Thanks once again Marko. The only thing that I am having trouble understanding is this:
1. What exactly happens when an item is passed to #(keyset-b (key-fn %)) ? Does > keyset-b looks up itself (because collections are functions) for the item > which contains *:id X *and returns true/false? It returns the argument if it contains it, and otherwise nil.<http://clojure.org/data_structures#Data%20Structures-Sets> Let's assume that key-fn is defined as #(.getID %) so we have: #(keyset-b #(.getID %)) And now let's assume that item-object is passed to it. So, #(.getID %) returns, let's say, the number 3 (which is the value of the id). How exactly is that number is being looked up in keyset-b? How does keyset-b knows we are looking for an item with key id and value 3? Apparently I am missing something here hence my confusion. Cheers On Thursday, March 21, 2013 8:48:51 PM UTC+2, Marko Topolnik wrote: > > On Thursday, March 21, 2013 5:21:53 PM UTC+1, Ryan wrote: > >> Thanks Marko. I do have couple more q's for you just to ensure I got >> everything right: >> >> (comp keyset-b key-fn) >>> This results in a function that first applies *key-fn*, then *keyset-b*. >>> So it's like #(keyset-b (key-fn %)). Let's call this function *predicate >>> *. >> >> >> 1. What exactly happens when an item is passed to #(keyset-b (key-fn %)) ? >> Does >> keyset-b looks up itself (because collections are functions) for the item >> which contains *:id X *and returns true/false? >> > > It returns the argument if it contains it, and otherwise > nil.<http://clojure.org/data_structures#Data%20Structures-Sets> > > >> 2. Isn't it more idiomatic to write #((key-fn %) keyset-b) ? >> > > No, because it doesn't work :) An arbitrary object cannot be applied as a > function. > > >> 3. Does remove loops list-a internally and applies the predicate to each >> item? (if the answer is no my head will definitely explode) >> > > *remove* is just like *filter*, only with reversed logic. Its > implementation in fact is literally > > (filter (complement pred) > coll))<http://clojuredocs.org/clojure_core/1.2.0/clojure.core/remove> > > >> On Thursday, March 21, 2013 6:09:42 PM UTC+2, Marko Topolnik wrote: >> >>> Personal preference. It causes less mental load because it more >>> obviously spells out what you are doing. >>> >>> On Thursday, March 21, 2013 4:58:08 PM UTC+1, Ryan wrote: >>>> >>>> Thanks a lot Marko. Much better now :) >>>> >>>> I also wanted to ask you why did you mention in a previous post that >>>> you prefer using *remove *than *filter + complement*. Is there a >>>> reason for this or just a personal preference? >>>> >>>> Ryan >>>> >>>> On Thursday, March 21, 2013 5:37:33 PM UTC+2, Marko Topolnik wrote: >>>>> >>>>> First we build a set of all the keys in *list-b*: >>>>> >>>>> (into #{} (map key-fn list-b)) >>>>> >>>>> Let's call that set *keyset-b. *Then we use *keyset-b* as a function >>>>> which returns truthy (non-nil) for any key that is contained in it, and >>>>> compose it with our *key-fn*: >>>>> >>>>> (comp keyset-b key-fn) >>>>> >>>>> This results in a function that first applies *key-fn*, then *keyset-b >>>>> *. So it's like #(keyset-b (key-fn %)). Let's call this function * >>>>> predicate*. >>>>> >>>>> Finally, we use *predicate* to *remove* any member of *list-a* for >>>>> which it is truthy: >>>>> >>>>> (remove predicate list-a) >>>>> >>>>> -marko >>>>> >>>>> On Thursday, March 21, 2013 4:14:46 PM UTC+1, Ryan wrote: >>>>> >>>>>> Marko, >>>>>> >>>>>> Can you please do me a favor and break down the function you >>>>>> suggested me? I understand partially how it works but I am having >>>>>> trouble >>>>>> to fully get it. >>>>>> >>>>>> (remove (comp (into #{} (map key-fn list-b)) key-fn) list-a) >>>>>>> >>>>>> -- -- 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 unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
