Thanks for your replies Marshal & Jim. I had not understood the 'laziness' concept correctly and was expecting it to behave differently. I thought that this post had somehow disappeared and I started another one on the same topic. Sorry about that.
--Vinay On Tuesday, 31 July 2012 18:17:00 UTC+5:30, Marshall Vandegrift wrote: > > "Vinay D.E" writes: > > > I am a newbie and was doing some exercises when I ran across something > that > > I don't understand. > > I am trying to count the number of elements in an array less than 100. > > > > My first attempt didn't work. The counter returns 0 > > > > (let [a (atom 0) > > i (take-while (fn[x] (swap! a inc) (< x 100)) [1 2 3 4 5])] > > [@a i]) => [0 (1 2 3 4 5)] > > The primary concrete problem is laziness, but fundamentally this sort of > mutable-imperative algorithm doesn't play nice with Clojure's core > features. > > The sequence returned by `take-while` is lazy, and none of the function > applications required to generate that sequence actually happen until > something forces evaluation of the sequence elements. So when you get > the result of `[@a i]` what happens is: > > (1) `@a` is evaluated; nothing else has actually happened yet, so the > result is still the initial value of `0`. > > (2) `i` is evaluated; now the sequence is forced and generated, and as > a side-effect the atom is updated. > > Instead of depending on mutating side-effects to do these sorts of > calculations, most of the Clojure standard library assumes that you'll > be working with (largely) pure functions which operate primarily in > terms of their parameters and return values. As an aside, you probably > wanted `filter` instead of `take-while` even for what you were trying to > do in the first place -- you should be able to figure out the difference > from the docs. So, using pure functions, we can something as simple as: > > (count (filter #(< x 100) input-sequence)) > > HTH, > > -Marshall > > -- 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
