[HN Gopher] random(random(random(random())))
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random(random(random(random())))
Author : Alifatisk
Score : 109 points
Date : 2023-05-15 19:23 UTC (3 hours ago)
(HTM) web link (openprocessing.org)
(TXT) w3m dump (openprocessing.org)
| davesque wrote:
| Since `random(x)` returns a random number in [0,x), it will
| always output a number that is less than `x` (the initial value
| of x is 1 in this case). Therefore, the outcome where repeated
| applications of `random(...)` to its own result has little
| shrinking effect is vanishingly small. It's the result of these
| repeated applications that is being subtracted from the length of
| the unit vector that is used to position each pixel. So the
| pixels naturally accumulate around that unit length. No broken
| crypto here.
|
| It's sort of like asking, "What would happen if I multiply a
| bunch of numbers together that are guaranteed to be less than 1
| (and even much less than 1 much of the time)?"
| gabereiser wrote:
| Exactly, like Perlin noise, the output is not indicative of the
| function. Works as intended. The beauty is in the fact that
| when using pseudorandom number generation and the right output
| function, you can simulate a lot of things. Star systems,
| planets, stock market games, aim handicap, and itano circus.
| awegio wrote:
| In more formal terms, since it is a uniform distribution you
| can write random(x) = random()*x
|
| then you can easily see that
| random(random(random())) = random()*random()*random()
|
| The resulting distribution is not equal to random()^3, because
| e.g. the probability that all 3 random calls give you a number
| <0.5 is only 0.125.
| muti wrote:
| Fork wrapping the random radius in sqrt() so the first disk is
| uniformly distributed
|
| https://openprocessing.org/sketch/1929418
| chkas wrote:
| Shameless copied:
| https://easylang.dev/ide/#run=textsize%203%0Afor%20i%20%3D%2...
| jtsiskin wrote:
| This would be way clearer as a Cartesian plot with the output of
| random as X, and sample number as Y.
| dathinab wrote:
| It's a funny one.
|
| On the first look it looks that random is broken then you
| realize, no random(n) is a random value between [0;n) so it's
| behaving exactly right.
|
| It also shows really nice how "naively" choosing random points in
| a non rectangular shape can easily lead to a non uniform
| distribution.
| SideQuark wrote:
| Lots of math all over the place computing the expected value :)
|
| So, here's a derivation: if rand(x) returns uniformly a real in
| [0,x) or equivalently for expected value, in [0,x], then here are
| the first few iteration expected values from which you can see
| the general answer. 1 term E(r) = (1/1) *
| Integral_0^1 x_0 dx_0 = (x_0^2/2) from 0 to 1 = 1/2
|
| 2 terms E(r(r)) = (1/1) * Int_0^1 (1/x_0)
| Int_0^x_0 x_1 dx_1 dx_0 = 1/4
|
| 3 terms: E(r(r(r))) = (1/1) * Int_0^1 (1/x_0)
| Int_0^x_0 (1/x_1) Int_0^x_1 x_2 dx_2 dx_1 dx_0 = 1/8
|
| And so on. The nth term is exactly (1/2)^n
| tbalsam wrote:
| What language? What are the parameters for the random function
| here (seed? upper range value in a uniform distribution? stddev
| for a random distribution?) Why? What is this demonstrating?
|
| I can grok a lot of the dimensional effects of randomness but
| without these things specified, the picture is rather meaningless
| to me.
| maxbond wrote:
| 1. The language is Processing
|
| 2. The argument is the high end of the range (0 to n
| [https://processing.org/reference/random_.html])
|
| 3. I dunno why, it seems like it's just cool. It seems to
| demonstrate that random(random(...)) collapses to 0, which is
| exactly what I expected but it's pretty cool.
| justinpombrio wrote:
| Q4: What is it plotting? `random()` returns a float, right?
| Is it plotting... r*cis(theta), where r is the random
| invocation and theta varies uniformly across the circle,
| normalized so that all the circles are the same size (where
| cis(theta) = (cos(theta), sin(theta))?
| maxbond wrote:
| Nah, it's plotting polar vectors, and it uniformly picks an
| angle. The random(...) is just the magnitude of the vector.
| justinpombrio wrote:
| FYI, r*cis(theta) _is_ a polar vector. So it's the thing
| I said, but with the angle chosen randomly.
| [deleted]
| tbalsam wrote:
| I'm a little confused then what the value of this
| demonstration is.
|
| The expectation marches by half the distance towards the
| outer edge each time, so it becomes a soap bubble at a rate
| of 1/2^n
|
| Which is nice and sorta cool but I'm not quite sure what the
| "whoa" factor here is. It's like one of those zip cinch tops
| for those bags. Sorta cool how it works on observation,
| really cool in detail, but also sorta...meh? At the same
| time?
|
| Maybe I'm a bit jaded here? Admittedly, a continuous
| animation of this would be an improvement on the cool factor
| to me, personally.
| maxbond wrote:
| Well, all I can tell you is that I'm having a good time
| discussing the nitty gritty of it in the comments here, and
| it made the gears of my mind turn in a pleasant way. It
| didn't make me go, "wow," but it did make me go, "hmm...."
| tbalsam wrote:
| Gotcha, yeah, that makes sense to me now.
|
| Maybe the real article was the HN comments section that
| we made along the way....
| walterbell wrote:
| _> Maybe the real article was the HN comments section
| that we made along the way...._
|
| Nominating this comment for
| https://news.ycombinator.com/highlights
| [deleted]
| sobellian wrote:
| I think the sequence tends toward the edge at exp(-n), not
| 2^-n. The distance from the edge is a product of n I.I.D.
| variables, so the logarithm is a sum of n I.I.D. variables,
| and the central limit theorem gives us the result.
|
| You can confirm in a python terminal (or anything else)
| that the product of n `random()`s decreases more rapidly
| than 2^-n.
|
| So maybe there's some value in it after all :)
| tbalsam wrote:
| How do you mean?
|
| I'm going based on the propagation of expectations
| through the system.
|
| The expectation of a uniform distribution is half the
| bound. Unless there's some math shenanigans going on, I
| believe that the expected expected value of a
| distribution drawn from distributions should be 1/2 * 1/2
| * 1 in this case.
|
| Of course it's not a bound but right now I'm having a
| hard time determining otherwise.
|
| Is there a mathematical motivation for e^-n here? That
| seems an odd choice since the uniform distribution is 0-1
| bounded generally. However I could understand if it
| emerges in a few conditions.
| sobellian wrote:
| e shows up because we're doing an iterated product (of
| random variables, but still). If you look at the central
| limit theorem, sum(log(rand())) tends to N *
| E[log(rand())] for large N. Well, what's E[log(rand())]?
| -1!
|
| Like I said, it's fairly easy to test this in a python
| terminal. I encourage anyone who doubts me to try it :)
| tbalsam wrote:
| Alright, I tried it in the terminal and the numbers
| confirmed my earlier math. .5->.25->.125 mean as
| N->infinity. I chained the python uniform function like
| the above code in the originally linked post does and
| averaged the results.
|
| The snark to me and the other commenters is a bit
| unnecessary in either case. I'm not really sure where
| you're getting a natural log or an iterated product of
| random variables in this instance.
|
| Could you perhaps show where you're transforming
| uniform(uniform(uniform(uniform(0, 1)))) into the math
| you're showing above? I'm trying to follow along here but
| am having difficulty connecting your line of reasoning to
| the problem at hand.
| sobellian wrote:
| Not trying to be snarky by any means. I'm sorry if you
| interpreted it that way.
|
| I don't think the difference will show up for small N.
| This is an asymptotes thing. Try it for N = 100, that's
| what I did. For example: >>>
| np.product(np.random.random(100))
| 5.469939152265385e-43 >>> 1 / np.e**100
| 3.7200759760208555e-44 >>> 1 / 2**100
| 7.888609052210118e-31
|
| The underlying thing here is that random(random()) in
| this case is the same as random() * random(). So
| random(random(random(...))) is the same as random() *
| random() * random() and then the analysis goes on. And
| sure, random() * random() has a mean close to 1/4. But
| the dynamics change as N becomes large.
|
| Edit - and just in case you doubt whether random() *
| random() * ... is a valid way of doing this, I also just
| checked the laborious way of doing it:
| >>> def foo(n): ... result = 1.0 ... for
| _ in range(n): ... result =
| np.random.uniform(0, result) ... return result
| ... >>> foo(100) 1.4267531652344414e-46
| >>> foo(100) 7.852496730908136e-49 >>>
| foo(100) 1.3216070221780724e-41
| tbalsam wrote:
| > I'm sorry if you interpreted it that way.
|
| is probably a good marker that it is time for me take my
| bow in this conversation, however, for an alternative
| approach I recommend SideQuark's comment on iterative
| random variables vs chains of multiplied random
| variables, which have different characteristics when
| defined as a sequence.
| sobellian wrote:
| I checked his comment, I think he's incorrect on the
| approaches being different. Using my function `foo` that
| does the iterative approach, we can compare the
| distributions and they're fairly identical.
| >>> np.mean([foo(100) for _ in range(100000)])
| 3.258425093913613e-33 >>>
| np.mean([np.product(np.random.random(100)) for _ in
| range(100000)]) 8.814732867008917e-33
|
| (There's quite a bit of variance on a log scale, so 3 vs
| 8 is not a huge difference. I re-ran this and got varying
| numbers with both approaches. But the iterated code is
| very slow...)
|
| Note that the mean is actually quite a bit closer to
| 2^-100 even though the vast majority of numbers from
| either distribution fall below 2^-100. Even so, the mean
| for both is approximately a factor of 100 less than
| 2^-100. Suspicious! Although I think we've both burned
| enough time on this.
| SideQuark wrote:
| >The distance from the edge is a product of n I.I.D.
| variables
|
| It is not a product of random variables; it is an
| iterated random variable. The output of one influences
| those higher in the chain. Redo your python code with
| rand(rand(rand...))) not rand() * rand() * rand()...
|
| The expectation of composition of functions is not the
| composition of expectations, so there is some work to do.
|
| For uniform over [0,1] for the innermost item, it becomes
| an iterated integral, with value (1/2)^n.
| sobellian wrote:
| I did redo it, and the distributions are identical. It's
| just a very heavily skewed distribution, so the expected
| value is not very intuitive. I still think even E[x]
| decreases faster than 2^-n though. See my other comments.
| Nevermark wrote:
| Except that your logarithms can have any consistent base,
| including 2.
|
| So I don't think using a logarithm to introduce a special
| dependence on e is warranted in this case.
| sobellian wrote:
| Well the expected value of E[log2(rand())] is not the
| same as E[log(rand())] and therein lies the difference.
| See my sibling comment.
|
| Like I said, you can check this in a python terminal.
| nimih wrote:
| Speaking personally, a webpage with nothing beyond a cool
| or interesting sequence of images is already in the 90th
| percentile of good HN posts.
| [deleted]
| [deleted]
| jlhawn wrote:
| This is a drawing app using P5.js
|
| Click the "</>" at the top to read the code.
|
| There's a popular "Let's Code" YouTube channel called The
| Coding Train that uses it. It's quite acccessible.
| dist-epoch wrote:
| The documentation suggests random() without arguments is not
| supported random(high) random(low,
| high)
|
| https://processing.org/reference/random_.html
| TaylorAlexander wrote:
| Oooh it's a high value, I thought it was a seed and that this
| was demonstrating a flaw in the algorithm.
| tbalsam wrote:
| Same, at first at least.
| romwell wrote:
| It's almost criminal to post things like that without any
| annotations.
|
| The first picture plots points where radius and angle (in polar
| coordinates) are uniformly distributed. Of course, this is _not_
| uniform in the disc.
|
| I don't have the time right now, but if I do later, I'll type up
| the math behind what a uniform distribution in the disc looks
| like, and what's going on in other pictures.
| [deleted]
| klyrs wrote:
| the bigger crime is that this is plotting
| 1-Random(Random(Random(Random())))
|
| and not Random(Random(Random(Random())))
| fsckboy wrote:
| just in terms of pure randomness (which is not what this is
| testing), random(random()) doesn't make more sense than random().
| If your random number generator is good, one is enough. If it's
| not good, multiple times is not going to help.
|
| I'm putting this in the past tense, it randumb enough times for
| me.
| the_af wrote:
| The argument to random() in this case is the upper bound, so
| `random()` and `random(random())` are different.
|
| See: https://processing.org/reference/random_.html
| alhirzel wrote:
| Yep, just about what one would expect from randomly cutting the
| top of your uniform distribution off multiple times!
|
| What's interesting is the transforms used to sample in some new
| space from e.g. uniform random inputs. For instance, disks [1] or
| hemispheres [2] or an arbitrary density function [3]. Common
| stuff in light transport simulation, and easy to get wrong.
|
| [1]:
| https://stats.stackexchange.com/questions/481543/generating-...
|
| [2]: https://alexanderameye.github.io/notes/sampling-the-
| hemisphe...
|
| [3]: https://en.wikipedia.org/wiki/Inverse_transform_sampling
| maxbond wrote:
| If you're wondering why the dots approach the circumference and
| not the center, the length of the vector is `diameter * (1 -
| random(random(...)))`
|
| It's interesting that the second circle is a pretty uniform
| distribution. `1 - random(random())` must be approximately
| `sqrt(random())`, iirc that's how you correct for the error
| demonstrated in the first circle
| (https://youtube.com/watch?v=4y_nmpv-9lI).
| bonzini wrote:
| ... That means each point is at a distance of sqrt(f(x)) from
| the circumference. And nesting random() n times is similar to
| random()^n, which explains why the first plot has them amassed
| in the center and the second has a more uniform distribution.
|
| Why are they similar? For a rough idea, random() is a random
| number between 0 and 1, which on average is 0.5;
| random(random()) is a random number between 0 and on average
| 0.5, which on average is 0.25; and so on.
| ozfive wrote:
| Oh man if my math teachers taught math with application, I
| would have been so much better off. It's so much easier to
| understand all of this after writing code for a while.
| avodonosov wrote:
| A better title would be "1 - Random(Random(Random(Random())))",
| than it's more understandable why the points lean to the
| circumference.
| [deleted]
| contravariant wrote:
| The cumulative distribution of random(random(...)) repeated k
| times is 1-(1-x)^k, so 1 - random(random()) should have a
| cumulative distribution of x^(2).
|
| I think that should even make it exactly uniform, but somehow
| it doesn't look like it. I may have missed a bit somewhere.
| maxbond wrote:
| Perhaps it looks nonuniform because we're not using literal
| points (our dots have nonzero area), so when they get too
| close they blend together and make the distribution look
| denser than it is in that area
| contravariant wrote:
| Could be, but it is also possible I'm missing something
| somewhere, especially because I can't explain why
| min(random(), random()) looks different from
| random(random()) or random()*random().
| penteract wrote:
| An attempt to explain the difference without just
| calculating the things:
|
| random(random()) has an identical distribution to
| random()*random() (it may even behave identically for a
| given rng state), although this is different to
| Math.pow(random(),2) since in that case there's 100%
| correlation between both parts which makes the expected
| value product bigger.
|
| random(random()) is also distributed equivalently to
| x=random() y=random() while(y>=x)
| y=random() return min(x,y)
|
| (the last line could also read 'return y') Comparing that
| to min(random(),random()), we can see that if the second
| call to random is smaller than the first, they will
| return the same result; otherwise, the program equivalent
| to random(random()) will return a smaller value,
| therefore the expected value of random(random()) must be
| lower that that of min(random(),random()).
| awegio wrote:
| According to the internet what you wrote is the
| distribution of min(random(), random()), but the
| distribution of the product is different.
|
| Product:
| https://math.stackexchange.com/questions/659254/product-
| dist...
|
| Minimum:
| https://stats.stackexchange.com/questions/220/how-is-the-
| min...
|
| I can't explain it either, but I also don't think there
| is a reason they should look the same.
| paulddraper wrote:
| Sorry to be blunt, but what is of interest here?
|
| Aesthetics?
|
| P.S. In case anyone is confused, the arg to random is the upper
| limit (not the seed), and r = diameter * (1 - value)
| kibwen wrote:
| I find it cool that this website lets you upload code for
| visualizations, and then lets any user modify that code and re-
| run it. It's like if Gist let you render images from code,
| rather than from just SVG.
| cheeze wrote:
| Yeah, this was a very fun illustrative view of why this is a
| bad idea.
| edrxty wrote:
| Yeah the code under the </> tab is very important, on first
| blush it looks like random isn't a uniform distribution for
| some reason. It's just because they plotted it with polar
| coordinates. It would be much clearer what's actually going on
| here if an XY plot was used.
| paulddraper wrote:
| No, it's because random(random(random())) is iterating the
| upper limit, not the seed.
| qingcharles wrote:
| This is the answer. I think the author is confused, as most
| platforms do have the seed as the lone parameter to their
| Random function.
| maxbond wrote:
| It is uniform I'm pretty sure, that's what plotting a polar
| coordinates look like when you use a uniform distribution for
| the length.
| edrxty wrote:
| Isn't that what I said?
| maxbond wrote:
| Oh I misunderstood, my bad. Upon rereading I see what you
| meant.
| andybak wrote:
| Most things are interesting.
| utunga wrote:
| This would've been a lot less confusing if they'd used
|
| let radius = d * random(random(random()))
|
| instead of
|
| let radius = d * (1 - random(random(random())))
|
| But, if so, probably so much less confusing that folks wouldn't
| be talking about it here, to be honest.
| [deleted]
| rjbwork wrote:
| Just whipped this up in Linqpad real quick to see how quickly
| random(random(...)) tends to converge to 0.
|
| Fun lil script.
|
| https://media.discordapp.net/attachments/519790546601115649/...
|
| EDIT: Realized I cut off the full script. First two lines are:
| var ran = new Random(); var counts = new
| Dictionary<int,int>();
| thriftwy wrote:
| I like using Math.min(nextInt(), nextInt()) for non-uniformly
| distributed random data.
| [deleted]
| gnramires wrote:
| Tips: The source code can be seen by clicking on '</>'; The
| random() function takes an upper limit of a desired uniform
| distribution (from 0 to x), this shows the effect of the
| distribution of those iterated random functions.
|
| I think this isn't so intuitive because the polar coordinates
| already imply greater density near center (which could be
| corrected with a suitable map).
| em-bee wrote:
| right, if the points are distributed in a square then the
| distribution looks more like what one would expect:
|
| (i am to lazy to register to make a fork, here is the code
| change):
|
| replace: let a = random(Math.PI*2); let r
| = d * (1 - random()); point(cos(a)*r, sin(a)*r);
|
| with: let x = random(d); let y = d * (1 -
| random()); point(x, y);
|
| in every block
|
| you could even simplify it to: let x =
| random(d); let y = random(d); point(x, y);
|
| and for more fun apply the nested random() call to both axis:
| let x = random(random(d)); let y = random(random(d));
| point(x, y);
| acjohnson55 wrote:
| The reason it's counterintuitive is because the randomly chosen
| distance variable is 1 minus the nested randoms. Each random
| call becomes the upper bound of future random calls, so the
| more times you nest it, the closer to zero you're likely to be.
| Subtracting that from one puts you close to the unit circle.
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