Hi Randall, yes, I am very aware that it is not an efficient solution ;-)
It is only an exercise just to make (Scheme) students aware of the possibilities of mutual definitions. The assignment in Kent's book reads: ;;; Exercise 2.8.6. ;;; All of the recursive procedures shown so far have been directly ;;; recursive. That is, each procedure directly applies itself to a new ;;; argument. It is also possible to write two procedures that use each ;;; other, resulting in indirect recursion. Define the procedures odd? ;;; and even?, each in terms of the other. [Hint: What should each return ;;; when its argument is 0?] Thanks for the reply! Arie 2008/12/5 Randall R Schulz <[EMAIL PROTECTED]> > > On Friday 05 December 2008 08:07, Arie van Wingerden wrote: > > Hi, > > > > I tried to use definitions which call each other, like so: > > > > (defn is-odd? [n] > > (if (= n 0) > > false > > (is-even? (- n 1)))) > > > > (defn is-even? [n] > > (if (= n 0) > > true > > (is-odd? (- n 1)))) > > > > but obviously Clojure rejects this, saying: > > Exception in thread "main" java.lang.Exception: Unable to resolve > > symbol: is-even? in this context > > which refers to "is-even?" in the definition of is-odd? > > > > Since I am very new to Clojure I may have missed a language construct > > to resolve this issue :-) > > You can (def is-even?) before your (defn is-odd? ...), which will allow > it to compile. > > However, this implementation is very inefficient, since it' will call > mutually recursively back and forth (without any kind of tail-call > optimization, at least as written) decrementing the initial argument > until it gets to zero. > > The trampoline mechanism can fix the fact that this code consumes an > amount of stack space linearly dependent on the initial argument, but > clearly this is not the way to test for evenness or oddness! > > > > Thx, > > Arie > > > Randall Schulz > > > > --~--~---------~--~----~------------~-------~--~----~ 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 [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
