Methods and Conditional Statements

Issues on Methods and Conditional Statements

Returning From a Method

So far, when defining a void method, we did not return a value. Here is an example:

using System;

public class Exercise
{
    private void Show()
    {
        Console.WriteLine("C# Programming is fun!!!");
    }

    public static int Main()
    {
        Exercise exo = new Exercise();

        exo.Show();
        return 0;
    }
}

In reality a void method can perform a return, as long as it does not return a true value. This is used to signal to the compiler that it is time to get out of the method. To add such as flag, in the appropriate section of the void method, simply type return;. Here is an example:

using System;

public class Exercise
{
    private void Show()
    {
        Console.WriteLine("C# Programming is fun!!!");
        
        return;
    }

    public static int Main()
    {
        Exercise exo = new Exercise();

        exo.Show();
        return 0;
    }
}

In this case, the return; operation doesn't serve any true purpose. In the next section, we will see how it can be made useful when associated with a conditional statement.

1. Start Microsoft Visual Studio
2. To create a new application, on the main menu, click File -> New Project...
3. In the middle list, click Empty Project
4. Change the Name to NationalBank5 and press Enter
5. To create a new file, on the main menu, click Project -> Add New Item...
6. In the middle list, click Code File
7. Change the Name of the file to Customer
8. Click Add
9. Complete the Customer.cs file as follows:
   

   public enum AccountType { Checking, Saving, Other }
   
   public class Customer
   {
       public string AccountNumber;
       public AccountType Type;
       public string FullName;
       public double Balance;
       public short  PIN;
   
       public Customer(string acnt = "000-000000-000",
                       AccountType category = AccountType.Other,
                       string name = "John Doe")
       {
           AccountNumber = acnt;
           Type = category;
           FullName = name;
           PIN = 0;
           Balance = 0.00D;
       }
   }

10. To create a new file, in the Solution Explorer, right-click NationalBank5 -> Add -> New Item...
11. In the middle list, make sure Code File is selected.
    Change the Name to Management and press Enter
12. Complete the file as follows:

    using System;
    
    public class Management
    {
        private Customer CreateNewAccount()
        {
            byte typeOfAccount = 0;
            Customer client = new Customer();
    
            Console.WriteLine("===========================================");
            Console.WriteLine("==-= National Bank =-======================");
            Console.WriteLine("-------------------------------------------");
            Console.Write("Enter a number for the new account(000-000000-000): ");
            client.AccountNumber = Console.ReadLine();
            Console.WriteLine("What type of account the customer wants to open");
            Console.WriteLine("1 - Checking Account");
            Console.WriteLine("2 - Savings Account");
            Console.Write("Enter account type: ");
            typeOfAccount = byte.Parse(Console.ReadLine());
            if (typeOfAccount == 1)
                client.Type = AccountType.Checking;
            else if (typeOfAccount == 2)
                client.Type = AccountType.Saving;
            else
                client.Type = AccountType.Other;
            Console.Write("Enter customer name: ");
            client.FullName = Console.ReadLine();
            Console.Write("Ask the customer to enter a PIN: ");
            client.PIN = short.Parse(Console.ReadLine());
    
            return client;
        }
    
        public double GetMoney()
        {
            double amount = 0;
    
            Console.Write("Amount: ");
            amount = double.Parse(Console.ReadLine());
            return amount;
        }
    
        private void ShowAccountInformation(Customer cust)
        {
            Console.WriteLine("===========================================");
            Console.WriteLine("==-= National Bank =-======================");
            Console.WriteLine("Customer Account Information");
            Console.WriteLine("-------------------------------------------");
            Console.WriteLine("Account #:    {0}", cust.AccountNumber);
            Console.WriteLine("Account Type: {0}", cust.Type);
            Console.WriteLine("Full Name:    {0}", cust.FullName);
            Console.WriteLine("PIN #:        {0}", cust.PIN);
            Console.WriteLine("Balance:      {0:F}", cust.Balance);
            Console.WriteLine("===========================================");
            return;
        }
    
        public static int Main()
        {
            double amount = 0;
            byte nextAction = 0;
            Customer accountHolder = null;
            Management registration = new Management();
    
            Console.Title = "National Bank";
    
            accountHolder = registration.CreateNewAccount();
            Console.WriteLine("Enter the customer's initial deposit");
            accountHolder.Balance = registration.GetMoney();
            Console.Clear();
    
            registration.ShowAccountInformation(accountHolder);
    
            do
            {
                Console.WriteLine("What do you want to do now?");
                Console.WriteLine("1 - Check account balance");
                Console.WriteLine("2 - Make a deposit");
                Console.WriteLine("3 - Withdraw money");
                Console.WriteLine("4 - Transfer money from one account to another");
                Console.Write("Enter your choice: ");
                nextAction = byte.Parse(Console.ReadLine());
                Console.Clear();
    
                switch (nextAction)
                {
                    case 1:
                        Console.Clear();
                        registration.ShowAccountInformation(accountHolder);
                        Console.Write("Press Enter for next operation");
                        Console.ReadKey();
                        break;
    
                    case 2:
                        Console.Write("Enter Deposit ");
                        amount = registration.GetMoney();
                        accountHolder.Balance = accountHolder.Balance + amount;
                        Console.Clear();
                        registration.ShowAccountInformation(accountHolder);
                        break;
    
                    case 3:
                        Console.Write("Enter Withdrawal ");
                        amount = registration.GetMoney();
    
                        if (amount > accountHolder.Balance)
                        {
                            Console.WriteLine(
               "You are not allowed to withdraw more money than your account has.");
                            Console.ReadKey();
                        }
                        else
                            accountHolder.Balance = accountHolder.Balance - amount;
    
                        Console.Clear();
                        registration.ShowAccountInformation(accountHolder);
                        break;
    
                    case 4:
                        Console.WriteLine(
                        "Operation not available: You have only one account with us");
                        break;
                }
    
    
                if ((nextAction < 1) || (nextAction > 4))
                    Console.WriteLine(
                    "Invalid Action: Please enter a value between 1 and 4");
            } while ((nextAction >= 1) && (nextAction <= 4));
    
            Console.ReadKey();
            return 0;
        }
    }

Some functions are meant to return a value that is conditional of their processing. The fact that a function indicates the type of value it would return may not be clear at the time the function is closed but a function defined other than void must always return a value. You can write a conditional statement, such as if, inside of a function and return a value from that condition. Here is an example:

using System;

public class Program
{
    enum HouseType { Unknown, SingleFamily, Townhouse, Condominium };

    public static int Main()
    {
        var type = GetHouseType();

        switch (type)
        {
            case HouseType.SingleFamily:
                Console.WriteLine("\nType of Home: Single Family");
                break;
            case HouseType.Townhouse:
                Console.WriteLine("\nType of Home: Townhouse");
                break;
            case HouseType.Condominium:
                Console.WriteLine("\nType of Home: Condominium");
                break;
            case HouseType.Unknown:
                Console.WriteLine("\nType of Home. Unknown");
                break;
        }

        return 0;
    }

    private static HouseType GetHouseType()
    {
        var type = 0;

        Console.WriteLine("What Type of House Would you Like to Purchase?");
        Console.WriteLine("1 - Single Family");
        Console.WriteLine("2 - Townhouse");
        Console.WriteLine("3 - Condominium");
        Console.Write("Your Choice? ");
        type = int.Parse(Console.ReadLine());

        if (type == 1)
            return HouseType.SingleFamily;
        else if (type == 2)
            return HouseType.Townhouse;
        else if (type == 3)
            return HouseType.Condominium;
    }
}

This GetHouseType() method indicates when one of three values is returned. In reality, this method could get a value other than the three that are considered. If the user enters such a value, the current version of the method would not know what to do. For this reason, the program will not compile. In Microsoft Visual C#, you would receive the following error:

'Program.GetHouseType()': not all code paths return a value

To solve this problem, you must provide a statement that would include any value other than those considered. You can do this by writing a final return that has its own value. Here is an example:

using System;

public class Program
{
    enum HouseType { Unknown, SingleFamily, Townhouse, Condominium };

    public static int Main()
    {
        var type = GetHouseType();

        switch (type)
        {
            case HouseType.SingleFamily:
                Console.WriteLine("\nType of Home: Single Family");
                break;
            case HouseType.Townhouse:
                Console.WriteLine("\nType of Home: Townhouse");
                break;
            case HouseType.Condominium:
                Console.WriteLine("\nType of Home: Condominium");
                break;
            case HouseType.Unknown:
                Console.WriteLine("\nType of Home. Unknown");
                break;
        }

        return 0;
    }

    private static HouseType GetHouseType()
    {
        var type = 0;

        Console.WriteLine("What Type of House Would you Like to Purchase?");
        Console.WriteLine("1 - Single Family");
        Console.WriteLine("2 - Townhouse");
        Console.WriteLine("3 - Condominium");
        Console.Write("Your Choice? ");
        type = int.Parse(Console.ReadLine());

        if (type == 1)
            return HouseType.SingleFamily;
        else if (type == 2)
            return HouseType.Townhouse;
        else if (type == 3)
            return HouseType.Condominium;
        else
            return HouseType.Unknown;
    }
}

1. Change the Management.cs file as follows:

   using System;
   
   public class Management
   {
       private AccountType SpecifyAccountType()
       {
           byte type = 0;
   
           Console.WriteLine("What type of account the customer wants to open");
           Console.WriteLine("1 - Checking Account");
           Console.WriteLine("2 - Savings Account");
           Console.Write("Enter account type: ");
           type = byte.Parse(Console.ReadLine());
   
           if (type == 1)
               return AccountType.Checking;
           else if (type == 2)
               return AccountType.Saving;
           else
               return AccountType.Other;
       }
   
       private Customer CreateNewAccount()
       {
           Customer client = new Customer();
   
           . . . No Change
   
           return client;
       }
   
       public double Deposit()
       {
           double amount = 0;
   
           Console.Write("Amount to deposit: ");
           amount = double.Parse(Console.ReadLine());
   
           return amount;
       }
   
       public double Withdraw(Customer cust)
       {
           double amount = 0;
   
           Console.Write("Amount to withdraw: ");
           amount = double.Parse(Console.ReadLine());
   
           if (amount > cust.Balance)
           {
               Console.WriteLine(
               "You are not allowed to withdraw more money than your account has.");
               Console.ReadKey();
               return 0.00D;
           }
           else
               return amount;
       }
   
       private void ShowAccountInformation(Customer cust)
       {
           . . . No Change
       }
   
       public static int Main()
       {
           double amount = 0;
           byte nextAction = 0;
           Customer accountHolder = null;
           Management registration = new Management();
   
           Console.Title = "National Bank";
   
           accountHolder = registration.CreateNewAccount();
           Console.WriteLine("Enter the customer's initial deposit");
           accountHolder.Balance = registration.Deposit();
           Console.Clear();
   
           registration.ShowAccountInformation(accountHolder);
   
           do
           {
               Console.WriteLine("What do you want to do now?");
               Console.WriteLine("0 - Close the application");
               Console.WriteLine("1 - Check account balance");
               Console.WriteLine("2 - Make a deposit");
               Console.WriteLine("3 - Withdraw money");
               Console.WriteLine("4 - Transfer money from one account to another");
               Console.Write("Enter your choice: ");
               nextAction = byte.Parse(Console.ReadLine());
               Console.Clear();
   
               switch (nextAction)
               {
                   case 1:
                       Console.Clear();
                       registration.ShowAccountInformation(accountHolder);
                       Console.Write("Press Enter for next operation");
                       Console.ReadKey();
                       break;
   
                   case 2:
                       amount = registration.Deposit();
                       accountHolder.Balance += amount;
                       Console.Clear();
                       registration.ShowAccountInformation(accountHolder);
                       break;
   
                   case 3:
                       amount = registration.Withdraw(accountHolder);
                       accountHolder.Balance -= amount;
                       Console.Clear();
                       registration.ShowAccountInformation(accountHolder);
                       break;
   
                   case 4:
                       Console.WriteLine(
                      "Operation not available: You have only one account with us");
                       break;
   
                   default:
                       return 0;
               }
   
               if ((nextAction < 1) || (nextAction > 4))
                   Console.WriteLine(
                   "Invalid Action: Please enter a value between 1 and 4");
           } while ((nextAction >= 1) && (nextAction <= 4));
   
           Console.ReadKey();
           return 0;
       }
   }

2. Press F5 to execute
3. Enter information as follows:
    

   Account #:	257-484902-444
   Account Type:	1
   Customer Name:	Josh Nay
   PIN:	5008
   Initial Deposit	250

   ===========================================
   ==-= National Bank =-======================
   -------------------------------------------
   New Account Number (000-000000-000): 257-484902-444
   What type of account the customer wants to open
   1 - Checking Account
   2 - Savings Account
   Enter account type: 1
   Enter customer name: Josh Nay
   Ask the customer to enter a PIN: 5008
   Enter the customer's initial deposit
   Amount to deposit: 250

4. Press Enter

   ===========================================
   ==-= National Bank =-======================
   Customer Account Information
   -------------------------------------------
   Account #:    257-484902-444
   Account Type: Checking
   Full Name:    Josh Nay
   PIN #:        5008
   Balance:      250.00
   ===========================================
   What do you want to do now?
   0 - Close the application
   1 - Check account balance
   2 - Make a deposit
   3 - Withdraw money
   4 - Transfer money from one account to another
   Enter your choice:

5. When asked to specify the next action, type 3 to withdraw money and press Enter
6. Type 164.37 as the amount to deposit and press Enter

   ===========================================
   ==-= National Bank =-======================
   Customer Account Information
   -------------------------------------------
   Account #:    257-484902-444
   Account Type: Checking
   Full Name:    Josh Nay
   PIN #:        5008
   Balance:      85.63
   ===========================================
   What do you want to do now?
   0 - Close the application
   1 - Check account balance
   2 - Make a deposit
   3 - Withdraw money
   4 - Transfer money from one account to another
   Enter your choice:

7. For the next action, type 2 and press Enter
8. Type 116.18 as the amount to deposit and press Enter

   ===========================================
   ==-= National Bank =-======================
   Customer Account Information
   -------------------------------------------
   Account #:    257-484902-444
   Account Type: Checking
   Full Name:    Josh Nay
   PIN #:        5008
   Balance:      201.81
   ===========================================
   What do you want to do now?
   0 - Close the application
   1 - Check account balance
   2 - Make a deposit
   3 - Withdraw money
   4 - Transfer money from one account to another
   Enter your choice:

9. For the next action, type 3 and press Enter
10. Type 300 as the amount to withdraw and press Enter

    Amount to withdraw: 300
    You are not allowed to withdraw more money than your account has.

11. Press Enter
12. Type 0 as the next action and press Enter

For the different situations in which we used a while condition so far, we included a means of checking the condition. Instead, you can include juste the true Boolean constant in the parentheses of true. Here is an example:

using System;

public class Exercise
{
    public static int Main()
    {
        while (true)
            Console.WriteLine("C# Programming is fun!!!");

	return 0;
    }
}

This type of statement is legal and would work fine. But it has no way of stopping because it is telling the compiler "As long as this is true, ...". The question is, what is "this"? As a result, the program would run for ever. Therefore, if you create a while(true) condition, in the body of the statement, you should provide a way for the compiler to stop, that is, a way for The condition to be false. This can be done by include an if condition. Here is an example:

using System;

public class Exercise
{
    public static int Main()
    {
        int i = 0;

        while (true)
        {
            if (i > 8)
                break;
            Console.WriteLine("C# Programming is fun!!!");

            i++;
        }
 
        return 0;
    }
}

This time, the program is asking the compiler to keep displaying a sentence in the console screen by counting from 0 and up; but if the count reaches 8, to stop. The above program would produce:

C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
C# Programming is fun!!!
Press any key to continue . . .

In the above program, we used a break operator. You can use any other mechanism that can stop the looping, as long as at one time the condition would be false. Therefore, here is another way of stopping a while(true) loop:

using System;

public class Exercise
{
    public static int Main()
    {
        int i = 0;

        while (true)
        {
            if (i < 8)
                Console.WriteLine("C# Programming is fun!!!");
            else
                return 0;

            i++;
        }
    }
}

This would produce the same result as previously.

Instead of using while(true), you can first declare and initialize a Boolean variable, or you can use a Boolean variable whose value is already known. The value can come from a method or by other means.

1. To start a new application, on the main menu, click File -> New Project...
2. In the middle list, click Empty Project
3. Change the name to DepartmentStore8
4. To create a new class, on the main menu, click Project -> Add Class...
5. Change the Name to StoreItem and click Add
6. Change the document as follows:

   using System;
   using System.Collections.Generic;
   using System.Linq;
   using System.Text;
   
   namespace DepartmentStore8
   {
       public class StoreItem
       {
           public int     itemNumber;
           public string  itemName;
           public string  itemSize;
           public decimal unitPrice;
   
           public StoreItem()
           {
               itemNumber = 0;
               itemName   = "";
               itemSize   = "";
               unitPrice  = 0.00M;
           }
   
           public StoreItem(int code, string name,
                            string size, decimal price)
           {
               itemNumber = code;
               itemName   = name;
               itemSize   = size;
               unitPrice  = price;
           }
       }
   }

7. In the Solution Explorer, right-click DepartmentStore8 -> Add -> New Item...
8. In the middle list, click Code File
9. Change the Name to Inventory and press Enter
10. Change the file as follows:

    using System;
    using DepartmentStore8;
    
    public class Inventory
    {
        static int Main()
        {
            StoreItem si = null;
            bool itemIsValid = true;
    
            Console.Title = "Department Store Inventory";
            Console.WriteLine("Department Store Inventory");
            while (itemIsValid)
            {
                si = new StoreItem();
    
                Console.WriteLine("------------------------");
                Console.Write("Enter Item #:     ");
                si.itemNumber = int.Parse(Console.ReadLine());
                if (si.itemNumber < 0)
                    itemIsValid = false;
                else
                {
                    Console.Write("Enter Item Name:  ");
                    si.itemName = Console.ReadLine();
                    Console.Write("Enter Item Size:  ");
                    si.itemSize = Console.ReadLine();
                    Console.Write("Enter Unit Price: ");
                    si.unitPrice = decimal.Parse(Console.ReadLine());
    
                    Console.WriteLine("=-= Store Item =-=");
                    Console.WriteLine("Item #:     {0}", si.itemNumber);
                    Console.WriteLine("Item Name:  {0}", si.itemName);
                    Console.WriteLine("Size:       {0}", si.itemSize);
                    Console.WriteLine("Unit Price: {0}", si.unitPrice);
                }
            }
    
            return 0;
        }
    }

11. Execute the application to test it
12. When requested, provide the data as follows:
     

    Item Number:	279475
    Item Name:	Wool Jersey Ruffle Dress
    Size:	8
    Unit Price:	150
    Item Number:	729070
    Item Name:	Mid-Weight Flat-Front Wool Trouser Pants
    Size:	36W - 30L
    Unit Price:	69.95
    Item Number:	297004
    Item Name:	Ponte Pencil Skirt with Pleated Detail
    Size:	16
    Unit Price:	80
    Item Number:	-1

    Department Store Inventory
    ------------------------
    Enter Item #:     279475
    Enter Item Name:  Wool Jersey Ruffle Dress
    Enter Item Size:  8
    Enter Unit Price: 150
    =-= Store Item =-=
    Item #:     279475
    Item Name:  Wool Jersey Ruffle Dress
    Size:       8
    Unit Price: 150
    ------------------------
    Enter Item #:     729070
    Enter Item Name:  Mid-Weight Flat-Front Wool Trouser Pants
    Enter Item Size:  36W - 30L
    Enter Unit Price: 69.95
    =-= Store Item =-=
    Item #:     729070
    Item Name:  Mid-Weight Flat-Front Wool Trouser Pants
    Size:       36W - 30L
    Unit Price: 69.95
    ------------------------
    Enter Item #:     297004
    Enter Item Name:  Ponte Pencil Skirt With Pleated Detail
    Enter Item Size:  16
    Enter Unit Price: 80
    =-= Store Item =-=
    Item #:     297004
    Item Name:  Ponte Pencil Skirt With Pleated Detail
    Size:       16
    Unit Price: 80
    ------------------------
    Enter Item #:     -1

13. Close the DOS window and return to your programming environment

Imagine that you want to count the positive odd numbers from a certain maximum to a certain minimum. For example, to count the odd numbers from 1 to 9, you would use:

9, 7, 5, 3, and 1

Notice that, to perform this operation, you consider the highest. Then you subtract 2 to get the previous. Again, you subtract 2 from the number to get the previous. What you are simply doing is to subtract a constant to what you already have and you invent very little. In computer programming, you can solve this type of problem by first writing a function, and then have the function call itself. This is the basis for recursion.

1. To start a new application, on the main menu, click File -> New Project...
2. In the middle list, click Empty Project
3. Change the nema to Recursions and press Enter
4. In the Solution Explorer, right-click Recursions -> Add -> New Item
5. In the middle list, click Code File
6. Change the name to Calculator and press Enter

 A type of formula to create a recursive method is:

ReturnValue Function(Arguments, if any)
{
    Optional Action . . .
    Function();
    Optionan Action . . .
}

A recursive method starts with a return value. If it would not return a value, you can define it with void. After its name, the method can take one or more arguments. Most of the time, a recursive method takes at least one argument that it would then modify. In the body of the method, you can take the necessary actions. There are no particular steps to follow when implementing a recursive method but there are two main rules to observe:

• In its body, the method must call itself
• Before or after calling itself, the method must check a condition that would allow it to stop, otherwise, it might run continuously

For our example of counting decrementing odd numbers, you could start by creating a method that takes an integer as argument. To exercise some control on the lowest possible values, we will consider only positive numbers. In the body of the method, we will display the current value of the argument, subtract 2, and recall the method itself. Here is our function:

using System;

public class Exercise
{
    static void OddNumbers(int a)
    {
        if (a >= 1)
        {
            Console.Write("{0}, ", a);
            a -= 2;
            OddNumbers(a);
        }
    }
    
    public static int Main()
    {
        const int number = 9;

        Console.WriteLine("Odd Numbers");
        OddNumbers(number);
        
        Console.WriteLine();
        return 0;
    }
}

Notice that the method calls itself in its body. This would produce:

Odd Numbers
9, 7, 5, 3, 1,
Press any key to continue . . .

1. To create a recursive function, change the document as follows:

   using System;
   
   public class Calculator
   {
       private long Factorial(long number)
       {
           if (number <= 1)
               return 1;
           return number * Factorial(number - 1);
       }
   
       public static int Main()
       {
           long factor = 0;
           Calculator exo = new Calculator();
   
           Console.Write("To calculate a factorial, enter a (small positive) number: ");
           factor = long.Parse(Console.ReadLine());
   
           Console.WriteLine("The factorial of {0} = {1}",
               factor, exo.Factorial(factor));
   
           System.Console.ReadKey();
           return 0;
       }
   }

2. Execute the application to test it
3. When asked to provide a number, type 8 and press Enter:

   To calculate a factorial, enter a (small positive) number: 8
   The factorial of 8 = 40320
   Press any key to continue . . .

4. Press Enter to close the DOS window

Recursive methods provide a valuable mechanism for building lists or series, which are value that are either increment or decrement but follow a pattern. Imagine that, instead of simply displaying odd numbers as we did above, you want to add them incrementally. If you have 1, it would also produce 1. If you have 5, you would like to add 1 to 3, then the result to 5, and so on. This can be illustrated as follows:

                1 = 1

            1 + 3 = 4

        1 + 3 + 5 = 9

    1 + 3 + 5 + 7 = 16

1 + 3 + 5 + 7 + 9 = 25

To perform this operation, you would consider 1. If the number is less than or equal to 1, the method should return 1. Otherwise, add 2 to 1, then add 2 to the new result. Continue this until you get to the value of the argument. The method can be implemented as follows:

using System;

public class Exercise
{
    static int AdditionalOdd(int a)
    {
        if (a <= 1)
            return 1;
        return a + AdditionalOdd(a - 2);
    }

    static void OddNumbers(int a)
    {
        if (a >= 1)
        {
            Console.Write("{0}, ", a);
            a -= 2;
            OddNumbers(a);
        }
    }
    
    public static int Main()
    {
        const int Number = 9;

        Console.WriteLine("Odd Numbers");
        OddNumbers(Number);

        Console.WriteLine();
        Console.WriteLine("Sum of Odds: {0}\n", AdditionalOdd(Number));

        return 0;
    }
}

This would produce:

Odd Numbers
9, 7, 5, 3, 1,
Sum of Odds: 25

Press any key to continue . . .

1. Change the document as follows:

   using System;
   
   public class Calculator
   {
       private long Factorial(long number)
       {
           if (number <= 1)
               return 1;
           return number * Factorial(number - 1);
       }
   
       private long Permutation(long n, long r)
       {
           if (r == 0)
               return 0;
           if (n == 0)
               return 0;
           if ((r >= 0) && (r <= n))
               return Factorial(n) / Factorial(n - r);
           else
               return 0;
       }
   
       private long Combinatorial(long a, long b)
       {
           if (a <= 1)
               return 1;
   
           return Factorial(a) / ((Factorial(b) * Factorial(a - b)));
       }
   
       public static int Main()
       {
           long factor = 0;
           long second = 0;
           Calculator exo = new Calculator();
   
           Console.Write("To calculate a factorial, enter a (small positive) number: ");
           factor = long.Parse(Console.ReadLine());
           Console.Write("To calculate a permutation and the combination, enter a second (small positive) number: ");
           second = long.Parse(Console.ReadLine());
   
           Console.WriteLine("Factorial:   F({0}) = {1}",
               factor, exo.Factorial(factor));
           Console.WriteLine("Permutation: P({0}, {1}) = {2}",
               factor, second, exo.Permutation(factor, second));
           Console.WriteLine("Combination: C({0}, {1}) = {2}",
               factor, second, exo.Combinatorial(factor, second));
   
           System.Console.ReadKey();
           return 0;
       }
   }

2. Execute the application to test it
3. When asked to provide a number, type 20 and press Enter
4. When asked to provide a second number, type 5 and press Enter:

   To calculate a factorial, enter a (small positive) number: 20
   To calculate a permutation and the combination, enter a second (small positive)
    number: 5
   Factorial:   F(20) = 2432902008176640000
   Permutation: P(20, 5) = 1860480
   Combination: C(20, 5) = 15504
   

5. Press Enter to close the DOS window
6. On the main menu, click File -> Close Solution
7. When asked whether you want to save, click No