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# VBA: Getting a random number

Posted on 2013-05-25
Medium Priority
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I am trying to get a random number with 4 significant figures using this formula

num = Round(Rnd(), 4) * 10 ^ Int((Rnd * 10) - 5)

Although the components work out nicely the final result comes up with lesser significant figures. How can I get only 4 significant figures in VBA?
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Question by:Saqib Husain, Syed
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LVL 81

Expert Comment

ID: 39196995
num = Round(Rnd() * 10 ^ Int((Rnd * 10) - 5), 4)

Kevin
0

LVL 81

Expert Comment

ID: 39196997
What is the range of values you are seeking? Maximum and minimum?

Kevin
0

LVL 43

Author Comment

ID: 39197000
10^ (+/-5)
0

LVL 81

Expert Comment

ID: 39197012
Does this work?

Round(Rnd() * 10 ^ Int(Rnd() * 10 - 5), 4)

Kevin
0

LVL 81

Expert Comment

ID: 39197022
Something weird is going on. There appears to be a bug in the VBA interpreter.

This:

Round(Rnd() * 10000, 4)

does not round to four places. It doesn't seem to round at all.

This works:

Dim Value As Double
Value = Rnd() * 10000
Value = Round(Value, 4)

Kevin
0

LVL 43

Author Comment

ID: 39197051
Currently I have applied the same formula to an excel cell and used the value of the cell. But my question still stands....how to get it directly through VBA?
0

LVL 81

Expert Comment

ID: 39197080
I showed you how above. That's all VBA dude.

Kevin
0

LVL 43

Author Comment

ID: 39197269
This does not give me the desired results

pwr = Rnd()
pwr = 10 ^ Int((pwr * 10) - 5)
num = Rnd()
num = Round(num, 4)
num = num * pwr
0

LVL 46

Expert Comment

ID: 39200183
Convert to currency data type, which has four decimal places.
Example:
``````num = CCur(Rnd() * 10 ^ Int((Rnd * 10) - 5))
``````
0

LVL 43

Author Comment

ID: 39200221
aikimark, I am looking for numbers in the range

0.00001000 to 999900

and I do not think the currency will do this as 0.00001001 is a possibility.
0

LVL 22

Expert Comment

ID: 39200320
so are you looking for something like this?

cdbl(rnd()*1000000)

sample result:  289562.463760376
0

LVL 17

Accepted Solution

andrewssd3 earned 2000 total points
ID: 39200459
Is this what you want? - just a bit more complication getting the original number, so it gets a random number between 1 and 9999 then multiplies it by 10^2 through 10^-5:
``````Dim pwr As Double
Dim num As Double

pwr = Rnd()
pwr = 10 ^ Int(pwr * 8 - 5)

num = Int(Rnd() * 9999 + 1)
num = num * pwr

Debug.Print num
``````
0

LVL 46

Expert Comment

ID: 39201175
@ssaqibh

You stated that you wanted a "number with 4 significant figures".  That generally means that you want a floating point number with a precision of four digits to the right of the decimal point.

0.00001000 and 0.00001001 go beyond your stated precision.  If displaying such values with a precision of four decimal places, both of these would be zero.
0

LVL 58

Expert Comment

ID: 39201208
Not sure what Excel has built-in, but in Access VBA, I wrote the code below to round to a significant number of digits.  Probably better ways of doing this, but it did achieve the correct results.

Just feed it a random number and I believe it will give you what you want.

Jim.

Public Function RoundSignificant(varValue As Variant, intNumSignificantDigits As Integer) As String

Dim strPrefix As String

Dim dblFactor As Double
Dim dblABSofValue As Double
Dim strFormatedValue As String

Dim lngPos As Long
Dim bolDecimalPoint As Boolean
Dim strChr As String

Dim lngNumOfDigits As Long

' Check for a prefix ('>' or '<').  If there is one,
' strip it off for now.
If left(varValue, 1) = ">" Or left(varValue, 1) < "<" Then
strPrefix = left(varValue, 1)
varValue = Val(Mid(varValue, 2))
Else
strPrefix = ""
End If

' Check for null
varValue = Nz(varValue, 0)

' If value of zero, return "N/A"
If varValue = 0 Then
RoundSignificant = "N/A"
Else

' Get the factor
dblFactor = 10 ^ Int(Log(Abs(varValue)) / Log(10) - intNumSignificantDigits + 1)

' Based on the factor, get an absolute value that's rounded.
If dblFactor = 1 Then
dblABSofValue = CLng((Abs(varValue) / dblFactor))
ElseIf dblFactor > 1 Then
dblABSofValue = CLng((Abs(varValue) / dblFactor) + (0.5 / dblFactor))
Else
dblABSofValue = CLng((Abs(varValue) / dblFactor) + (0.5 * dblFactor))
End If
dblABSofValue = dblABSofValue * dblFactor
' Format the value as a string.
strFormatedValue = Format((IIf(varValue >= 0, 1, -1) * dblABSofValue), "#0.00000000000000000000")

' Do we have a decimal point?
If InStr(strFormatedValue, ".") > 0 Then
' If so, chop off all zeros on the right
While right(strFormatedValue, 1) = "0"
strFormatedValue = left(strFormatedValue, Len(strFormatedValue) - 1)
Wend
End If

' Scan for the number of digits in the string
For lngPos = 1 To Len(strFormatedValue)
strChr = Mid(strFormatedValue, lngPos, 1)

If strChr >= "0" And strChr <= "9" Then
lngNumOfDigits = lngNumOfDigits + 1
End If

If strChr = "." Then
bolDecimalPoint = True
End If
Next

' Is the number of digits found less then the significance required?
If lngNumOfDigits < intNumSignificantDigits Then
' If so and we have decimal point, add some zeros
If bolDecimalPoint = True Then
strFormatedValue = strFormatedValue & String(intNumSignificantDigits - lngNumOfDigits, "0")
End If
End If

' Do we have anything to the right of the decimal?
' If not, remove the decimal point
If right(strFormatedValue, 1) = "." Then
strFormatedValue = left(strFormatedValue, Len(strFormatedValue) - 1)
End If

' Add the prefix back in if we have one
If strPrefix <> "" Then
RoundSignificant = strPrefix & strFormatedValue
Else
RoundSignificant = strFormatedValue
End If

End If

End Function
0

LVL 43

Author Comment

ID: 39201256
rspahitz
sample result:  289562.463760376
Nope Only 4 significant figures

andrewssd3
I still have to test your solution

aikimark
You stated that you wanted a "number with 4 significant figures".  That generally means that you want a floating point number with a precision of four digits to the right of the decimal point.
examples of 4 significant figures are
1234000000
1234000
12340
123.4
12.34
1.234
0.1234
0.001234
0.000001234

JDettman
I am too scared to try your solution unless you confirm that it meets my requirements as per the list ending four lines above
0

LVL 58

Expert Comment

ID: 39201262
FYI, just found the formula for significant rounding:

round(10-nÂ·x)Â·10n, where n = floor(log10 x) + 1 - p

x being a positive number to a precision of p significant digits.  I believe this will work in Excel.  For negatives use the abs() value.

Jim.
0

LVL 58

Expert Comment

ID: 39201269
oops, no super scripts.  That's the round of 10 raised to the negative n times X times 10 rasied to n.

Not sure if that will work or not, but you can try it.

Jim.
0

LVL 58

Expert Comment

ID: 39201280
<<I am too scared to try your solution unless you confirm that it meets my requirements as per the list ending four lines above >>

Just cut and paste into a module and then call it from a cell, but the formula I just posted is a lot more conscise for Excel.

Jim.
0

LVL 46

Expert Comment

ID: 39201575
This will give you a four digit integer.
``````Int(Rnd() * 1000)
``````

It is up to you to multiply it by some power of 10 that will move the decimal place to the right or left.  In your example, you wanted the power raised (decimal moved) from +2 (right) to -8 (left).
``````Num = Int(Rnd() * 1000) * 10^(Int(Rnd() *11)-8)
``````

Note: your Num variable should be a Double data type.
0

LVL 17

Expert Comment

ID: 39201656
Hi - I have just tested my solution in 100,000 iterations, and it gives

Max - 999900
Min - 0.00001

Which I think is what you wanted.
0

LVL 22

Expert Comment

ID: 39201682
So you want 4 digits, but randomly placed within powers of 10?
Int(Rnd()*10000) will produce a number from 0 to 9999 (although some times it may round to 10000 so you may want a separate check for that).

then just multiple or divide by powers of 10.

num= Int(Rnd()*10000) * 10 ^ Int((Rnd * 10) - 5)

However, sometimes you might get random numbers like 0123 so does that count as only 3 significant digits?  If so you really want a range of 1000-9999 so here:
``````num= Int(Rnd()*9000+1000) * 10 ^ Int((Rnd * 16) - 8)
``````
Sample data from the above:

0.004129
93110
0.0009643
67.09
0.0005886
62970000
285400000
140.4
3.291
311.7

I just noticed that sometimes you will get less than 4 digits...that's because after multiplying by 9000, the right-most digit might be zero.  I guess you can prevent that by ensuring that your resulting number after adding 1000 is not divisible by 10.  the easiest way to handle that is to simply generate a new number since 90% of the time you'll be okay.

``````Do
tempNum=Int(Rnd()*9000+1000)
Loop Until (tempNum Mod 10)<>0
num= tempNum * 10 ^ Int((Rnd * 16) - 8)
``````
0

LVL 22

Expert Comment

ID: 39201769
If it helps, here's what I used for testing:

``````    Dim num As Double
Dim origNum As Double
Dim tempNum As Double
Dim iCntr As Integer

For iCntr = 1 To 100
Do
origNum = Rnd
tempNum = Int(origNum * 9000 + 1000)
'If tempNum Mod 10 = 0 Then
'    Debug.Print ;
'End If
Loop Until (tempNum Mod 10) <> 0
num = tempNum * 10 ^ Int((Rnd * 16) - 8)
Debug.Print origNum; tempNum; num 'Int(num * 9000 + 1000) * 10 ^ Int((Rnd * 16) - 8)
Next
``````

The Immediate window will show the original number, the number before the power (but after multiplying by 9000 and adding 1000, then the final number.

Sample results:

0.775829255580902  7982  798200000
0.893206655979156  9038  903.8
0.255511462688446  3299  3.299
0.769428551197052  7924  792400000
0.173246800899506  2559  25.59
0.314691126346588  3832  0.003832
0.651410758495331  6862  0.00006862

So this will produce a 4-digit number where neither the leading nor trailing digit is zero, since apparently that is not considered significant.  i.e. 1230 is no good because it only has 3 significant digits.  Likewise for 0.01230 but okay for 1001.
0

LVL 43

Author Closing Comment

ID: 39201919
First solution that passes.

Thanks
0

LVL 22

Expert Comment

ID: 39202086
Good idea, but it will fail on numbers that are powers of 10, like 9370, which might appear as 9370000 or .00937
It will also produce numbers from 1 to 999, which will leave you without those "4 significant digits"
0

LVL 43

Author Comment

ID: 39202109
rspahitz, those numbers are part of the range of 4 significant figures. In one of my clarifications I specified the range

0.00001000 to 999900

which indicates that 0.00001000 is a member.
0

LVL 22

Expert Comment

ID: 39202116
not to argue, but then that means that 0.00001 is valid, since it's the same as 0.00001000 (but formatted differently.)
0

LVL 43

Author Comment

ID: 39202143
No problem with getting things clarified.

Yes it is valid.
0

LVL 43

Author Comment

ID: 39202154
My original question says

How can I get only 4 significant figures in VBA?

Which probably should have been

How can I get not more than 4 significant figures in VBA?

to eliminate the confusion.
0

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