C# - RECURSION

Recursion is a technique where a Method calls itself.

Let us understand Recursion using the program to find out the Factorial of a number.

Below is how we calculate the factorial of a number.

Say, we want to calculate the factorial of 3.

3 * 2 * 1 = 6

Similarly, the factorial of 6 is,

6 * 5 * 4 * 3 * 2 * 1 = 720

Below is a program to calculate the factorial of 3 using Recursion.

Example :

public class MyApplication
{
    public static void Main(string[] args)
    {
        int n = 3;
        int f = fact(n);

        System.Console.WriteLine("The factorial of "+n+" is "+f);
    }

	static int fact(int x) 
	{
        if (x == 1) 
        {
            return 1;
        } 
        else 
        {
            return (x * fact(x-1));
        }
	}
}

Output :

The factorial of 3 is 6

In the above code, we are calculating the factorial of 3.

So, we have initialised the value of n with 3.

int n = 3;

And called the fact(n) Method, passing the value of n(i.e. 3).

int f = fact(n);

And the execution of fact() Method begins.

static int fact(int x)
{
	if (x == 1)
	{
		return 1;
	}
	else
	{
		return (x * fact(x-1));
	}
}

So, the value of x is 3(That is passed as argument).

Then in the next line, we have checked if the value of x is 1. If so, the method execution stops and 1 is returned.

if (x == 1) {
	return 1;
}

Now, since the value of x is 3. We come to the else part and come across the recursive call, where the fact Method calls itself.

else {
	return (x * fact(x-1));
}

But the thing to note is that there is a return statement. A return statement actually means to return the value and finish the Method execution.

But in this case the value will not be returned immediately. Instead, the fact Method calls itself.

And C#, maintains an internal Stack that is hidden from the user.

So, in the return statement, if we substitute the value of x with 3.

return (x * fact(x-1));

It becomes return (3 * fact(2)). And that is kept in the internal stack maintained by C#.

For fact(3), C# keeps a note in stack that the return value is return (3 * fact(2)).And the value of fact(2) is yet to be determined.

Now, fact(2) Method from return (3 * fact(2)) with the new Argument, 2 is called. And the first pass begins.

1st Pass

So, the value of x is 2(That is passed as recursive argument).

And the fact(2)(i.e. After substituting the value of x by 2) Method execution begins with new Argument 2.

static int fact(int x)
{
	if (x == 1)
	{
		return 1;
	}
	else
	{
		return (x * fact(x-1));
	}
}

Similarly, the control comes to the else statement and the return statement is encountered.

else
{
	return (x * fact(x-1));
}

Even in this case C# finds a recursive call to the fact Method.

So, it stores the value in the stack.

So, in the return statement, if we substitute the value of x with 3.

return (x * fact(x-1));

It is return (2 * fact(1)).

Now, fact(1) Method with the new Argument, 1 is called. And the second pass begins.

2nd Pass

So, the value of x is 1(That is passed as recursive argument).

And the fact(1) Method execution begins with new Argument 1.

static int fact(int x)
{
	if (x == 1)
	{
		return 1;
	}
	else
	{
		return (x * fact(x-1));
	}
}

Now, the control comes to the if statement

if (x == 1) {
	return 1;
}

and the return 1 statement is encountered.

Now, C# doesn't find a recursive call to the fact Method.

And the return value for fact(1) is 1.

And now, it is time for C# to Backtrack

i.e. To get the values out of the Stack.

So C# goes to the top element of the stack and takes it out.

And the topmost element is fact(2) with value return (2 * fact(1)). So, it takes out fact(2) and substitutes the value of fact(1) with 1.

And the value of fact(2) becomes,

'fact(2)' -->  'return (2 * fact(1))' -->  return (2*1) --> return 2

So, the value of fact(2) is 2.

So C# goes to the stack again.

And the topmost element is fact(3) with value return (3 * fact(2)). So, it takes out fact(3) and substitutes the value of fact(2) with 2.

And the value of fact(3) becomes,

'fact(3)' -->  'return (3 * fact(2))' -->  return (3*2) --> return 6

So, the value of fact(3) is 6.

Now since, the Stack is empty,

C# returns the value of fact(3) i.e. 6.

And finally, the value is fact(3) is returned.

int f = fact(n);

And the returned value is assigned to f.

And the print statement,

System.Console.WriteLine("The factorial of "+n+" is "+f);

Prints the output.

Output :

The factorial of 3 is 6

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