Random number

[kirkm]

Public Function randomValue(top As Integer, low As Integer)
    Randomize    ' Initialize random-number generator.
    randomValue = Int((top - low + 1) * Rnd) + low ' Generate random value between low and top.
    randomValue = Int(top * Rnd) + low
End Function

Which calc is correct? They can't both be. can they ?
If a number wanted between 10 and 22 then it'd be int(13*rnd)+ 10. That's different to the 2nd calc.

 

[plog]

Here's the documentation:

Rnd function (Visual Basic for Applications)

Office VBA reference topic

docs.microsoft.com

Says this is the formula for a number from lowerbound thru upperbound (which means includes both upperbound and lowerbound):

Int((upperbound - lowerbound + 1) * Rnd + lowerbound)

 

[kirkm]

That's a third variety of the same (??) thing. But if it's right I'll use it. It's harder than you'd think to eliminate wrong ones.

 

[theDBguy]

That's a third variety of the same (??) thing. But if it's right I'll use it. It's harder than you'd think to eliminate wrong ones.

Is it a third method, or is it the same as the first one in your original post?

 

[MajP]

Reason that Plog is correct is that rnd gives a number between 0 and 1, but I do not think it includes 0 and 1. So it is some number greate than 0 and less than 1.
if that is the case lets assume the top is 10 and bottom is 0
the 1st formula would give you
Int(10-0 + 1) * Rnd) + 0
or
int(11) * rnd
so this gives you 0 - 10 since int truncates

The second formula is wrong
Int(top * Rnd) + low
int(10 * rnd) + 0

this will give you a value between 0 and 9. Assume the rnd gave you .99999
You multiply by ten and you get 9.999
int gives you
9
you would never get 10.

And agree that is the same formula.

 

[The_Doc_Man]

@MajP - you are correct in that RND excludes certain numbers in its range. The way it is computed, the number can never be 0 and can never be exactly 1.00000...000 either. The number starts (via Randomize) as a bit pattern plucked out of the middle bytes of the time-of-day clock multiplied by the seed number and then truncated. Thereafter, they extract and integer-multiply the middle bits of that number by the seed (again) and drop the results into a floating-point number, bytes 2-4, as the mantissa. Because of "hidden-bit" normalization, if you ever had 0 or 1.0000...000 your next number and all numbers thereafter would 0.

 

[Cronk]

I'd always thought Rnd could return 0. Just checked on the MS documentation

The Rnd function returns a value less than 1 but greater than or equal to zero.

@kirkm

 randomValue = Int(top * Rnd) + low

can return values higher than top

 

[The_Doc_Man]

Rnd can return zero under some circumstances, but I thought it "broke" if it ever reached zero.