Positive integer to Roman Numeral conversion function

Question

For the past two hours I was searching for a built-in function for converting an integer to an equivalent Roman numeral, but it failed to achieve the target. Hence I wrote my own routine for performing this conversion. I think it needs some more clarifications.

Public Class Form1
    Dim ind() As Integer = {1, 2, 3, 4, 5, 10, 50, 100, 500, 1000}
    Dim rom() As String = {"I", "II", "III", "IV", "V", "X", "L", "C", "D", "M"}
    Dim limit As Integer = 9
    Dim output As String = ""
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim num As Integer
        output = ""
        num = CInt(txt1.Text)
        While num > 0
            num = find(num)
        End While
        txt2.Text = output
    End Sub
    Public Function find(ByVal Num As Integer) As Integer
        Dim i As Integer = 0
        While ind(i) <= Num
            i += 1
        End While
        If i <> 0 Then
            limit = i - 1
        Else
            limit = 0
        End If
        output = output & rom(limit)
        Num = Num - ind(limit)
        Return Num
    End Function
End Class

Note: This function can only be used for range of 1-999

Answer 1

• This code is bound far too tight to your form. The first thing we need to do is separate the logic for converting an integer to a roman numeral from the event procedures of the form. This will allow us to reuse this code anywhere.

  Module IntegerExtensions
  
      Dim ind() As Integer = {1, 2, 3, 4, 5, 10, 50, 100, 500, 1000}
      Dim rom() As String = {"I", "II", "III", "IV", "V", "X", "L", "C", "D", "M"}
      Dim limit As Integer = 9
      Dim output As String = ""
  
      Private Function ConvertToRomanNumeral(ByVal input As Integer) As String
          output = ""
          While input > 0
              input = find(input)
          End While
          Return output
      End Function
  
      Private Function Find(ByVal Num As Integer) As Integer
          Dim i As Integer = 0
          While ind(i) <= Num
              i += 1
          End While
          If i <> 0 Then
              limit = i - 1
          Else
              limit = 0
          End If
          output = output & rom(limit)
          Num = Num - ind(limit)
          Return Num
      End Function
  
  End Module
  
  Public Class Form1
      Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  
          txt2.Text = IntegerExtensions.ConvertToRomanNumeral(999)        
  
      End Sub
  End Class
  

• Next we need to make sure that the argument passed to ConvertToRomanNumeral is within the accepted range of 1-999. Receiving a Index out of bounds message is not nearly as useful as knowing that ConvertToRomanNumeral doesn't accept the value you passed into it.

  Private Function ConvertToRomanNumeral(ByVal input As Integer) As String
  
      If (input < 1 Or input > 999) Then
          Throw New ArgumentOutOfRangeException("input","Input must be in the range 1-999")
      End If
  
      output = ""
      While input > 0
          input = find(input)
      End While
      Return output
  End Function
  

• ind and rom are not very good variable names. I get that rom stands for Roman Numerals, but I have no idea what ind means. Indices maybe? Also, I like plural names for arrays. Let's replace those. ind >> indices and rom >> romanNumerals.

• There are way too many class level variables. Limit should be strictly declared inside of Find. I also don't find it a good idea to set the value of output from the Find function. I would consider this to be a side effect, and side effects are confusing and hard to debug. The only way for me to properly scope output was to merge Find into ConvertToRomanNumeral.

  Module IntegerExtensions
  
      Private Function ConvertToRomanNumeral(ByVal input As Integer) As String
  
          If (input < 1 Or input > 999) Then
              Throw New ArgumentOutOfRangeException("input","Input must be in the range 1-999")
          End If
  
          Dim indices() As Integer = {1, 2, 3, 4, 5, 10, 50, 100, 500, 1000}
          Dim romanNumerals() As String = {"I", "II", "III", "IV", "V", "X", "L", "C", "D", "M"}
          Dim limit As Integer = 9
          Dim output As String = ""
  
          While input > 0
              Dim number as Integer = input
              Dim i As Integer = 0
              While indices(i) <= number
                  i += 1
              End While
              If i <> 0 Then
                  limit = i - 1
              Else
                  limit = 0
              End If
              output = output & romanNumerals(limit)
              number = number - indices(limit)
              ' this was previously our return statement
              input = number                  
          End While
          Return output
      End Function
  
  End Module
  

• You may have noticed earlier that I named the module IntegerExtensions. That's because I've been planning on making this exactly that the entire time. Extension methods will allow us to call the function directly on any integer, instead of passing an integer into it. It looks a little cleaner when in use. I changed the function name to ToRomanNumberal and adjusted the ArgumentOutOfRangeException message accordingly.

  Module IntegerExtensions
  
      <Extension()>
      Public Function ToRomanNumeral(ByVal input As Integer) As String
  
          If (input < 1 Or input > 999) Then
              Throw New ArgumentOutOfRangeException("input","Cannot convert integers outside of the range 1-999")
          End If
  
          Dim indices() As Integer = {1, 2, 3, 4, 5, 10, 50, 100, 500, 1000}
          Dim romanNumerals() As String = {"I", "II", "III", "IV", "V", "X", "L", "C", "D", "M"}
          Dim limit As Integer = 9
          Dim output As String = ""
  
          While input > 0
              Dim number as Integer = input
              Dim i As Integer = 0
              While indices(i) <= number
                  i += 1
              End While
              If i <> 0 Then
                  limit = i - 1
              Else
                  limit = 0
              End If
              output = output & romanNumerals(limit)
              number = number - indices(limit)
              ' this was previously our return statement
              input = number                  
          End While
          Return output
      End Function
  
  End Module
  

Excercises for you:

• How could this be done using a dictionary instead.
• How can this be changed so that numbers outside of 1-999 can be converted?

Answer 2

To catch up where ckuhn203 got tired, I would like to finish ckuhn203's review.

• Validating the input

  As it is senseless to check wether input > 999 in the case that input < 1 we can use the short-circuiting boolean operator OrElse

  If (input < 1 OrElse input > 999) Then
      Throw New ArgumentOutOfRangeException("input","Cannot convert integers outside of the range 1-999")
  End If
  

• Initializing String variables

  For readability you should initialize String variables by assigning String.Empty.

  Dim output As String = ""
  

  will become

  Dim output As String = String.Empty
  

• Inside the while loop

  This

  If i <> 0 Then
      limit = i - 1
  Else
      limit = 0
  End If
  

  is logically wrong. What would happen if i=-1 ? As it is <>0 limit would become -2. So it needs to be changed to

  If i > 0 Then
      limit = i - 1
  Else
      limit = 0
  End If
  

  next we get rid of the number variable as it isn't needed

  While input > 0
  
      Dim i As Integer = 0
      While indices(i) <= input
          i += 1
      End While
      If i > 0 Then
          limit = i - 1
      Else
          limit = 0
      End If
      output = output & romanNumerals(limit)
      input = input - indices(limit)
  
  End While
  

  And because we have the validation of the input number, I don't see a reason to keep the limit variable at all, so let us remove it

  While input > 0
  
      Dim i As Integer = 0
      While indices(i) <= input
          i += 1
      End While
      If i > 0 Then
          i -= 1
      Else
          i = 0
      End If
      output = output & romanNumerals(i)
      input = input - indices(i)
  
  End While
  

  and because i < 0 can't happen, we simplify the if i > 0 by removing the else part and get the result

  <System.Runtime.CompilerServices.Extension()>
  Public Function ToRomanNumeral(ByVal input As Integer) As String
  
      If (input < 1 OrElse input > 999) Then
          Throw New ArgumentOutOfRangeException("input", "Cannot convert integers outside of the range 1-999")
      End If
  
      Dim indices() As Integer = {1, 2, 3, 4, 5, 10, 50, 100, 500, 1000}
      Dim romanNumerals() As String = {"I", "II", "III", "IV", "V", "X", "L", "C", "D", "M"}
      Dim output As String = String.Empty
  
      While input > 0
  
          Dim i As Integer = 0
          While indices(i) <= input
              i += 1
          End While
          If i > 0 Then
              i -= 1
          End If
          output = output & romanNumerals(i)
          input = input - indices(i)
  
      End While
  
      Return output
  End Function