Question

How can I create a lazy sequence of random numbers?

My current code:

(import '(java.util Random))

(def r (new Random))
(defn rnd [_] 
    (.nextInt r 10))

(defn random-numbers [max] 
    (iterate #(.nextInt r max) (.nextInt r max)))

(println (take 5 (random-numbers 10)))

executing it throws an exception:

(Exception in thread “main” clojure.lang.ArityException: Wrong number of args (1) passed to: user$random-numbers$fn
at clojure.lang.AFn.throwArity(AFn.java:437)
at clojure.lang.AFn.invoke(AFn.java:39)
at clojure.core$iterate$fn__3870.invoke(core.clj:2596)
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:60)
at clojure.lang.RT.seq(RT.java:466)
at clojure.core$seq.invoke(core.clj:133)
at clojure.core$take$fn__3836.invoke(core.clj:2499)
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:60)
at clojure.lang.Cons.next(Cons.java:39)
at clojure.lang.RT.next(RT.java:580)
at clojure.core$next.invoke(core.clj:64)
at clojure.core$nthnext.invoke(core.clj:2752)
at clojure.core$print_sequential.invoke(core_print.clj:57)
at clojure.core$fn__4990.invoke(core_print.clj:140)
at clojure.lang.MultiFn.invoke(MultiFn.java:167)
at clojure.core$pr_on.invoke(core.clj:3264)
at clojure.core$pr.invoke(core.clj:3276)
at clojure.lang.AFn.applyToHelper(AFn.java:161)
at clojure.lang.RestFn.applyTo(RestFn.java:132)
at clojure.core$apply.invoke(core.clj:600)
at clojure.core$prn.doInvoke(core.clj:3309)
at clojure.lang.RestFn.applyTo(RestFn.java:137)
at clojure.core$apply.invoke(core.clj:600)
at clojure.core$println.doInvoke(core.clj:3329)
at clojure.lang.RestFn.invoke(RestFn.java:408)
at user$eval7.invoke(testing.clj:12)
at clojure.lang.Compiler.eval(Compiler.java:6465)
at clojure.lang.Compiler.load(Compiler.java:6902)
at clojure.lang.Compiler.loadFile(Compiler.java:6863)
at clojure.main$load_script.invoke(main.clj:282)
at clojure.main$script_opt.invoke(main.clj:342)
at clojure.main$main.doInvoke(main.clj:426)
at clojure.lang.RestFn.invoke(RestFn.java:408)
at clojure.lang.Var.invoke(Var.java:401)
at clojure.lang.AFn.applyToHelper(AFn.java:161)
at clojure.lang.Var.applyTo(Var.java:518)
at clojure.main.main(main.java:37)
[Finished in 3.8s with exit code 1]

Is this a completey wrong approach, because I am using state, namely r is an instance of java.util.Random, or is it just a nooby syntax error?

I just studing clojure on myself, so please bear with me 🙂 .

Answer 1

repeatedly is great for repeatedly running a function and gathering the results in a seq

user> (take 10 (repeatedly #(rand-int 42)))
(14 0 38 14 37 6 37 32 38 22)

as for your original approach: iterate takes an argument, feeds it to a function and then takes the result of that and passes it back to the same function. I’t not quite what you want here because the function you are using doesn’t need any arguments. You can of course give it a placeholder for that argument and get it working, though repeatedly is likely a better fit.

(defn random-numbers [max]
  (iterate (fn [ignored-arg] (.nextInt r max)) (.nextInt r max)))
#'user/random-numbers

user> (println (take 5 (random-numbers 10)))
(3 0 0 2 0)