#!markdown

# Chapter 4 - Writing, Debugging, and Testing Functions

#!csharp

// allow simplified WriteLine and other console method calls
using static System.Console;

#!markdown

## Writing a times table function

#!csharp

static void TimesTable(byte number)
{
  WriteLine($"This is the {number} times table:");

  for (int row = 1; row <= 12; row++)
  {
    WriteLine($"{row} x {number} = {row * number}");
  }
  WriteLine();
}

#!csharp

TimesTable(6) // semicolon is optional in a .NET notebook

#!csharp

TimesTable(number: 87) // optional to specify the parameter name

#!markdown

## Writing a function that returns a value

#!csharp

static decimal CalculateTax(
  decimal amount, string twoLetterRegionCode)
{
  decimal rate = 0.0M;

  switch (twoLetterRegionCode)
  {
    case "CH": // Switzerland
      rate = 0.08M;
      break;
    case "DK": // Denmark
    case "NO": // Norway
      rate = 0.25M;
      break;
    case "GB": // United Kingdom
    case "FR": // France
      rate = 0.2M;
      break;
    case "HU": // Hungary
      rate = 0.27M;
      break;
    case "OR": // Oregon
    case "AK": // Alaska
    case "MT": // Montana
      rate = 0.0M;
      break;
    case "ND": // North Dakota
    case "WI": // Wisconsin
    case "ME": // Maine
    case "VA": // Virginia
      rate = 0.05M;
      break;
    case "CA": // California
      rate = 0.0825M;
      break;
    default: // most US states
      rate = 0.06M;
      break;
  }

  return amount * rate;
}

#!csharp

decimal taxToPay = CalculateTax(149, "FR"); 
WriteLine($"You must pay {taxToPay} in tax.");

#!markdown

## Converting numbers from cardinal to ordinal

#!csharp

/// <summary>
/// Pass a 32-bit integer and it will be converted into its ordinal equivalent.
/// </summary>
/// <param name="number">Number is a cardinal value e.g. 1, 2, 3, and so on.</param>
/// <returns>Number as an ordinal value e.g. 1st, 2nd, 3rd, and so on.</ returns>
static string CardinalToOrdinal(int number)
{
  switch (number)
  {
    case 11: // special cases for 11th to 13th
    case 12:
    case 13:
      return $"{number}th";
    default:
      int lastDigit = number % 10;

      string suffix = lastDigit switch
      {
        1 => "st",
        2 => "nd",
        3 => "rd",
        _ => "th"
      };
      return $"{number}{suffix}";
  }
}

#!csharp

static void RunCardinalToOrdinal()
{
  for (int number = 1; number <= 40; number++)
  {
    Write($"{CardinalToOrdinal(number)} ");
  }
  WriteLine();
}

#!csharp

RunCardinalToOrdinal()

#!markdown

## Calculating factorials with recursion

#!csharp

static int Factorial(int number)
{
  if (number < 1)
  {
    return 0;
  }
  else if (number == 1)
  {
    return 1;
  }
  else
  {
    checked // for overflow
    {
      return number * Factorial(number - 1);
    }
  }
}

#!csharp

static void RunFactorial()
{
  for (int i = 1; i < 15; i++)
  {
    try
    {
      WriteLine($"{i}! = {Factorial(i):N0}");
    }
    catch (System.OverflowException)
    {
      WriteLine($"{i}! is too big for a 32-bit integer.");
    }
  }
}

#!csharp

RunFactorial()

#!markdown

## Using lambdas in function implementations

#!csharp

static int FibImperative(int term)
{
  if (term == 1)
  {
    return 0;
  }
  else if (term == 2)
  {
    return 1;
  }
  else
  {
    return FibImperative(term - 1) + FibImperative(term - 2);
  }
}

#!csharp

static void RunFibImperative()
{
  for (int i = 1; i <= 30; i++)
  {
    WriteLine("The {0} term of the Fibonacci sequence is {1:N0}.",
      arg0: CardinalToOrdinal(i),
      arg1: FibImperative(term: i));
  }
}

#!csharp

RunFibImperative()

#!csharp

static int FibFunctional(int term) => 
  term switch
  {
    1 => 0,
    2 => 1,
    _ => FibFunctional(term - 1) + FibFunctional(term - 2)
  };

#!csharp

static void RunFibFunctional()
{
  for (int i = 1; i <= 30; i++)
  {
    WriteLine("The {0} term of the Fibonacci sequence is {1:N0}.",
      arg0: CardinalToOrdinal(i),
      arg1: FibFunctional(term: i));
  }
}

#!csharp

RunFibFunctional()