Mark: that looks a lot like the "collector" I learned about in The Little Schemer. I actually do have How to Design Programs, but I wanted to finish Seasoned Schemer first. I was poking around in SICP because of a Project Euler problem. Thanks for the tip!
John: good catch. Can you confirm the difference between my original defn and your letfn? Both work, but as I understand it from the documentation, defn would actually define that local function globally, while letfn actually does bind it locally to all the expr which follow in the body.. On Sun, Nov 8, 2009 at 7:01 PM, John Harrop <jharrop...@gmail.com> wrote: > You have a bug: > (defn exp-mod [base exp m] > (cond > (zero? exp) 1 > (even? exp) (mod (Math/sqrt (exp-mod base (/ exp 2) m)) m) > :else (mod (* base (exp-mod base (inc exp) m)) m))) > should be > (defn exp-mod [base exp m] > (cond > (zero? exp) 1 > (even? exp) (mod (Math/sqrt (exp-mod base (/ exp 2) m)) m) > :else (mod (* base (exp-mod base (dec exp) m)) m))) > The recursion depth is logarithmic in exp so TCO is probably unnecessary > here. But with the bug, it gets locked into a cycle of exp being alternately > 2 and 1 and never reaching zero, the base case, so the recursion continues > until the stack blows up. > Meanwhile, > (defn fermat-test [n] > (defn try-it [a] > (= (exp-mod a n n) a)) > (try-it (inc (rand-int (dec n))))) > should really be > (defn fermat-test [n] > (letfn [(try-it [a] > (= (exp-mod a n n) a))] > (try-it (inc (rand-int (dec n)))))) > in Clojure, or even > (defn fermat-test > "Performs the Fermat test on n with a, if specified, or with a random > value from 2 to n." > ([n a] > (= (exp-mod a n n) a)) > ([n] > (fermat-test n (inc (rand-int (dec n)))))) > > > > --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
