Percent of change calculation VB.NET

Posted on 2010-11-12
Last Modified: 2012-05-10
Attached is a screenshot of what should seemingly be a simple piece of code. I am attempting to calculate percent of change and avoid a divide by zero.....My code to get around the zero works, however i am getting the wrong answers with what i thought to be the base calculation
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    End Sub
    Private Function CalcPercent(ByVal NewVal As Integer, ByVal OldVal As Integer) As Decimal

        If NewVal = 0 And OldVal > 0 Then
            CalcPercent = -100
            Exit Function
        End If

        If NewVal > 0 And OldVal = 0 Then
            CalcPercent = 100
            Exit Function
        End If

        If NewVal = 0 And OldVal = 0 Then
            CalcPercent = 0
            Exit Function
        End If

        CalcPercent = (NewVal - OldVal) / OldVal * 100

    End Function
    Private Sub TestCalc()
        Dim myConnection As SqlConnection
        Dim myCommand As SqlCommand
        Dim myReader As SqlDataReader
        Dim prm As SqlParameter
        Dim NEWVALUE As Integer
        Dim OLDVALUE As Integer
        Dim EXPECTED As String
            myConnection = New SqlConnection(ConfigurationSettings.AppSettings("connectionString"))
            myCommand = New SqlCommand("TestPercentCalcsRTN", myConnection)
            myCommand.CommandType = CommandType.StoredProcedure

            myReader = myCommand.ExecuteReader()

            While myReader.Read
                OLDVALUE = myReader("OLDVALUE")
                NEWVALUE = myReader("NEWVALUE")
                EXPECTED = myReader("EXPECTED")
                Response.Write("OLDVALUE : " + Right("_____________" + Str(OLDVALUE), 10) + "   ")
                Response.Write("NEWVALUE : " + Right("_____________" + Str(NEWVALUE), 10) + "   ")
                Response.Write("CALCULATED : " + Right("_____________" + Str(CalcPercent(NEWVALUE, OLDVALUE)), 10))
                Response.Write("____EXPECTED : " + Right("_____________" + EXPECTED, 10) + "   " + "  <BR> ")
            End While

        Catch ex As Exception


            If Not myReader Is Nothing Then
                myReader = Nothing
            End If

            If Not myConnection Is Nothing Then
            End If

        End Try
    End Sub

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Question by:shelbyinfotech
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LVL 37

Expert Comment

ID: 34123126
You just want
CalcPercent = NewVal/OldVal * 100
LVL 37

Expert Comment

ID: 34123132
Actually, I think you are a little confused. You are getting the percentage increase currently. Going from 50 to 100 is a 100% increase.
LVL 37

Expert Comment

ID: 34123174
Your data is correct, your expectations are wrong.
You don't need
        If NewVal = 0 And OldVal > 0 Then
            CalcPercent = -100
            Exit Function
        End If

It will do that automatically.

        If NewVal > 0 And OldVal = 0 Then
            CalcPercent = 100
            Exit Function
        End If

This shouldn't be 100, it should be infinite. Maybe you should use 99999 or something to make the ouput more indicative of the actual result.

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LVL 37

Expert Comment

ID: 34123183
So ignore my first post. That will give you what the new value is as a percent of the old value.  Your original formula gives you the percentage change.

Author Comment

ID: 34123269
Case 1 is not giving me -5%
LVL 37

Expert Comment

ID: 34123328
Case 1 should not give you 5% If you want from 100% to 95%, that would be a 5% decrease.

When you go from 105% to 100% you are dropping 5 of the 105 so 5/105 is the correct drop amount. Converted to a percent gives 4.76% and some change, just like it is showing.
LVL 37

Accepted Solution

TommySzalapski earned 500 total points
ID: 34123387
You do have a formatting issue though.
Try this
CalcPercent = Round((NewVal - OldVal) / OldVal * 100,2)

Author Closing Comment

ID: 34123795

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