It's a pet peeve of mine but, please, try hard not to use indices :-) (or if you need indices pick a better suited data structure) The code you try to write is hard to write because you are going against the grain.
If you try to write swap for vectors (which support efficient random lookup and assoc) it's pretty simple: (defn swap [v i j] (-> v (assoc i (v j)) (assoc j (v i)))) But if you want to make it work with any kind of collection and return a seq (it's the code you have written) it's far less pleasant... and efficient. hth Christophe On Fri, Nov 13, 2009 at 7:07 AM, Mark Tomko <mjt0...@gmail.com> wrote: > Let's try this again: > > (defn swap [coll i j] > (if (= i j) coll > (let [li (min i j) ui (max i j)] > (let [[pre-li post-li] (split-at li coll)] > (let [[post-li-pre-ui post-li-post-ui] > (split-at (- ui 1 li) (rest post-li))] > (concat > pre-li > (list (nth coll ui)) > post-li-pre-ui > (list (nth coll li)) > (rest post-li-post-ui))))))) > > The code is actually even more complicated. I'm sure with a little > more time I could clean it up. > > On Nov 12, 9:59 pm, Mark Tomko <mjt0...@gmail.com> wrote: >> Oh, I posted too soon. My implementation has a bug. >> >> On Nov 12, 9:56 pm, Mark Tomko <mjt0...@gmail.com> wrote: >> >> >> >> > I came up with a way to do it, but I'm sure there's a simpler way. >> > Here's what I have: >> >> > (defn swap [coll i j] >> > (let [li (min i j) ui (max i j)] >> > (let [[pre-li post-li] (split-at li coll)] >> > (let [[post-li-pre-ui post-li-post-ui] (split-at (- ui 1) (rest >> > post-li))] >> > (concat >> > pre-li >> > (list (nth coll j)) >> > post-li-pre-ui >> > (list (nth coll i)) >> > (rest post-li-post-ui)))))) >> >> > Basically, I find the lower index and the upper index. I then find >> > the elements in the collection that appear before the lower index, and >> > the elements that appear between the lower index and the upper index, >> > and the elements that appear after the upper index. I then create a >> > new list that is the concatenation of these sublists, in order. It's >> > sort of the definition of swap on an immutable data structure, but it >> > feels like an awful lot of code. >> >> > I considered using subvec: >> >> >http://clojure.org/api#toc548 >> >> > But I didn't want to require that my input collection be a vector, or >> > convert it to one. This the precursor to my implementing a heap data >> > structure, as a little toy application. > > -- > 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 > -- Professional: http://cgrand.net/ (fr) On Clojure: http://clj-me.cgrand.net/ (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
