

// Recursive function to find the sum of the first n natural numbers
int sumOfNaturalNumbers(int n) {
if(n == 0)
return 0; // Base condition
else
return n + sumOfNaturalNumbers(n - 1); // Recursive call
}

int main() {
int num;

// Taking input from the user
printf("Enter a positive integer: ");
scanf("%d", &num);

// Ensuring that the number entered is non-negative
if(num < 0) {
return 1;
}

// Displaying the result
printf("Sum of the first %d natural numbers is: %d", num, sumOfNaturalNumbers(num));

return 0;
}
``````

### Output:

```Enter a positive integer: 5
Sum of the first 5 natural numbers is: 15
```

# 4. Step By Step Explanation

1. Recursive Function:

- The function sumOfNaturalNumbers is defined to calculate the sum recursively.

- It takes an integer 'n' as its argument.

- If n is 0, it returns 0 (base case).

- Otherwise, it returns the sum of 'n' and the sum of the first 'n-1' natural numbers. This is achieved by the function calling itself.

2. User Input:

- The user is prompted to input a positive integer.

- A check is performed to ensure the number entered is non-negative.

3. Displaying Result:

- The result, i.e., the sum of the first 'n' natural numbers, is displayed using the printf function.

By using recursion, we have efficiently solved the problem in a logical and concise manner. This example demonstrates the elegance and power of recursive techniques in programming.