Let’s do this. • I know that lazy sequences only evaluate the items in the sequence that are asked for, how does it do this? Lazy sequences (henceforth LS, because I am a LP, or Lazy Person) are composed of parts. The head, or the part(s, as really 32 elements are evaluated at a time, … Read more
Here’s another approach that celebrates Clojure’s Java interop. This takes 374ms on a 2.4 Ghz Core 2 Duo (running single-threaded). I let the efficient Miller-Rabin implementation in Java’s BigInteger#isProbablePrime deal with the primality check. (def certainty 5) (defn prime? [n] (.isProbablePrime (BigInteger/valueOf n) certainty)) (concat [2] (take 10001 (filter prime? (take-nth 2 (range 1 Integer/MAX_VALUE))))) … Read more
Let me move the whitespace in your code so it’s obvious to you what is wrong: (defn factors [x] ((loop [n x i 2 acc []] (if (prime? n) (conj acc n) (if (zero? (mod n i)) (recur (/ n i) 2 (conj acc i)) (recur n (inc i) acc)))))) You see that weird (( … Read more
Here’s my take on it. I made a point of only using Clojure data structures to see how that would work out. Note that it would have been perfectly usual and idiomatic to take a blocking queue from the Java toolbox and use it here; the code would be easy to adapt, I think. Update: … Read more
I normally call (keys (ns-publics ‘foo)) to list Vars exported by the namespace foo; e.g. for clojure.contrib.monads this returns (defmonad censor m-when-not m+write+m maybe-m maybe-t …) (the … stands for quite a lot more). More generally, there’s a bunch of functions whose names start in ns- which list Vars by namespace, with certain additional criteria … Read more
doall is all you need. Just because the seq has type LazySeq doesn’t mean it has pending evaluation. Lazy seqs cache their results, so all you need to do is walk the lazy seq once (as doall does) in order to force it all, and thus render it non-lazy. seq does not force the entire … Read more
Symbols look themselves up in a map, much as keywords do. See Symbol’s implementation: … 122 public Object invoke(Object obj) { 123 return RT.get(obj, this); 124 } 125 126 public Object invoke(Object obj, Object notFound) { 127 return RT.get(obj, this, notFound); 128 } … (RT is clojure.lang.RT, which does just about everything. “RunTime”?) In the … Read more
Built-in: user> (def v [“one” “two” “three” “two”]) #’user/v user> (.indexOf v “two”) 1 user> (.indexOf v “foo”) -1 If you want a lazy seq of the indices for all matches: user> (map-indexed vector v) ([0 “one”] [1 “two”] [2 “three”] [3 “two”]) user> (filter #(= “two” (second %)) *1) ([1 “two”] [3 “two”]) user> … Read more
CORRECTED TWICE: A simpler way to un-chunk a lazy sequence: (defn unchunk [s] (when (seq s) (lazy-seq (cons (first s) (unchunk (next s)))))) First version omitted (when … so it returned an infinite seq of nil’s after the input sequence ended. Second version used first instead of seq so it stopped on nil. RE: your … Read more
require loads libs (that aren’t already loaded), use does the same plus it refers to their namespaces with clojure.core/refer (so you also get the possibility of using :exclude etc like with clojure.core/refer). Both are recommended for use in ns rather than directly.