C# 0 The Complete Reference

FactR( )

 is a bit more complex. When 

FactR( )

 is called with 

an argument of 1, the method returns 1; otherwise, it returns the product of 

FactR(n–1)*n

. To 

evaluate this expression, 

FactR( )

 is called with 

n–1

. This process repeats until 

n

 equals 1 and 

the calls to the method begin returning. For example, when the factorial of 2 is calculated, the 

first call to 

FactR( )

 will cause a second call to be made with an argument of 1. This call will 

return 1, which is then multiplied by 2 (the original value of 

n

). The answer is then 2. You 

might find it interesting to insert 

WriteLine( )

 statements into 

FactR( )

 that show the level of 

recursion of each call and what the intermediate results are.

When a method calls itself, new local variables and parameters are allocated storage on 

the system stack, and the method code is executed with these new variables from the start. 

A recursive call does not make a new copy of the method. Only the arguments are new. As 

each recursive call returns, the old local variables and parameters are removed from the 

stack, and execution resumes at the point of the call inside the method. Recursive methods 

could be said to “telescope” out and back.

Here is another example of recursion. The 

DisplayRev( )

 method uses recursion to 

display its string argument backward.

// Display a string in reverse by using recursion.

using System;

class RevStr {

  // Display a string backward.

  public void DisplayRev(string str) {

    if(str.Length > 0)

      DisplayRev(str.Substring(1, str.Length-1));

    else

      return;

    Console.Write(str[0]);

  }

}

class RevStrDemo {

  static void Main() {

    string s = "this is a test";

    RevStr rsOb = new RevStr();

    Console.WriteLine("Original string: " + s);

    Console.Write("Reversed string: ");

    rsOb.DisplayRev(s);

    Console.WriteLine();

  }

}

Here is the output:

www.freepdf-books.com

---