Datagridview formula in a unbound column

Hello People I have a bit of a problem and was wondering if someone could help out. I have a datagridview with 5 bound columns and one unbound column. One of the columns is called total meter readings (Black) and is has all the values of the meter reading recorded throughout the month The purpose of the unbound columns is to calculate the difference between each reading. for e.g. one day, the reading might be 100, then the week after it will be 125. The row in the unbound column should calculate and display 25. and should continue to calculate and display a new difference value every time a new meter readings is entered. So Far I have typed this peace of code

Code:

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For Each DataGridView In dataGridView4.Rows
            Dim i As Integer
            dataGridView4.Rows(i).Cells(5).Value = dataGridView4.Rows(i).Cells(3).Value - dataGridView4.Rows(i).Cells(3).Value
            i += 1
        Next

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and this is the result i'm getting

Attachment 118285

As you can see it looping but the difference is zero. The total meter readings are subtracting themselves. What I want is for the bottom reading to subtract the one above. Can anyone help with this please.

When are you calling this code? after you've populated the dgv or before?
have you refreshed the dgv after you've run your code?

you would be better creating a datatable with all your data in ( and calculations ) then write them to the dgv once you have your figures.

You shouldn't be using a loop at all. You want to do this on a "per row" basis. Handle the CellValueChanged event of the grid, use the e.ColumnIndex property to see if it's one of the source columns that has changed and, if it is, get the data from the source columns, perform the calculation and populate the destination column(s). You use the e.RowIndex property to work with cells all in the same row.

i'm calling it after I populated the dgv using the following code

Code:

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Private Sub btnSearch_Click(sender As System.Object, e As System.EventArgs) Handles btnSearch.Click
        If DateTimePicker1.Value > DateTimePicker2.Value Then
            MsgBox("Your 'start date' is at a later date than your 'end date'.  Please chosose a later 'end date' or a earlier 'start date'")
            Return
        End If

        Me.DsMeterReadings1.Clear()
        Me.SqlSelectCommand1.CommandText = "SELECT ID, date, meterReadingBlack, materReadingColour, calloutReading FROM Table_meterReadings WHERE (machineRef = '" & txtMachineRef.Text & "' AND date BETWEEN '" & DateTimePicker1.Value.ToString("yyyyMMdd") & "' AND '" & DateTimePicker2.Value.ToString("yyyyMMdd") & "')"
        Me.SqlDataAdapter1.Fill(Me.DsMeterReadings1.Table_meterReadings)

        For Each DataGridView In dataGridView4.Rows
            Dim i As Integer
            dataGridView4.Rows(i).Cells(5).Value = dataGridView4.Rows(i).Cells(3).Value - dataGridView4.Rows(i).Cells(3).Value
            i += 1
        Next
    End Sub

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I used the following code when someone wants to look at the meter reading during the last 30 days. This is then supposed yto calculate the difference between the readings taken during that month