Grouping numbers with arrays

[DosBoss]

Hi guys!
Stuck on a bit of homework and hoping someone here could point me in the right direction.

This is the first question I've done (and It works fine btw!),

basically what it's doing is generating 100 random numbers, and then dividing each number by 10 to get a 1 digit number, this number is then used to count how many numbers per group are generated (for example, numbers between 0-9,10-19,20-29...etc) and then displaying them in a list box

 Private Sub btn_generate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_generate.Click
        Dim generator As Random = New Random(DateTime.Now.Millisecond)
        Dim ageCount(0 To 99) As Short
        Dim ageGroupNo As Short
        Dim Age As Short

        lst_output.Items.Clear()
        lst_output.Items.Add("Age:" + vbTab + "Count:")

        For i = 0 To 99  - For 100 iterations
            Age = generator.Next(0, 100)  - Generate a random num between 0-99
            ageGroupNo = Age \ 10  - Divide number by 10
            ageCount(ageGroupNo) += 1  - Increment Group
        Next


        Dim groups As New List(Of String)

        For i = 0 To 9  - Generates group titles (0-9, 10-19...etc)
            groups.Add(CStr(i * 10) + "-" + CStr((i * 10) + 9))
        Next

        For i = 0 To 9  - Displays data
            lst_output.Items.Add(CStr(groups(i)) + vbTab + CStr(ageCount(i)))
        Next

    End Sub

so that's all well and good, but now the challenge
do the same thing but with 100 Decimal numbers between 1.0 and 2.75.

Private Sub btn_generate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_generate.Click
        Dim generator As Random = New Random(DateTime.Now.Millisecond)
        Dim heightCount(0 To 99) As Decimal
        Dim heightGroupNo As Decimal
        Dim Height As Decimal



        lst_output.Items.Clear()
        lst_output.Items.Add("Height:" + vbTab + "Count:")

        For i = 0 To 99  - For 100 iterations
Generate:
            Height = Math.Round(generator.NextDouble() * 2.75, 2)  - Random Num no greater than 2.75

            If Height < 1.0 Then  - Make sure number is at least 1.0
                GoTo Generate
            End If

            heightGroupNo = (Height / 0.9)  - THIS is the problem part

            heightCount(heightGroupNo) += 1 
        Next

        For i = 0 To 8
            lst_output.Items.Add(CStr(heightCount(i)))
        Next
  

    End Sub

I need the generated decimals to be broken into 9 groups

< 1.20
1.20 - 1.39
1.40 - 1.59
1.60 - 1.79
1.80 - 1.99
2.00 - 2.19
2.20 - 2.39
2.40 - 2.59
>2.60

I've tried converting the decimals to whole numbers and then dividing by 9, but nothing seems to be getting me the desired results, I tried a select/case statement, which worked but obviously inefficient as hell,
I know there's a better way to do it then that!

I get the feeling I'm missing something obvious and perhaps a fresh pair of eyes tomorrow morning will do the trick, but as it stands I'm pulling my hair out :P

thanks for your time, hope you can assist!

[.paul.]

array indices must be integer. here's an improvement for you. be sure to read the comments.

Public Class Form1

    Private Sub btn_generate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_generate.Click
        Dim generator As Random = New Random(DateTime.Now.Millisecond)
        Dim ranges(,) As Decimal = {{1D, 1.19D}, {1.2D, 1.39D}, {1.4D, 1.59D}, {1.6D, 1.79D}, {1.8D, 1.99D}, {2D, 2.19D}, {2.2D, 2.39D}, {2.4D, 2.59D}, {2.6D, 2.75D}}
        Dim heightCount(0 To 99) As Decimal
        Dim heightGroupNo As Integer
        Dim Height As Decimal

        lst_output.Items.Clear()
        lst_output.Items.Add("Height:" + vbTab + "Count:")

        For i as Integer = 0 To 99  '- For 100 iterations

            Do 'improvement here
                Height = CDec(Math.Round(generator.NextDouble() * 2.75, 2))  '- Random Num no greater than 2.75
            Loop While Height < 1D

            heightGroupNo = getIndex(ranges, Height)  '- fixed
            If heightGroupNo = -1 Then i -= 1 : Continue For

            heightCount(heightGroupNo) += 1
        Next

        For i = 0 To 8 'fixed outputting
            lst_output.Items.Add(If(i = 0, "< 1.20", If(i = 8, "> 2.60", String.Format("{0:00} - {1}", ranges(i, 0), ranges(i, 1)))) & vbTab & CStr(heightCount(i)))
        Next

    End Sub

    Private Function getIndex(ByVal d(,) As Decimal, ByVal value As Decimal) As Integer
        For x As Integer = 0 To d.GetUpperBound(0)
            If value >= d(x, 0) And value <= d(x, 1) Then Return x
        Next
        Return -1
    End Function

End Class

[stanav]

Because the differences between groups are not the same (1st group is 1.2 while other groups is only 0.2), you need to use select case or if...elseif...esle statement.

[Shaggy Hiker]

I'm not sure about the exact speed benefit, but what I would try would be generating random numbers between 0 and your maximum size. Basically, if you took all of your categories and multiplied by 100, you'd be converting all of your categories into integers.

You could then make an array of Integer equal to the range of numbers you wanted to generate. Yeah, it might be 1000, or more, but so what, memory is cheap. Your loop would then look like this:

For x = 0 to 99
 yourArray(generator.Next(maxValue)) +=1
Next

Then you'd just sum up your ranges with a series of loops, one per category. The first loop would look like this:

Dim sum as Integer
For x = 0 to 119
 sum += yourArray(x)
Next

The pieces are totally trivial, as there are no conversions and no conditionals in the whole code. The code trades memory for speed. The drawback would be that with only 100 items, a very large number of the array slots would be 0, so you'd be performing a bunch of extra += 0 statements, but you'd have to test it to see whether or not it mattered. I would expect that the first loop would cost more in time than all the others, regardless of the number of categories you used, just because of the cost of the random number generator.

[DosBoss]

Wow, brilliant guys,

thanks a lot to all of you!, you've taught me more than you know in a single post, I really appreciate it!