On Dec 13, 4:51 am, levand <luke.vanderh...@gmail.com> wrote: > So, I'm trying to understand functional programming, particularly as > it relates to the seq abstraction, and I'm hitting a slight difficulty > as I'm playing around - something that seems as if it ought to be > simple, is not. > > I'm playing with copying one seq into another. I've found several > different ways to do it, but the most obvious one, in my opinion, is > missing. > > Here's the straight-up, high performance tail recursive version that > was my first thought: > > (defn copy-seq-r [source] > (loop [so-far '() to-go source] > (if (nil? to-go) > so-far > (recur (cons (first to-go) so-far) > (rest to-go))))) > > The obvious problem with this is that because seqs are last-in first- > out, this returns the seq in reverse order. Obviously not what we > wanted. And using (reverse) or consing the (last) is awful for > performance. >
We'll this version actually does reverse. So one way to copy a sequence could be to (reverse (reverse seq)): (defn copy-seq (seq) (reverse (reverse seq))) or (def copy-seq (comp reverse reverse)) But, as mago pointed out, copying persistent structures doesn't make that much sense. --~--~---------~--~----~------------~-------~--~----~ 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 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 -~----------~----~----~----~------~----~------~--~---
