On Tue, Aug 9, 2011 at 12:50 PM, Kevin Sookocheff <kevin.sookoch...@gmail.com> wrote: > Hi, > > I have a question regarding the map data structure. I'm trying to program a > Sieve of Eratosthenes using the algorithm at Wikipedia: > > Input: an integer n > 1 > > Let A be an array of bool values, indexed by integers 2 to n, > initially all set to true. > > for i = 2, 3, 4, ..., while i^2 ≤ n: > if A[i] is true: > for j = i^2, i^2 + i, i^2 + 2i, ..., while j ≤ n: > A[j] = false > > Now all i such that A[i] is true are prime. > > I'm having a problem creating the data structure A. > > What I want to do is create a map of integers from 2 to n all initialized to > true that I can then prune using the algorithm. > > Any ideas?
Since the keys are consecutive integers, you might consider a vector of booleans: (def A (vec (repeat (inc n) true))) You can then use assoc to set any particular index to false: (assoc A 5 false) Differences using a vector instead of a hash-map: will stay in numeric order, doesn't allow arbitrary dissoc, nth and assoc may be faster, use less memory, (vector-of :bool) will definitely use less memory, and probably others. Depending on your goals, a vector may or may not be preferable to a hash-map. --Chouser -- 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
