I'm fairly new too, but I'll take a stab. I think the koan's for 19-21 are trying to make you do exactly what you are doing: consider the performance characteristics for various operations on a vector vs a list or sequence.
Problem 19, for example, is highlighting that 1 - Last is slow for a vector. The last function is really meant to operate on sequences, and therefore is restricted to only being able to use first/next in its implementation, so for a vector, it has to walk the entire vector to get to the end. 2 - Vectors can efficiently get the last value with "peek". So, for 19, it seems to make sense to convert to a vector and just peek: #(peek (vec %)) For 21, the same is true: nth is a sequence operation and therefore must walk the vector. The solution here is actually what you tried (use get), but you'd need to convert the lists to a vector first: #(get (vec %) %2) So, my "lesson learned" here is: If you need random access or, mostly need access to the end of a fixed-length (ie non-infinite), use, or convert to a vector. But, again I'm new too :) On Thu, Jun 9, 2011 at 8:50 PM, clojurefanxx <neuzhou...@gmail.com> wrote: > i'm a newbie working thru 4clojure.com's problems #19 thru #21 where > i'm asked to write a function, which when given a list or vector, > returns the last, penultimate, or an arbitrary nth element, > respectively. > > for problem #19 (return last element), using the function last was not > accepted as a good solution for both lists and vectors, but the > following was accepted: > > user=> (#(nth % (- (count %) 1)) '(5 4 3)) > 3 > user=> (#(nth % (- (count %) 1)) '[1 2 3 4 5]) > 5 > > for problem #21, (return arbitrary element) , using the function nth > or get was not accepted. > --------------------------------------------------------------------------------------------- > Write a function which returns the Nth element from a sequence. > > (= (__ '(4 5 6 7) 2) 6) > > (= (__ [:a :b :c] 0) :a) > > (= (__ [1 2 3 4] 1) 2) > > (= (__ '([1 2] [3 4] [5 6]) 2) [5 6]) > ------------------------------------------------------------------------------------------------ > > > i think using the nth or get functions on vectors to return an element > at index position n should be fine performance-wise, > but what's a general solution for lists that will perform better than > O(n)? (i 'm assuming that's the reason why > last, nth, and get were rejected when I tried them out as candidate > solutions??) > > thanks for any help! > > > > -- > 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 -- 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
