20bits - Latest Comments in Random Weighted Elements in PHP

Re: Random Weighted Elements in PHP

Lance — Sun, 06 Jun 2010 23:29:03 -0000

Excellent, exactly what I needed. Doing a genetic algorithm AI and need to pick a "gene" (candidate solution) weighted according to its score or suitability to solving the problem at hand.
The prize, of course, is the opportunity to mate with another gene from the pool. :)
Very sexy stuff we're dealing with here.

Thanks

Re: Random Weighted Elements in PHP

data recovery — Wed, 26 May 2010 09:18:10 -0000

very concise and accurate. Well done

Re: Random Weighted Elements in PHP

Dan Morgan — Fri, 30 Apr 2010 00:42:04 -0000

I found a much simpler way about this : http://www.danmorgan.net/pr...

Re: Random Weighted Elements in PHP

Meta — Fri, 16 Apr 2010 14:45:01 -0000

As far as I know in general case this can be only done via some sort of approximation for float random number. For integers in fixed space it's pretty easy task:
For example I have some random distribution where "1" probability is two times less then "2", "2" two time less then "3" and so on to max of "10".
I fill an array
[probablity of "1", probabilty of ("1" or "2"),...]
then give some number (just to have numbers) to each probability. Numbers will be relative. For simplicity let's say that probablity of "1" is equal to one:
[1,1+2,1+2+4, ..,1+2+4+..+..512] =
[1,3,7,...,1023]
Then use such array as a random map by generating random number between 1 and 1023. i.e in case i = rnd(1,1023) = 5 it maps to 3th cell of random map (3 < 5 <= 7) and leads to number "3" in the result.

Re: Random Weighted Elements in PHP

Andy — Thu, 26 Nov 2009 06:11:06 -0000

Oh! just realised:
4. it should be $r = mt_rand(0,1000) or mt_rand(0,$factor) [NOT mt_rand(1,1000)] as the whole affair is merely a scaling of mt_rand(0,1)

Re: Random Weighted Elements in PHP

Andy — Thu, 26 Nov 2009 05:59:59 -0000

Thanks for this.
A few thoughts, in response to this and comments:
1. Yes, the 1000 factor is arbitrary and should be replaced if not approp to your context.
2. if the same weights are to be used repeatedly in a session, it may be worth setting up array of factored cumulative vals from the original weighted. This makes the loop a straight lookup rather than doing the same cumulative calcs each time.
3. I keep thinking about the inefficiency of the 'bottom to top' comparison order. I wonder how much difference this makes with a large number of elements?

Re: Random Weighted Elements in PHP

crhatfield — Fri, 20 Nov 2009 03:27:44 -0000

Add sorting to further increase accuracy, but at a cost of speed. (not alot though!)

arsort($weights,SORT_NUMERIC);

Re: Random Weighted Elements in PHP

leeoniya — Wed, 18 Nov 2009 14:02:47 -0000

Konrad, your implementation is not accurate. see for yourself. accumulate 100,000 cycles of a 50/50 weight. you will always get A around 49.5 and B around 50.5.

Re: Random Weighted Elements in PHP

Konrad — Tue, 21 Jul 2009 10:20:48 -0000

There are some improvements I noticed when I converted this to Python.
First instead of adding onto the extra variable offset you could just substract from r.
Also replacing 1000 by the sum of all weights makes things more flexible cause you can easily go by just having counts instead of weights also you don't have to make the sum up to 1. And even leave out the multiplication in line 5.
This is what I have implemented in Python

def wchoice(lst):
wtotal = sum(lst)
n = random.uniform(0, wtotal)

for i, weight in enumerate(lst):
if n < weight:
break
n = n - weight
return i

Re: Random Weighted Elements in PHP

Richard Lynch — Fri, 22 May 2009 13:06:37 -0000

As far as I can tell, the choice of 1000 within the function is fairly arbitrary.

I replaced it with 10, 100, 10000, and 42 and got seemingly equally random results (just eyeballing them).

I was careful, however, to provide an array with weights between 0 and 1, and whose sum was 1.
I.e., $ws = array(array(0.1,1),array(0.2,2),array(0.3,3),array(0.4,4));

But I think you'll be in trouble no matter what if your array input does not give a full 100% values.

I'd have to test more to be sure, but it's easier to just validate the input. :-)

Re: Random Weighted Elements in PHP

niabi — Thu, 23 Apr 2009 20:58:03 -0000

This is great, looking for something similar, the only thing that can't quite work on my side, is what if i have about 1,000 elements and I want certain ones to have more weight than others?

I would have to write
A = [1,2-999,1000]

but what about the weight? it would be kind of hard I guess to divide 100 / 1,000 and just give weight to certain items...

any solution for this kind of problem?

something like: in my sql i have 1,000 items, in the table i have 3 rows, ID, NAME, WEIGHT then the items that have a higher number (1,2,3,4 ...) have more chance of showing up, 2 has double the chance of 1 and 3 double the chance of 2 but triple the chance of 1 and so on...

any idea?

Re: Random Weighted Elements in PHP

blueskiwi — Thu, 19 Mar 2009 14:45:16 -0000

for anyone wondering... you need to adjust the random range and the 1000 multiplier according to your weights

ie the min and max rand value should (I think) correspond to the smallest possible weight value and the sum of all your weight values respectively (?)

and if you use a multiplier that would be...

min rand value = smallest possible weight * multiplier
max rand value = sum of all weight values in set * multiplier

Re: Random Weighted Elements in PHP

blueskiwi — Thu, 19 Mar 2009 14:35:08 -0000

awesome, you rock, this is just what I needed, thanks!!

Re: Random Weighted Elements in PHP

Matt — Sat, 14 Feb 2009 02:04:38 -0000

Thank you! Exactly what I was looking for.

Re: Random Weighted Elements in PHP

Porkins — Thu, 12 Feb 2009 09:08:40 -0000

This is great - a nice, tight function that does exactly what I was looking for - with one possible exception. Does this work well if you are drawing from an array with, say, 200 elements? Does adjusting the weight to the thousandths instead of the hundredths result with any degree of accuracy?

Re: Random Weighted Elements in PHP

Ishaan — Sat, 24 Jan 2009 01:35:42 -0000

I've been looking for something which does exactly this for a very long time now, and this is by far more elegant than anything I've been able to make so far. Thanks!

Re: Random Weighted Elements in PHP

pj — Tue, 16 Dec 2008 01:18:56 -0000

I'm going to implement this for my python program, thanks!

Re: Random Weighted Elements in PHP

Kareem — Tue, 09 Sep 2008 20:00:44 -0000

Thank you very much for this awesome trick, i was working on a small application, and i was using the array method, but i didn't like it, as you said it takes lots of resources if there was a big set of data, which is the case for me.
Wish you the best.
Regards.

Re: Random Weighted Elements in PHP

Venkat Koduru — Sun, 11 May 2008 08:18:19 -0000

Hmmm. I like your function, but here's what I'm looking to do...

I want to generate a random number from 0 to 6000. I want their to be the highest probability (40% chance) of getting between 100 and 2000, inclusive. I want the probability of getting a number from 99 to 0, to gradually decrease, so that it would be almost impossible to get 0. Just to be clear the probability of getting 98 should be less than that of 99, the probability of getting 97 should be less than the probability of getting 98, etc. Similarly, I want the probability of getting a number from 2001 to 6000 (again including 2001 and 6000) to gradually decrease, so that it's almost impossible to get 6000.

Anyone know how I could do this?

Re: Random Weighted Elements in PHP

Jeremy — Sat, 26 Apr 2008 18:59:03 -0000

Brilliant, and superbly clean code and explanation!

Re: Random Weighted Elements in PHP

Jesse — Thu, 07 Feb 2008 19:18:58 -0000

Mike,

X = f(W) is the random variable in question, if you want to make it even more formal. The specific properties of X, e.g., its CDF, are determined entirely by W.

Re: Random Weighted Elements in PHP

Mike — Thu, 07 Feb 2008 19:13:20 -0000

Great post. Now I'm trying to remember what I learned in my courses on probability in college... seems like there is a probability distribution for this, or something like that... what's the difference between a PDF and a CDF again? In any case, I think your method is essentially the same thing a stats class would have you do. :P

Re: Random Weighted Elements in PHP

Jesse — Thu, 07 Feb 2008 17:33:32 -0000

Chris,

Yeah, I'm trying to get back into it. Ever since I sold Adonomics I've been working more than full-time on it. Between that and a desire for an active social life I don't have much time for blogging.

I'm going to try to write one article at least once every other week, though.

Re: Random Weighted Elements in PHP

Chris — Thu, 07 Feb 2008 17:26:13 -0000

I'm glad you're still blogging. It's been a while since your previous post. I've enjoyed reading your posts on programming (especially the one on MySQL improvements). Thanks for writing these useful posts.