VB 2010 - Unit Converter

[00brandnew]

I am attempting to make a unit converter that converts between km, m, cm, miles, yards, feet, and inches. I have a textbox that is used to input the value to convert followed by a space and the units (i.e., 56 km) and a second textbox where the user types what unit they want to convert to (i.e., cm). When converting, the output will be displayed in the second textbox. (I know this is complicated, but this is the way it is to be done).
I am doing this using sub and function procedures and If blocks and Select Case blocks (i.e., keeping it as simple as possible).

I'm not sure how to declare the variables because they contain both numbers and letters so I declared as strings (Dim unitFrom, unitTo As String = txtUnitTo.Text, etc.). I then tried to use case blocks where:
Select Case unitTo
Case "cm"
txtUnitTo.Text = CStr(txtUnitFrom.Text) & "cm" [to display the number and unit -- i.e, 5,600,000 cm]
This, however, is not working and I think I am looking at this all wrong. I have also tried using the Double variable but am running into the same problem.

I also need to add a comma every 3 digits in the answer, which i am unsure on how to do. Any help would be appreciated on where to start or how to accomplish this. As an added note, I am a beginning programmer so if explanations could be kept in simple terms, I would appreciate it.

[akhileshbc]

Hi... Welcome to the forums...

I made a small very basic example:

Public Class Form1
 
    '~~~ On clicking the button
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'Variables
        Dim dblInput As Double
        Dim dblOutput As Double
        Dim dblFactor As Double
 
        '~~~ Converting the input entered by user to Double type
        If Double.TryParse(TextBox1.Text, dblInput) = False Then
            MessageBox.Show("Can't convert the input value to Double !")
        Else
            '~~~ Checking what conversion is selected by the user, from the ComboBox
            Select Case ComboBox1.SelectedIndex
                Case 0  '~~~ cm to m
                    dblFactor = 1 / 100
                Case 1  '~~~ m to cm
                    dblFactor = 100
                Case 2  '~~~ cm to mm
                    dblFactor = 10
                Case 3  '~~~ mm to cm
                    dblFactor = 1 / 10
            End Select
 
            '~~~ Do the math
            dblOutput = dblInput * dblFactor
 
            '~~~ Display the result
            MessageBox.Show("Output = " & dblOutput.ToString("###,###,##0.00"))
        End If
    End Sub
 
    '~~~ During loading, fill the ComboBox with some text
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ComboBox1.Items.Add("cm to m")  '~~~ At index 0 of ComboBox
        ComboBox1.Items.Add("m to cm")  '~~~ At index 1 of ComboBox
        ComboBox1.Items.Add("cm to mm") '~~~ At index 2 of ComboBox
        ComboBox1.Items.Add("mm to cm") '~~~ At index 3 of ComboBox
        '..etc..
    End Sub
End Class

Hope you'll get some ideas.

[jmcilhinney]

I disagree with using a single Select statement with a Case for every conversion. Each time you add a unit, the number of Cases grows exponentially. You should pick a single base until and then you just need a single conversion factor for each other unit to that base. Say you pick metres as the base. If the user wants to convert from kilometres to inches, you convert the kilometres to metres and the metres to inches. Three units only need to two conversion factors.

[dbasnett]

Starting point for parsing the user input

        Dim smplTB As String = "1,000.045 km" 'simulated textbox entry
        Dim parts() As String = smplTB.Split(New Char() {" "c}, StringSplitOptions.RemoveEmptyEntries)
        parts(0) = parts(0).Replace(",", "")
        Dim valPart As Double
        Dim unitPart As String
        Dim numDecs As Integer = 0
        If Double.TryParse(parts(0), valPart) Then
            'valid number
            unitPart = parts(1)
            Debug.WriteLine(unitPart & " " & valPart.ToString)
            If parts(0).IndexOf("."c) <> -1 Then
                numDecs = parts(0).Length - parts(0).IndexOf("."c) - 1
            End If
            Debug.WriteLine(numDecs.ToString)
        End If

[dbasnett]

Convert the from value to a common base. Then the to Value can be computed from there.

This code (building on previous post) shows a start at the process

        Dim meters As Double = 0
        'from unit converted to meters
        Select Case unitPart
            Case ""
            Case "km"
                meters = valPart * 1000
            Case "m"
                meters = valPart
            Case "cm"
                meters = valPart / 100
            Case "mm"
                meters = valPart / 1000
            Case "in"
                meters = valPart * 0.00254
            Case "ft"
                meters = valPart * 0.3048
            Case "mi"
                meters = valPart * 1609.3
        End Select

        Dim toUnit As String = "in"
        Dim toVal As Double = 0
        'to unit based on meters
        Select Case toUnit
            Case "in"
                toVal = meters * 39.37
            Case "ft"
                toVal = meters * 3.28
        End Select

http://inotek.com/conversions.html