I recently needed a way to convert decimal values into their fraction counterparts. A search on Google turned up this little gem: http://www.geocities.com/oosterwal/c...onversion.html originally written for VBA/VB6. Naturally I'm working in .NET, so I converted the code to VB.NET.

VB Code:
  1. Function dec2frac(ByVal dblDecimal As Double) As String

  2.         '

  3.         '   Excel function to convert decimal values to integer fractions.

  4.         '

  5.         '   Original Written by:     Erik Oosterwal

  6.         '   Converted to .NET by:     Chris Anderson

  7.         '   Started on:     November 16, 2006

  8.         '   Completed on:   November 16, 2006

  9.         '   Converted on:   February 25, 2007

  10.         '

  11.  
  12.         Dim intNumerator, intDenominator, intNegative As Integer

  13.         ' Declare integer variables 

  14.         Dim dblFraction, dblAccuracy As Double

  15.         ' Declare floating point variables as double precision.

  16.         Dim txtDecimal As String

  17.         ' need a string representation of the input value

  18.         ' in order to determine the required accuracy.

  19.  
  20.         ' Find the accuracy needed for the output by checking the number of digits behind the decimal point

  21.         '   of the input value.

  22.         '

  23.         '    dblAccuracy = 1 / 10 ^ (Len(CStr(dblDecimal - Fix(dblDecimal))) - 2)

  24.         '

  25.         '   While the formula above should work, there is a serious error in the way Excel handles

  26.         '   decimal numbers and there's a huge rounding error issue.  Subtracting the int() of

  27.         '   12.1 from 12.1 produces 0.0999999999 or something similar.  Obviously that won't

  28.         '   work for our desired accuracy of the magnitude of the fractional part of the number

  29.         '   so a slower more cumbersome method has to be used...

  30.  
  31.         dblAccuracy = 0.1                                       ' Set the initial Accuracy level.

  32.         txtDecimal = dblDecimal.ToString                        ' Get a  string representation of the input number.

  33.  
  34.         For i As Integer = 0 To (txtDecimal.Length - 1)                                 ' Check each character to see if it's a decimal point...

  35.             If txtDecimal.Substring(i, 1) = "." Then                    ' if it is then we get the number of digits behind the decimal

  36.                 dblAccuracy = 1 / 10 ^ (txtDecimal.Length - i)    '   assign the new accuracy level, and

  37.                 Exit For                                            '   exit the for loop.

  38.             End If

  39.         Next

  40.  
  41.         intNumerator = 0                                ' Set the initial numerator value to 0.

  42.         intDenominator = 1                              ' Set the initial denominator value to 1.

  43.         intNegative = 1                                 ' Set the negative value flag to positive.

  44.  
  45.         If dblDecimal < 0 Then

  46.             intNegative = -1 ' If the desired decimal value is negative,

  47.             '   then set the negative value flag to

  48.             '   negative.

  49.         End If

  50.  
  51.         dblFraction = 0                                 ' Set the fraction value to be 0/1.

  52.  
  53.         Do While Math.Abs(dblFraction - dblDecimal) > dblAccuracy   ' As long as we're still outside the

  54.             '   desired accuracy, then...

  55.             If Math.Abs(dblFraction) > Math.Abs(dblDecimal) Then      ' If our fraction is too big,

  56.                 intDenominator += 1         '   increase the denominator

  57.             Else                                            ' Otherwise

  58.                 intNumerator += intNegative   '   increase the numerator.

  59.             End If

  60.  
  61.             dblFraction = intNumerator / intDenominator     ' Set the new value of the fraction.

  62.  
  63.         Loop

  64.  
  65.         Return intNumerator.ToString & "/" & intDenominator.ToString ' Display the numerator and denominator

  66.     End Function

It's not the prettiest, and it could probably be written a little more streamlined, but it does work. If anyone has any suggestions for improvement, please do.

-tg