On Wed, Mar 24, 2010 at 8:21 PM, Glen Rubin <rubing...@gmail.com> wrote: > I wrote the following code to produce a lazy sequence of the triangle > numbers. (triangle numbers are the series of numbers: 1, 1+2, 1+2+3, > etc...) > > (defn tri-nums [] > "prduce a lazy sequence of triangle numbers" > (let [triangles (map #(range 1 %) (iterate inc 2))] > (map #(reduce + %) triangles))) > > However, I now have produced a large triangle number (76576500) and > want to know which triangle number it is (e.g. the first, second, > third, etc...)
Why not index the sequence with the 'indexed' function up front? > So, I wrote the following code which should accept a triangle number > as input and tell you which one it is in the series: > > (defn which-tri [z] > (loop [x 1 y (first (tri-nums))] > (cond > (= y z) > x > (< y z) > (recur (inc x) (first (rest (tri-nums)))) > true > (println "no such tri-num")))) > > > The problem is that this function works for the first two triangle > numbers (1, 3), but then just sits there doing nothing on any higher > (6, 10, etc...). You're calling rest at the same rate as inc in your loop, which means that x is increasing at a linear rate while y is increasing at a roughly quadratic rate. Thus as soon as y overtakes x, x never has a hope of catching up. Like I said above, you probably don't need to do this to find the index. But supposing you did, this is an instance of a more general problem: determining the index (if any) of a number in a sorted sequence. Something like this: (defn index-in-sorted-seq [x s] (loop [s s, i 0] (when (seq s) (cond (= x (first s)) i (> x (first s)) (recur (rest s) (inc i)) :else nil)))) -Per -- 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 To unsubscribe from this group, send email to clojure+unsubscribegooglegroups.com or reply to this email with the words "REMOVE ME" as the subject.