How to Convert a Fraction to a Decimal

This little test program has a lot of user input validation.

About 10 years ago, while working as a pipefitter, I was called upon to figure out how to fit a riser pipe onto a reducer. Since the reducer is tapered, there was no normal way to do it.

I had to calculate as a best guess as to what my dimensions would be based on what I already knew about fitting pipe to pipe. It didn’t turn out near as pretty as I would have hoped, but a grinder does wonders to finish getting it closer to where it needed to be.

While I’m currently laid off, even though I haven’t had to do one of these since then, I finally decided to set down and figure out all of the math involved to create the 8 points and lengths needed for half of the riser circle and to be able to add the differences in the taper of the reducer. It took about 3 days working on and off to complete the formula to do all of the calculations required.

Do to the massive amount of calculations required to output the final 8 lengths, I wanted to create a program that does all of the math for the user.

My first hurdle is the user input validation. I need to validate if the person is inputting a decimal, a fraction or some off the wall entry.

Since the normal user would input a fraction, I searched for a premade function on the internet for the fraction to decimal conversion. I found this one here from a March 14, 2008 blog post. You find very few that go from fraction to decimal. Most samples go from decimal to fraction.

After removing an extra EndIf, the code worked fine for converting a fraction to a decimal (see code below).

Private Function FractionToDecimal(ByVal frac As String) As String
    Dim decimalVal As String = "0"
    Dim upper As Decimal = 0
    Dim lower As Decimal = 0
    Dim remain As Decimal = 0
    If frac.IndexOf("/") <> -1 Then

        'End If < ----Extra End if
        If frac.IndexOf(" ") <> -1 Then
            remain = CType(frac.Substring(0, frac.IndexOf(" ")), Decimal)
            frac = frac.Substring(frac.IndexOf(" "))
        End If
        upper = CType(frac.Substring(0, frac.IndexOf("/")), Decimal)
        lower = CType(frac.Substring(frac.IndexOf("/") + 1), Decimal)
        decimalVal = (remain + (upper / lower)).ToString
    End If
    Return decimalVal
End Function

After getting the code to work, I then started thinking about user input validation. I added validation for every type of malformed input I could think of and gave the ability to return an error or return the result of the conversion.

As you can see, the first screenshot is a normal conversion. The second screenshot shows the fraction with the enumerator larger than the denominator, I could have allowed this type of input, but in this case I am catching for when people swap the two numbers.

Now the Final Code

Imports System.Text.RegularExpressions

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) _
           Handles MyBase.Load
        Me.Text = "Convert Fraction to Decimal  " & My.Application.Info.Version.ToString
    End Sub

    Private Sub btnConvert_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles btnConvert.Click
        'Starts at the begining and checks for any upper or lower case letters.
        Dim regx As New Regex("^[a-zA-Z]")
        ' Regx class http://msdn.microsoft.com/en-us/library/
        '                  system.text.regularexpressions.regex(v=vs.90).aspx
        Dim inputNfo As String 
        ' The String from the input text box
        inputNfo = tbUserInput.Text
        Dim outputStr As String = Nothing
        'Here we check if it is a reconised decimal number,
        'if true then just return the decimal number
        If IsNumeric(inputNfo) = True Then
            outputStr = inputNfo
            tbOutput.ForeColor = Color.Green
            lblInputValidation.Text = "Input is Valid"
            lblInputValidation.ForeColor = Color.Green
            ' Verify user is not spelling out the number
        ElseIf regx.IsMatch(inputNfo) = True Then
            outputStr = "Please check you are not Spelling your whole number"
            lblInputValidation.Text = "Input is Not Valid"
            lblInputValidation.ForeColor = Color.Red
            ' if not a decimal, verify it is not a malformed fraction
        ElseIf inputNfo.Contains("/") And inputNfo.Contains(".") = False Then
            Dim ReturnString As String
            ReturnString = FractionToDecimal(inputNfo)
            'this part checks in the function if the enumerator is larger than the denominator
            If ReturnString = "Error Please Check Fraction" Then
                outputStr = ReturnString
                tbOutput.ForeColor = Color.Red
                lblInputValidation.Text = "Input is Not Valid"
                lblInputValidation.ForeColor = Color.Red
            Else
                'If all is good then return the converterd fraction as a decimal
                outputStr = ReturnString
                tbOutput.ForeColor = Color.Green
                lblInputValidation.Text = "Input is Valid"
                lblInputValidation.ForeColor = Color.Green
            End If

            'this checks that the input in not blank
        ElseIf inputNfo = Nothing Or inputNfo = " " Then
            outputStr = "Nothing was input"
            tbOutput.ForeColor = Color.Red
            lblInputValidation.Text = "Nothing was input"
            lblInputValidation.ForeColor = Color.Red
        Else
            'returns this if the input number is realy messed up.
            outputStr = "Input not reconised"
            tbOutput.ForeColor = Color.Red
            lblInputValidation.Text = "Input not reconised "
            lblInputValidation.ForeColor = Color.Red
        End If

        'Returns the final output
        tbOutput.Text = outputStr.ToString
    End Sub

    Private Function FractionToDecimal(ByVal frac As String) As String
        'Code from http://amrelgarhytech.blogspot.com/2008/03/fraction-to-decimal.html 
        ',,Fixed error to get to work.
        Dim decimalVal As String = "0"
        Dim upper As Decimal = 0
        Dim lower As Decimal = 0
        Dim remain As Decimal = 0
        If frac.IndexOf("/") <> -1 Then

            If frac.IndexOf(" ") <> -1 Then
                remain = CType(frac.Substring(0, frac.IndexOf(" ")), Decimal)
                frac = frac.Substring(frac.IndexOf(" "))
            End If

        End If
        upper = CType(frac.Substring(0, frac.IndexOf("/")), Decimal)
        lower = CType(frac.Substring(frac.IndexOf("/") + 1), Decimal)
        If upper > lower Then
            Return "Error Please Check Fraction"
        Else
            decimalVal = (remain + (upper / lower)).ToString
        End If

        Return decimalVal
    End Function

End Class

Starting from the top, we need the “Imports System.Text.RegularExpressions” for checking user input in the second test.

Next, we set inputNfo as a string and equal it to the input text.

Then, we run the inputNfo through the VB.NET built-in IsNumeric function to see if it is a recognized decimal number, if it is, then just return that number to the output textbox. No conversion needs to be done.

If inputNfo is not recognized as a number, we then pass it to the regular expression test and make sure the user is not trying to spell out the number (three instead of 3), if they are, it returns an error, if not, then it passes on to the next test.

The next test would be to find out if the character “/” is in the string and also to make sure the character “.” is not in the string, if it is, then the user input looks something like:

“22. 3/4” , with the decimal in place, it will return an error.

We could add more code to handle that problem and fix it but, why baby the user, make them input the information correctly so they don’t get used to inputting the information incorrectly.

If it passes all of those tests, then the string / number gets passed to the actual function that does the conversion. While in the conversion function we verify that the fraction does not have a larger enumerator than a denominator. Our users should not need to input the fraction in this way. (For the final program that this sample app. was built to test.)

If it fails the test, then it returns an error message, (as in the screenshot above). If it passes, then it returns the result of the Conversion as a decimal.

Conclusion

Again, due to the massive amount of calculations, my final program will require, I need to validate every possible way a user could input a number so the end result would be a decimal, and then I would be able to do the calculations on the input parameters.

If I catch all possible input errors, then any output errors should be easier to trace and fix.

The output of the 8 lengths in my final application will then be converted back from a decimal to a fraction rounded to the nearest 1/16^th of an inch.