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

• SoEnLion: Does this help ? (Format numbers to roman numerals) stackoverflow.com/questions/11305822/…
• Bjørn-Roger Kringsjå: Why do you need clarification? This is just a reversed version of your own answer. The code is almost identical. With that being said, it failed with an IndexOutOfRangeException at the first run. Try enter 1234.
• Sujith Karivelil: SoEnLion : defenitly work but need to extend. now the function is capable of provide answer below 1000.
• RubberDuck: The code functions as advertised from 1-999. I can review this. Please leave it open.

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?

• RubberDuck: I would just like to note that there is still a fair bit of improvement that can be made. I was just getting a bit tired.

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