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From: Stefan Behnel on 20 Jun 2010 06:44 southof40, 20.06.2010 12:19: > I have list of of N Vehicle objects - the only possible vehicles are > cars, bikes, trucks. > > I want to select an object from the list with a probability of : cars > 0.7, bikes 0.3, trucks 0.1. > > I've currently implemented this by creating another list in which each > car object from the original list appears 7 times, each bike 3 times > and each truck once. I then pick at random from that list. > > This works but seems very clunky to me. Why? It's a very simple, generic, easy to understand and fast solution to the problem. Stefan
From: Cameron Simpson on 20 Jun 2010 06:58 On 20Jun2010 12:44, Stefan Behnel <stefan_ml (a)behnel.de> wrote:| southof40, 20.06.2010 12:19: | >I have list of of N Vehicle objects - the only possible vehicles are | >cars, bikes, trucks. | > | >I want to select an object from the list with a probability of : cars | >0.7, bikes 0.3, trucks 0.1. | > | >I've currently implemented this by creating another list in which each | >car object from the original list appears 7 times, each bike 3 times | >and each truck once. I then pick at random from that list. | > | >This works but seems very clunky to me. | | Why? It's a very simple, generic, easy to understand and fast | solution to the problem. Only 3 out of 4, if you want to be precise in your selections. Supposing he wants probabilities 0.7432, 0.3765, 0.1087654 ? The required list needs to be Very Long to achieve an accurate representation, and thus Very Slow to construct/populate. A faster approach is to make a list represention the sum of the proportions as one counts along the choices, thus 0.7, 1.0, 1.1 in the example given (0.7, 0.7+0.3, 0.7+0.3+0.1). Then choose a value in the range 0.0 to the total (1.1) using the pseudo-random function of your choice, such as that in the random module. Then binary search the list for the matching item. The list scales linearly as the number of choices, not exponentially with the precision of the proportions. The search is logarithmic with the number of choices. Beyond a very small number of choices the former will dominate. Cheers, -- Cameron Simpson <cs (a)zip.com.au> DoD#743http://www.cskk.ezoshosting.com/cs/ .... you could spend *all day* customizing the title bar. Believe me. I speak from experience. - Matt Welsh
From: duncan smith on 20 Jun 2010 12:25 southof40 wrote: > I have list of of N Vehicle objects - the only possible vehicles are > cars, bikes, trucks. > > I want to select an object from the list with a probability of : cars > 0.7, bikes 0.3, trucks 0.1. > > I've currently implemented this by creating another list in which each > car object from the original list appears 7 times, each bike 3 times > and each truck once. I then pick at random from that list. > > This works but seems very clunky to me. Can anyone suggest a better > data structure which would support the 'weighted randomness' I'm > after ? > > I'm not fixed on the idea of using a list - could be a dictionary, > tree whatever . > > Thanks in advance. > Try googling for "alias table". Efficient if you're selecting many random objects from the same mass function. Better than binary search on the cumulative mass function in big-O terms (but maybe not in practical terms for reasonable sized problems). Neither approach is as efficient as the one you outlined, but the required list size might be an issue for some sets of probabilities. Duncan
From: duncan smith on 20 Jun 2010 22:26
duncan smith wrote: > southof40 wrote: >> I have list of of N Vehicle objects - the only possible vehicles are >> cars, bikes, trucks. >> >> I want to select an object from the list with a probability of : cars >> 0.7, bikes 0.3, trucks 0.1. >> >> I've currently implemented this by creating another list in which each >> car object from the original list appears 7 times, each bike 3 times >> and each truck once. I then pick at random from that list. >> >> This works but seems very clunky to me. Can anyone suggest a better >> data structure which would support the 'weighted randomness' I'm >> after ? >> >> I'm not fixed on the idea of using a list - could be a dictionary, >> tree whatever . >> >> Thanks in advance. >> > > Try googling for "alias table". Efficient if you're selecting many > random objects from the same mass function. Better than binary search > on the cumulative mass function in big-O terms (but maybe not in > practical terms for reasonable sized problems). Neither approach is as > efficient as the one you outlined, but the required list size might be > an issue for some sets of probabilities. > > Duncan BTW, the alias table approach is basically a means of getting round the problem of needing large lists. Assuming your probabilities should be 0.7, 0.2 and 0.1 you could construct a list of 3 objects. The first would be 100% car, the second would be 60% bike and 40% car, the third would be 30% truck and 70% car. Choose an object at random, then the vehicle type according to the mass function associated with the object. The alias table approach only requires the generation of a single uniform random variate and a single comparison (once you've constructed it). Duncan |