A recent post to StackOverflow <http://stackoverflow.com/questions/42798706/clojure-clojurescript-elegant-way-to-generate-a-deck-of-cards/42801711#42801711> asked for clarification on different ways of generating a deck of cards. The OP had used nested `for` with `flatten` and `map-indexed` but thought there must be a better (clearer & more concise) solution. In addition to the first two solutions offered, this problem also shows an example of when Python-style *generator functions* with a *yield* statement may also be considered. Pasted below is the example:
--------------------------------------------------------- For a third take on this problem, you can use the generator/yield style of programming popular in Python. This solution makes use of the *lazy-gen* & *yield* combination from the Tupelo library <https://github.com/cloojure/tupelo#generator-functions-for-lazy-sequences-a-la-python> : (ns tst.clj.core (:use clojure.test tupelo.test) (:require [tupelo.core :as t] )) (t/refer-tupelo) (defn new-deck [] (lazy-gen (let [id (atom 0)] (doseq [suit [:spade :heart :diamond :club]] (doseq [rank [:2 :3 :4 :5 :6 :7 :8 :9 :10 :jack :queen :king :ace] ] (yield {:id (swap! id inc) :suit suit :rank rank})))))) (pprint (new-deck)) ({:suit :spade, :rank :2, :id 1} {:suit :spade, :rank :3, :id 2} {:suit :spade, :rank :4, :id 3} {:suit :spade, :rank :5, :id 4} {:suit :spade, :rank :6, :id 5} {:suit :spade, :rank :7, :id 6} {:suit :spade, :rank :8, :id 7} {:suit :spade, :rank :9, :id 8} {:suit :spade, :rank :10, :id 9} {:suit :spade, :rank :jack, :id 10} {:suit :spade, :rank :queen, :id 11} {:suit :spade, :rank :king, :id 12} {:suit :spade, :rank :ace, :id 13} {:suit :heart, :rank :2, :id 14} {:suit :heart, :rank :3, :id 15} {:suit :heart, :rank :4, :id 16} {:suit :heart, :rank :5, :id 17} <snip> {:suit :club, :rank :10, :id 48} {:suit :club, :rank :jack, :id 49} {:suit :club, :rank :queen, :id 50} {:suit :club, :rank :king, :id 51} {:suit :club, :rank :ace, :id 52}) I just had to use keywords for the card names since using :rank 13 for the ace (or is it the king?) is just wrong...(shudder)...on so many levels! ;) Note that it is legal for a keyword in Clojure to consist of only digits. Since *yield* is putting the individual items on the output queue, we don't need to return a sequence from for and can just use doseq. Also, we don't need to worry about a multi-variate for, nor about using flatten, concat, or mapcat constructs. Use of the atom for id is as simple as it gets, although you could build the whole thing out of loop/recur instead of the atom & 2 doseq forms if you really wanted to be "pure". Under the covers *lazy-gen/yield* uses core.async to create an output stream with a default buffer size of 32 (modeled after lazy chunking in clojure.core). While for and map-indexed solutions work fine for this example, sometimes it may be clearer to the reader to be extra explicit about the looping constructs, etc. Also, if there were other operations in the inner loop before & after the yield statement, it might be awkward to force the solution into a map or for style solution. Enjoy! Alan P.S. I noticed after searching that Alex Engelberg worked on something similar a few years ago. This version is slightly different from his (hopefully improved!), although I did switch some symbols to gensyms after seeing his implementation. :) P.P.S. Before the purists complain about the use of mutable state & imperative loops, please remember that *for some problems* (not all!) this may yield a simpler solution with fewer mental stack frames required. Also, the solution is a pure function overall as seen from the outside, since it returns a non-side-effecting lazy-sequence, and neither the state atom nor the imperative loops escape the function body. This is similar to the implementation of many functions in clojure.core itself. -- 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/d/optout.
