Solutions for those exercises.

Multiple Choices

1. What is the simplest definition of recursion?

   • A method is recursive if it does not take arguments.
   • A property is recursive if it has a backing field.
   • A method is recursive if it calls itself.
   • A method is recursive if it calls a constructor.
   • A class is recursive if it inherits from another class.

2. Consider the following code:

   void Test(int n)
       {
       if (n != 0){
           Console.Write($"{n} ");
           Test(n - 1);
           }
       }
   Test(3);

   What will it display?

   • Nothing
   • n
   • 3 2 1
   • 3 2 1 0
   • 3 2 1 0 -1 -2 -3 -4 -5 … until the program crashes.
   • 2 1 0 -1 -2 -3 -4 -5 … until the program crashes.
   • 1 2 3
   • 0 1 2 3

3. Consider the following code:

   void Test(int n)
       {
           Console.Write($"{n} ");
           Test(n - 1);
       }
   Test(3);

   What will it display?

   • Nothing
   • n
   • 3 2 1
   • 3 2 1 0
   • 3 2 1 0 -1 -2 -3 -4 -5 … until the program crashes.
   • 2 1 0 -1 -2 -3 -4 -5 … until the program crashes.
   • 1 2 3
   • 0 1 2 3

4. Consider the following code:

   void Test(int n)
       {
           Test(n - 1);
           Console.Write($"{n} ");
       }
   Test(3);

   What will it display?

   • Nothing
   • n
   • 3 2 1
   • 3 2 1 0
   • 3 2 1 0 -1 -2 -3 -4 -5 … until the program crashes.
   • 2 1 0 -1 -2 -3 -4 -5 … until the program crashes.
   • 1 2 3
   • 0 1 2 3

Exercises

1. What would the following code display?

   int Myst1(int n)
   {
       if (n != 0)
       {
           return n + Myst1(n - 1);
       }
       else
       {
           return n;
       }
   }
    
   Console.WriteLine(Myst1(4));

2. What would the following code display?

   void Myst2(int n)
   {
       if (n == 0) { Console.WriteLine("Done"); }
       else if (n < 0)
       {
           Console.Write($"{n} ");
           Myst2(-n);
       }
       else
       {
           Console.Write($"{n} ");
           Myst2(-(n - 1));
       }
   }
   Myst2(3);

3. What would the following code display?

   void Myst3(int len)
   {
       MystH(0, 1, 1, len);
   }
   void MystH(int axP, int bxP, int counter, int len)
   {
       if (counter <= len)
       {
           Console.Write($"{axP} ");
           MystH(bxP, axP + bxP, counter + 1, len);
       }
   }
   Myst3(6);

4. Rewrite the following while loop as a recursive method.

       int i = 10;
       while (i > 0)
       {
         Console.Write($"{i} ");
         i--;
       }

   (Download this code)

5. Rewrite the following do loop as a recursive method.

       int n = 10;
       int sum = 0;
       do
       {
         sum += n;
         n--;
       } while (n > 0);
       Console.WriteLine($"Sum: {sum}");

   (Download this code)

6. Rewrite the following do loop as a recursive method.

   int n;
   do
   {
       Console.WriteLine("Enter an integer, 0 to exit.");
       n = int.Parse(Console.ReadLine());
   } while (n != 0);
   Console.WriteLine("Bye!");

7. Rewrite the following for loop as a recursive method.

       int[] test = { 1, 5, 2, 10, 11, -12, -11 };
       for (int l = 0; l < test.Length; l++)
       {
         if (test[l] % 2 != 0)
         {
           Console.Write($"{test[l]} ");
         }
       }
    
       // Display odd values using recursion
       void OddInArray(int[] arP, int cIndex)
       {
         if (cIndex < arP.Length)
         {
           if (arP[cIndex] % 2 != 0)
           {
             Console.Write($"{arP[cIndex]} ");
           }
           OddInArray(arP, cIndex + 1);
         }
       }
       OddInArray(test, 0);

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8. Write a recursive method that takes an int as argument, generates a random int between 0 and this argument, displays it and calls itself with that number. The method should stop when the int generated is 0.

9. Write a recursive method that takes a string as argument and returns true if it is a palindrome. Your method should return true on input "civic", "noon", "radar" and "" (empty string), and false on input "test" and "not a palindrome".

10. Write a recursive method that takes an int as argument and returns the number of even digits in it. For example, on input 631, the method should return 1 since only 6 is even.

11. Write a recursive method that displays a one-dimensional array of int. You can write an example usage if it helps you.