# Recursion Functions in C

**Recursion** in C programming refers to a programming technique where a function calls itself to solve a problem. It is a powerful and elegant approach to solving complex problems that can be broken down into smaller, similar subproblems. Recursion is based on the principle of "divide and conquer," where a larger problem is divided into smaller, more manageable subproblems, and the results are combined to solve the original problem.

#### How does a Recursion Function work in C:

**1. Function Call:**The recursive function is called with a specific input.

**2. Base Case Check:**Base Case Check: Inside the function, there's a check for a base case. The base case is a condition that, when met, stops the recursion. If the base case is satisfied, the function returns a result immediately.

**3. Recursive Case:**If the base case is not met, the function calls itself with a modified or simpler version of the original problem. The idea is to break down the problem into smaller, more manageable subproblems.

**4. Stack Frame:** Each function call creates a new stack frame, which contains the function's local variables, parameters, and return address. These stack frames are stored on the call stack.

**5. Return Values:** When the base case is reached, the function returns a value. If it's not the base case, the function returns the result of the recursive call.

**6. Backtracking**: As the recursive calls complete, the stack frames are popped off the call stack in a last-in, first-out (LIFO) fashion. This is the process of backtracking, which allows the program to continue and combine results from multiple recursive calls.

### Base Condition in Recursion Functions:

The base condition is the termination criterion that stops the recursion. It defines the simplest scenario where the function does not make a recursive call. Without a base condition, the recursion would continue indefinitely, leading to a stack overflow or infinite loop. The base condition provides a way to exit the recursion and ensures that the function stops calling itself when a specific condition is met.

##### Example:

#include <iostream>

// Function to perform countdown recursively

void countdown(int n) {

// Base condition: Stop when n becomes 0

if (n <= 0) {

std::cout << "Blastoff!" << "\n";

return;

}

// Recursive condition: Call the function with a smaller value

std::cout << n << " ";

countdown(n - 1);

}

// Main function

int main() {

// Call the countdown function with an initial value

countdown(5);

return 0;

}

In this example, the base condition is if (n <= 0), which stops the recursion when n becomes less than or equal to 0. The function prints "Blastoff!" and returns without making another recursive call.

### Recursion Case:

This is when the function continues to solve a smaller, similar part of the original problem by calling itself, progressively reducing the problem's size until it reaches a base condition.

#### Here's an example of Recursion Function :

int factorial(int n) {

if (n <= 1) {

return 1; // Base case

} else {

return n * factorial(n - 1); // Recursive case

}

}

int main() {

int num = 5;

int result = factorial(num);

printf("Factorial of %d is %d\n", num, result);

return 0;

}

In this example, the factorial function calls itself with a smaller value **(n <= 1)** until it reaches the base case **(n <= 1)**. At the base case, it **returns 1**, and as the stack frames are popped off, the results are combined to calculate the factorial.

It's important to define the base case correctly to avoid infinite recursion, and recursion depth should be considered to prevent a stack overflow. Understanding how the call stack works is essential for debugging and managing **recursive** functions effectively.