Armando's suggested change worked fine for me.
(use '[clojure.contrib.lazy-seqs :only (primes)])
(defn prime-factors [n]
(let [f (some #(if (= 0 (rem n %)) %) primes)]
(println "n:" n ", f:" f)
(if (= f n)
(sorted-set f)
(conj (prime-factors (/ n f)) f))))
user=> (prime-factors 600851475143)
n: 600851475143 , f: 71
n: 8462696833 , f: 839
n: 10086647 , f: 1471
n: 6857 , f: 6857
#{71 839 1471 6857}
On Jan 13, 10:01 am, Vitaly Peressada <[email protected]> wrote:
> Armando, thanks for a plausible explanation. Here is what happened
> after I made the suggested change:
>
> user=> (prime-factors 600851475143)
> n: 600851475143 , f: 71
> n: 8462696833 , f: 839
> n: 10086647 , f: 1471
> n: 6857 , f: 6857
> #<CompilerException java.lang.ClassCastException: java.lang.Integer
> cannot be cast to clojure.lang.IFn (REPL:91)>
>
> I guess this something subtle with unordered vs. sorted-set as
> implicit factors accumulator. May be that is why the author of the
> solution had to use unordered set followed by (apply max ...).
>
> On Jan 13, 11:23 am, Armando Blancas <[email protected]>
> wrote:
>
>
>
>
>
>
>
> > A literal set is a unordered hash-set. To get the factors in order
> > change #{f} for (sorted-set f).
>
> > On Jan 13, 7:09 am, Vitaly Peressada <[email protected]> wrote:
>
> > > The following solution by <b>mtgred</b> for <a href="http://clojure-
> > > euler.wikispaces.com/">Project Euler Clojure</a> problem 003 uses
> > > implicit recursion.
>
> > > <pre>
> > > (use '[clojure.contrib.lazy-seqs :only (primes)])
> > > (defn prime-factors [n]
> > > (let [f (some #(if (= 0 (rem n %)) %) primes)]
> > > (if (= f n) #{f} (conj (prime-factors (/ n f)) f))))
> > > (apply max (prime-factors 600851475143))
> > > </pre>
>
> > > Here is above with added println
>
> > > (defn prime-factors [n]
> > > (let [f (some #(if (= 0 (rem n %)) %) primes)]
> > > (println "n:" n ", f:" f)
> > > (if (= f n)
> > > #{f}
> > > (conj (prime-factors (/ n f)) f))))
>
> > > Which produces
>
> > > n: 600851475143 , f: 71
> > > n: 8462696833 , f: 839
> > > n: 10086647 , f: 1471
> > > n: 6857 , f: 6857
> > > #{71 839 6857 1471}
>
> > > Can anybody explain why 6857 comes 3rd? I would expect to be the last.
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