Hi, On Fri, Oct 9, 2009 at 7:22 PM, vishy <vishalsod...@gmail.com> wrote:
> > I was attempting problem no 4 at projecteuler.net. The solution I came > up with was http://paste.lisp.org/display/88432 > > But, it executes very slowly.Why so? > Your code: (use 'clojure.contrib.combinatorics) (apply max (map (fn [[x y]] (* x y)) (filter #(= (seq(str (* (nth %1 0) ( nth %1 1))))(reverse (str (* (nth %1 0) (nth %1 1)))))(selections (range 100 1000) 2)))) If we outline your steps: 1) generate all pairs of 3 digits numbers 2) multiply (twice), serialize to string (twice), reverse and compare 3) multiply (again) 4) find the max 1) since multiplicationis commutative you don't need to consider both [x y] and [y x], so you can halve the amount if computation by generating less pairs 2) put the multiplication and serialization in a let: (fn [[x y]] (let [s (str (* x y))] (= (seq s) (reverse s)))) 3) too bad we can't keep the previous result 4) use reduce instead of apply (except for str and concat) So fixing #1 and #2 should nearly divide by 4 the run time. (reduce max (map (fn [[x y]] (* x y)) (filter (fn [[x y]] (let [s (str (* x y))] (= (seq s) (reverse s)))) (for [x (range 100 1000) y (range x 1000)] [x y])))) You should notice that we are not interested in pairs but in their product, so it fixes complaint #3. (reduce max (filter #(let [s (str %)] (= (seq s) (reverse s))) (for [x (range 100 1000) y (range x 1000)] (* x y)))) And it's indeed 4x faster. You can also consider merging the filter in the comprehension: (reduce max (for [x (range 100 1000) y (range x 1000) :let [n (* x y) s (str n)] :when (= (seq s) (reverse s))] n)) (for stylistic reasons) hth, Christophe --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
