[FIXED] How to generate random number with dot?

Issue

I have classic integer generator of random numbers.

But I need a random number from 0 to three decimal numbers after the first, the smallest is 0.010 and biggest 9.910.

Example: 0.136 OR 1.539

My code function is this:

 Public Function RandomWtring(len As Integer) As String
        Dim chars = "0123456789"
        Dim str_len = len
        Dim randomstring = ""

        For i As Integer = 0 To len - 1
            Dim rNum = Math.Floor(Rnd() * chars.Length)
            randomstring += chars.Substring(rNum, 1)
        Next

        Return randomstring
    End Function

But I dont know how to do the 0.254 example... thx

Solution

Random number generators (RNGs) usually produce a number in the range zero up to (but not including) another number. To generate a random number in a range from min to max, you need to get the size of the range, use the RNG, then scale that back into the desired range.

The VB.NET Rnd() function is not actually a very good RNG; the .NET Random Class is a bit better (there are better in some aspects, but let's not worry about that for this question). All RNGs need some sort of initialisation to start them off, so what you will need to do is have the line

Dim rand As New Random()

somewhere in your code outside the actual function:

Function MyRandomNumber() As String
    Dim min As Decimal = 0.01D
    Dim max As Decimal = 9.91D
    Dim decimalPlaces = 3
    Dim multiplier = CDec(Math.Pow(10, decimalPlaces))

    Dim range = (max - min) * multiplier
    Dim val = rand.Next(0, CInt(range + 1))
    Return (val / multiplier + min).ToString()

End Function

I used variables for the minimum and maximum values, and the number of decimal places needed, just in case you need to change them. Also, it is good programming practise to used appropriately-named variables for things like that so that it is easier to understand what is going on if you go back to the code after a few months.

---

If you need a number rather than a string, change the declaration to

Function MyRandomNumber() As Decimal

and the return line to

    Return val / multiplier + min

Answered By - Andrew Morton

Answer Checked By - David Goodson (PHPFixing Volunteer)