# C# Recursion

A function that calls itself is known as a recursive function. And, this way is known as recursion.

A physical world example would be to place two parallel mirrors facing each other. Any object in between them would be reflected recursively.

## How Recursion Works?

In the above example, we have called the `recurse()` method from inside the `Main` method (normal method call). And, inside the `recurse()` method, we are again calling the same `recurse()` method. This is a recursive call.

To stop the recursive call, we need to provide some conditions inside the method. Otherwise, the method will be called infinitely.

Hence, we use the if...else statement (or similar approach) to terminate the recursive call inside the method.

## Example: Factorial of a Number Using Recursion

The factorial of a positive number `n` is given by:

``factorial of n (n!) = 1 * 2 * 3 * 4....n``

In C#, we can use recursion to find the factorial of a number. For example,

``````using System;
class Program
{
public static void Main()
{
int fact, num;
Console.Write("Enter a number: ");

// take input from user

Program obj = new Program();

// calling recursive function
fact = obj.factorial(num);

Console.WriteLine("Factorial of {0} is {1}", num, fact);
}

// recursive function
public int factorial(int num)
{
// termination condition
if (num == 0)
return 1;
else
// recursive call
return num * factorial(num - 1);
}
}``````

Output

```Enter a number: 4
Factorial of 4 is 24```

In the above example, we have a method named `factorial()`. We have passed a variable `num` as an argument in `factorial()`.

The `factorial()` is called from the `Main()` method. Inside `factorial()`, notice the statement:

``return num * factorial(num - 1);``

Here, the `factorial()` method is calling itself. Initially, the value of `num` inside `factorial()` is 4. During the next recursive call, 3 is passed to the `factorial()` method. This process continues until `num` is equal to 0.

When `num` is equal to 0, the `if` statement returns `true` hence 1 is returned. Finally, the accumulated result is passed to the `Main()` method.

## Working of Factorial Program

The image below will give you a better idea of how the factorial program is executed using recursion.

Disadvantages - When a recursive call is made, new storage locations for variables are allocated on the stack. As each recursive call returns, the old variables and parameters are removed from the stack. Hence, recursion generally uses more memory and is generally slow.