CODEWITHSUNDEEP
clojure
Mossmyr
Mossmyr

Reputation: 991

LazySeq cannot be cast to Number

I'm trying to implement a function that returns Bernoulli numbers:

(defn binom [N K]
    (if (or (= K 0) (= N K)) 
        (int 1)
        (int (+ (binom (- N 1) (- K 1))
                (binom (- N 1) K)))))

(defn bernoulli [M]
    (if (= M 0)
        (int 1)
        (for [K (range 0 (- M 1))] 
            (* (binom M K)
               (bernoulli K)))))

(println (bernoulli 5))

(Obviously it's not correctly implemented yet, but that is irrelevant to my question)

When I run this program, I get the exception java.lang.ClassCastException: clojure.lang.LazySeq cannot be cast to java.lang.Number.

However, if I replace the recursive call (bernoulli K) with (bernoulli 0), the code runs fine.

What is going on here? Why can my binom function handle K as a number alright, but my bernoulli function can't?

Here's the entire error message:

$ clojure bern.clj
(Exception in thread "main" java.lang.ClassCastException: clojure.lang.LazySeq cannot be cast to java.lang.Number
    at clojure.lang.Numbers.multiply(Numbers.java:146)
    at user$bernoulli$iter__3__7$fn__8$fn__9.invoke(x2.clj:21)
    at user$bernoulli$iter__3__7$fn__8.invoke(x2.clj:18)
    at clojure.lang.LazySeq.sval(LazySeq.java:42)
    at clojure.lang.LazySeq.seq(LazySeq.java:60)
    at clojure.lang.RT.seq(RT.java:473)
    at clojure.core$seq.invoke(core.clj:133)
    at clojure.core$print_sequential.invoke(core_print.clj:46)
    at clojure.core$fn__5270.invoke(core_print.clj:140)
    at clojure.lang.MultiFn.invoke(MultiFn.java:167)
    at clojure.core$pr_on.invoke(core.clj:3266)
    at clojure.core$pr.invoke(core.clj:3278)
    at clojure.lang.AFn.applyToHelper(AFn.java:161)
    at clojure.lang.RestFn.applyTo(RestFn.java:132)
    at clojure.core$apply.invoke(core.clj:601)
    at clojure.core$prn.doInvoke(core.clj:3311)
    at clojure.lang.RestFn.applyTo(RestFn.java:137)
    at clojure.core$apply.invoke(core.clj:601)
    at clojure.core$println.doInvoke(core.clj:3331)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at user$eval18.invoke(x2.clj:29)
    at clojure.lang.Compiler.eval(Compiler.java:6514)
    at clojure.lang.Compiler.load(Compiler.java:6955)
    at clojure.lang.Compiler.loadFile(Compiler.java:6915)
    at clojure.main$load_script.invoke(main.clj:283)
    at clojure.main$script_opt.invoke(main.clj:343)
    at clojure.main$main.doInvoke(main.clj:427)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at clojure.lang.Var.invoke(Var.java:415)
    at clojure.lang.AFn.applyToHelper(AFn.java:161)
    at clojure.lang.Var.applyTo(Var.java:532)
    at clojure.main.main(main.java:37)

Upvotes: 3

Views: 1702

Answers (1)

noisesmith
noisesmith

Reputation: 20194

The second condition in bernoulli is a call to for, which returns a lazy-seq, but you are trying to invoke an arithmetic operation to it, which is simply invalid. Perhaps you want to sum the result with (apply + (for ...)), which uses the values returned by for as args to + (I don't know the specific algorithm off the top of my head, maybe you want an average or a product rather than a sum...).

Upvotes: 4

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