[erlang-questions] On OTP rand module difference between OTP 19 and OTP 20
Richard A. O'Keefe
Fri Sep 1 04:57:49 CEST 2017
On 1/09/17 6:35 AM, Michael Truog wrote:
> As I argued in the original pull request for these recent 20.0 random
> number changes, a uniform distribution is much more intuitive if it is
> inclusive: [0,1]
Intuitive? For integers, I'll grant that. For reals? Not so much.
I certainly cannot grant "MUCH more intuitive".
I've had occasion to do (random next * 2 - 1) arcTanh, which of
course breaks down if *either* 0 or 1 is returned by (random next).
> For example, if you are dealing with probabilities, it is simpler to
> think in percentages from 0.00 to 1.00
Actually, when I'm dealing with probabilities, I never think
about them as percentages. Now the interesting thing here
is this. Suppose you want to get a true [false] outcome
with probability p [1-p]. Then random next < p does the
job perfectly, but ONLY if 1 is excluded.
The trick of generating a random integer from 1 to N by
doing (in C): (int)(random() * N) + 1 can of course give
you N+1 if random() can return 1.0, and this is a thing I very
often do. (Yes, if 0.0 is excluded, the probability of getting
1 is very slightly skewed, but it's _very_ slightly.)
> An example from the python documentation is at
> https://docs.python.org/3/library/random.html#random.uniform though they
> have ambiguity about the highest value due to a rounding problem they have.
Oh, the bit where they say "The end-point value b may or may not be
included in the range." Worst of both worlds. You cannot rely on it
being included and you cannot rely on it being excluded.
Let's face it, the usual expectation is that a uniform random number
generator will return a value in the half-open range [0,1).
I have uses for (0.0, 1.0).
Michael Truog has uses for [0.0,1.0], although I wasn't able to tell
from a quick scan of his code what they are.
I could personally live with a warning in the documentation that says
that the random number generator could return 0.0, and here's a little
loop you might use to avoid that, and another suggestion in the code
about how to get the result Michael Truog wants.
I just want it to be obvious that it's dangerous to assume that the
result will not be 0.
By the way, given that a common way to make random floats is to
generate a bitvector, consider
(0 to: 15) collect: [:each | ((each / 15) * 256) truncated].
You will notice that the spacing between the values is *almost*
uniform, but not at the end.
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