Mastering Recursion in C#: An In-Depth Guide

What is recursion?

Recursion is a concept in which method calls itself. Every recursive method needs to be terminated, therefore, we need to write a condition in which we check is the termination condition satisfied. If we don’t do that, a recursive method will end up calling itself endlessly.

Create an application which calculates the sum of all the numbers from n to m recursively:

class Program
{
    public static int CalculateSumRecursively(int n, int m)
    {
        int sum = n;
 
        if(n < m)
        {
            n++;
            return sum += CalculateSumRecursively(n, m);
        }
 
        return sum;
   }
 
    static void Main(string[] args)
    {
        Console.WriteLine("Enter number n: ");
        int n = Convert.ToInt32(Console.ReadLine());
 
        Console.WriteLine("Enter number m: ");
        int m = Convert.ToInt32(Console.ReadLine());
 
        int sum = CalculateSumRecursively(n, m);
 
        Console.WriteLine(sum);
 
        Console.ReadKey();
    }
}

Code Explanation

The method CalculateSumRecursively is our recursive method that calculates the sum of the numbers from n to m. The first thing we do is to set our sum to the value of n. Then, we check if the value of n is less then the value of m. If it is, we increase the value of n by 1 and add to our sum a result of the same method but with the increased n. If it is not, we just return the value of the sum variable.

The C# will reserve a memory storage for every recursive method so that the values from the previous method are not overridden.

So let’s see our example through the diagram:

Create an application which prints out how many times the number can be divided by 2 evenly:

class Program
{
    public static int CountDivisions(double number)
    {
        int count = 0;
 
        if(number > 0 && number % 2 == 0)
        {
            count++;
            number /= 2;
 
            return count += CountDivisions(number);
        }
 
        return count;
    }
 
    static void Main(string[] args)
    {
        Console.WriteLine("Enter your number: ");
        double number = Convert.ToDouble(Console.ReadLine());
 
        int count = CountDivisions(number);
        Console.WriteLine($"Total number of divisions: {count}");
 
        Console.ReadKey();
    }
}