You can store the numbers in a vector [2 5 3]. Instead of doing this store the cumulative weight v = [0 2 7 10]. What your problem comes down to is given a number n, find the the lowest index i such that v[i] <= n. This can be accomplished with binary search. You can also use a binary search tree instead of a vector. You also keep a vector w with the actual objects. Drawing is done like this: generate a random number n between 0 and total-weight = (last v). Then find the smallest index i such that v[i] <= n. The drawn object is w[i]. Hope this helps.
On 26 jun, 15:01, Nicolas Oury <nicolas.o...@gmail.com> wrote: > Dear all, > > for a project, I need a data structure to represent finite distribution, > that is a set of elements, each of which having a mass in the set, and two > functions: > > - total-mass : the sum of the masses of each element > - draw : return an element with a probability proportional to its mass > > I currently use the simple idea : a Persistent Hash Map from elt to mass: > - the total-mass is the reduce of (+ . second) > - drawing is choosing a random-number between 0 and the total-mass and > looping through the seq, to look for the corresponding element. > > Both operation are linear in the number of elements. This is a problem, > because these sets tend to be quite big in my project. > > I think that if I used a HashTrie with total-mass cached in each node, > total-mass would be in constant time and draw in log time. > > So here is my question : does anyone has, for example for the "clojure in > clojure" project, a sketch of an implementation of PersistentHashMap > in clojure itself? (I would be happy to contribute, if it needs more work). > Or has anyone a better idea that mine to have drawable sets? > > Thanks for your help, > > Best regards, > > Nicolas. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
