Help with arrays

[Erexshin]

Hey guys, brand new to this forum. I'm not sure if I'm posting in the right place.
I have looked in my textbook and searched online and cannot find anything that fits my situation. I know this is easier than I'm making it, but I'm new to coding and really need some help. For this project I am supposed to put theses particular values into an array {scores(19)}. Then I am supposed to write the program to find the mean and standard deviation of the values in the array. Finding the mean was easy. However, the standard deviation is a little more difficult. The formula is as follows:

SD = math.sqrt((x(1) - m)^2 + (x(2) - m)^2)...(x(20) - m)^2)/n)

where x is the array, (1,2...etc) is the element in the array, m is the mean and n is the total number of elements in the array.

My problem is figuring out how to tell VB to handle each element separately. I have no problem writing a loop to add the elements together, but I'm having big issues using each element as a standalone value.

Please help, please let me know if this is answered somewhere else, and please let me know if I need to clear this up. This project is due monday, November 30, 2009 and school is closed until then for the holidays.

Thanks for the help guys,
Erexshin

[jcis]

..I'm having big issues using each element as a standalone value...

What do you mean? you can use the elements in the array as you did in the formula: x(1) is element 1 in the array, x(2) is element 2 in the array (this assuming you loaded your Array starting from element 1, you could start at 0 since arrays are 0 based)

[Erexshin]

That is one way to do it. But, let's say this array was dynamic, and grew to have more than 20 elements; that formula would no longer work as it would only find the standard deviation of the first 20 elements. How would I write it to go from Lbound to Ubound and plug each element into the formula?

[jcis]

Ok, i understand, see this code, it fires with a CommandButton..

Private Const Items As Long = 19
Private x(Items) As Long

Private Sub Command1_Click()
Dim Mean As Long, SD As Long

    LoadArray
    Mean = CalculateMean
    SD = CalculateStandardDeviation(Mean)
    
    MsgBox SD
End Sub

Private Sub LoadArray()
Dim i As Long

    For i = 0 To UBound(x)
        x(i) = i 'The  Array will be loaded with numbers from 0 to 19
    Next
End Sub

Private Function CalculateMean() As Long ' Returns Mean
Dim i As Long, AuxVar As Long

    For i = 0 To UBound(x)
        AuxVar = AuxVar + x(i)
    Next
    AuxVar = AuxVar / Items
    CalculateMean = AuxVar
End Function


Private Function CalculateStandardDeviation(m As Long) As Long  ' Returns SD
Dim i As Long, AuxVar As Long, SD As Long

    For i = 0 To UBound(x)
        AuxVar = (x(i) - m) ^ 2
        SD = SD + AuxVar
    Next
    SD = Sqr(SD / Items)
    CalculateStandardDeviation = SD
End Function

20 items in an array that go from 0 to 19. I'm not sure if the SD is being calculated correctly, but at least you can get the idea on how to do it. Tell me if it works.

[Erexshin]

Thank you for the quick replies. I will try this out first thing in the morning. Gotta get some sleep. Thanks again and I will definitely post tomorrow regarding this.

[Ellis Dee]

Your formula for calculating standard deviation is wrong; you're supposed to divide the total of the squares by the number of elements (minus one) before you get the square root.

These functions will take any numeric array (Integer, Long, Currency, Single, Double) and return the mean and stddev as doubles.

Public Function Mean(pvarArray As Variant) As Double
    Dim lngCount As Long
    Dim dblTotal As Double
    Dim i As Long
 
    If Not IsArray(pvarArray) Then Exit Function
    For i = LBound(pvarArray) To UBound(pvarArray)
        lngCount = lngCount + 1
        dblTotal = dblTotal + pvarArray(i)
    Next
    Mean = dblTotal / CDbl(lngCount)
End Function
 
Public Function StdDev(pvarArray As Variant, pdblMean As Double) As Double
    Dim lngCount As Long
    Dim dblTotal As Double
    Dim i As Long
 
    If Not IsArray(pvarArray) Then Exit Function
    For i = LBound(pvarArray) To UBound(pvarArray)
        lngCount = lngCount + 1
        dblTotal = dblTotal + (pvarArray(i) - pdblMean) ^ 2
    Next
    If lngCount > 1 Then dblTotal = dblTotal / CDbl(lngCount - 1)
    StdDev = Sqr(dblTotal)
End Function

Sample usage:

Private Sub Command1_Click()
    Dim lngArray(20) As Long
    Dim dblMean As Double
    Dim dblStdDev As Double
    Dim i As Long

    ' Randomize should only go in your project's entrypoint (Sub Main or Form_Load)
    Randomize
    ' Populate array with random values (0-999)
    For i = LBound(lngArray) To UBound(lngArray)
        lngArray(i) = Int(1000 * Rnd)
    Next
    ' Get mean and stddev
    dblMean = Mean(lngArray)
    dblStdDev = StdDev(lngArray, dblMean)
    ' Display results
    MsgBox "Mean: " & Format(dblMean, "0.000") & vbNewLine & "StdDev: " & Format(dblStdDev, "0.000")
End Sub

[Erexshin]

Just an update. Still working the kinks out on this, and I looked in my textbook; this stuff is found nowhere in the book. It's a good thing I paid $300 for this book.

[jcis]

Just an update. Still working the kinks out on this, and I looked in my textbook; this stuff is found nowhere in the book. It's a good thing I paid $300 for this book.

It's normal having some doubts at the begining, i think now you have enough code to find the solution by yourself, if you can do it that would be the best, it's always better when you write and understand you're own code. I see you bought a book, that's really good, also use google and the forums when looking for info about specific topics, keep pushing and you'll get it.

[Ellis Dee]

What kinks?

[Erexshin]

OK. Thank you all for your help. You guys definitely got me pointed in the right direction. this is my first class in my major and we are only covering the basics of VB. A lot of the things you were showing me was out of our range, so to speak. But, it did help me see what the program was supposed to be doing. All in all, using what we have been taught, this is what I came up with:

Dim scores() As Double = {59, 60, 65, 75, 56, 90, 66, 62, 98, 72, 95, 71, 63, 77, 65, 77, 65, 50, 85, 62} 'Places the scores into the array
    Private Sub btnDisplay_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplay.Click
        Dim add, n, mean, sd As Double 'This declares which variables are to be used as a double data type
        Dim grade As String 'This declares which variables are to be used as a string data type
        add = 0 'assigns a base value of 0 to this variable
        lstDisplay.Items.Clear() 'clears the list box of any old data
        For n = LBound(scores) To UBound(scores) 'loop to establish the limits on the array and assigns the value 20 to the variable n
        Next 'ends the loop
        For a As Double = LBound(scores) To UBound(scores) 'loop to total the exam scores and calculate the standard deviation
            add += scores(a) 'totals the exam scores
            sd = Math.Sqrt((scores(a) - mean) ^ 2 / n) 'formula for finding the standard deviation
        Next
        mean = (add / n) 'formula for finding the mean
        outputInfo(add, mean, sd, n, grade) 'establishes the parameters for the sub procedure
    End Sub
    Sub outputInfo(ByRef add As Double, ByRef mean As Double, ByRef sd As Double, ByRef n As Double, ByVal grade As String)
        Dim fmtStrHeader As String = "{0,-10}{1,12}" 'establishes my string header
        lstDisplay.Items.Add("There were " & n & " exams.") 'displays the total amount of exams
        lstDisplay.Items.Add("Mean: " & mean) 'displays the mean
        lstDisplay.Items.Add("Std. Deviation: " & FormatNumber(sd, 2)) 'displays the standard deviation
        lstDisplay.Items.Add("") 'skips a line
        lstDisplay.Items.Add(String.Format(fmtStrHeader, "Score", "Grade")) 'displays the columns for score and grade
        For m As Integer = LBound(scores) To UBound(scores) 'Loop for establishing which letter grade goes with which scores
            If scores(m) >= mean + (1.5 * sd) Then 'determines what score gets an A
                grade = "A"
            ElseIf mean + (0.5 * sd) <= scores(m) And scores(m) < mean + (1.5 * sd) Then 'determines what score gets a B
                grade = "B"
            ElseIf mean - (0.5 * sd) <= scores(m) And scores(m) < mean + (0.5 * sd) Then 'determines what score gets a C
                grade = "C"
            ElseIf mean - (1.5 * sd) <= scores(m) And scores(m) < mean - (0.5 * sd) Then 'determines what score gets a D
                grade = "D"
            ElseIf (scores(m) < mean - (1.5 * sd)) Then 'determines what score gets a F
                grade = "F"
            End If
            lstDisplay.Items.Add(String.Format(fmtStrHeader, scores(m), grade)) 'displays the scores with their corresponding grade
        Next 'closes the loop
    End Sub 'tells the program where the sub procedure ends

I hope that is readable. Anyways, its working and you guys definitely helped. Thanks a ton.

[Erexshin]

Also, as for the formula for finding the standard deviation, I'm using the formula that they have written in the book. This is a VB book and not a statistics book. Plus, this particular book is riddled with typos. So, in an effort to save myself any extra headache, I am going to use this formula since it is given with the assigned project. Not that your formula doesn't work or isn't good enough, Ellis, its just that this is what the teacher will be using to determine the accuracy of my program. I'm sure you understand. Again, I couldn't have done it without your guidance. Thanks.

[Ellis Dee]

That's VB.NET; this is the classic VB (VB6 and VBA) forum. I'm guessing the things we posted that aren't in your book aren't actually part of VB.NET.

[Erexshin]

Haha. My apologies. I'm such a greenhorn.