Hello Andreas! Thanks for your swift reply. I'll try to explain the reasoning behind my code.
In the Clojure Reference I found the function into, which is described like so: Usage: (into to from) "Returns a new coll consisting of to-coll with all of the items of from-coll conjoined." I'm currently reading the book "Programming Clojure". In it, the author has a chapter on binding which states that vectors are indeed immutable. I'm not sure if this is a problem of scope, but I thought that I could coerce fib-vec into receiving a new vec and throwing the old one away (making the old one available for deletion from the GC). That is why I used 'into fib-vec'. I thought it would re-bind fib-vec to itself, but with one element less. I used the same idea with ansVec. Could you perhaps tell me why it cannot do that? Rather, is this behavior possible in Clojure? PS: Thanks for the book-tipp! I'll be sure to check it out. On Mar 20, 10:48 am, Andreas Kostler <andreas.koestler.le...@gmail.com> wrote: > Hi Christian, > What you're trying to do is to build up a vector with the last element of the > vector being the answer to your problem: > (last answer) > => 4613732 > You're trying to use cons (conj) to build up that list. > > Now, your function below never terminates because you're: > a) Not actually building up the ans-vec > b) Not actually changing fib-vec to be closer to termination > > Clojure datastructures are immutable. conj returns a new seq with the second > argument conj(s)ed onto the old sequence without changing the old seq e.g.: > > (def v [1 2 3]) > (conj v 4) > => [1 2 3 4] > > but: > v > => [1 2 3] > > The same applies to into. > The closest I can think of you're trying to achieve is: > > (defn answer [] > (loop [ans-vec [0] fib-v fib-vec] > (if (empty? fib-v) > ans-vec > (recur (conj ans-vec (+ (last ans-vec) (last fib-v))) (pop > fib-v))))) > > Now you're actually building up ans-vec in a tail-recursive fashion while > changing fib-v to be closer to termination. > > Having said that, what you really want is (reduce + fib-vec) ;) > > Andreas > P.S. To get your head around basic recursion dig into "The little Schemer" by > Friedman and Felleisen > > On 20/03/2011, at 7:22 PM, Christian wrote: > > > > > > > > > > > I've tried Project Euler 2 now. > > > For those unfamiliar, Project Euler Problem 2 states: find the sum of > > all even-valued fibonacci terms that are less than four million. > > > Let's assume that I have the code to fill the vector: > > > (def fib-seq > > ((fn rfib [a b] > > (lazy-seq (cons a (rfib b (+ a b))))) > > 0 1)) > > > (def fib-vec > > (into [] (filter even? (take 35 fib-seq)))) > > > Many people reading the Clojure Samples will be familiar with the > > first sample. My own implementation did not want the 'take' function > > to work. However, that is a question for another day. Assume that the > > magic number 35 in the second function does indeed give me all even- > > valued fibonacci terms under four million. (I did not know how to > > specify a predicate depending on the value of (fib-seq) to check if > > the value is under 4 million.) > > > With the function '(reduce + (fib-vec))' I effortlessly summed up all > > values in the vector. This is peculiar, because I first wanted to > > write a function that does this for me, but to no avail. > > > (defn answer [] > > (let [ansVec [0]] > > (while (not-empty fib-vec) > > (conj ansVec (+ (last ansVec) (last fib-vec))) > > (into fib-vec (pop fib-vec))))) > > > This function never terminates. Why not? Where am I thinking wrong? > > > -- > > 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 > > ************www.leica-geosystems.com************* > > when it has to be right, Leica Geosystems > > Please consider the environment before printing this email. -- 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
