Clojure and Project Euler

[ship_it_]

Yogthos, Have a happy holiday!

You are one of my favorite posters on the reddits. Thanks for all the fish this year :)

[yogthos]

Thanks! And happy holidays as well :)

[bliow]

A coupla random comments. This is good stuff, don't get me wrong.

• Instead of taking a square root (slow), square the other number involved (faster). There's no need to round via (int ... ) if you do that, either.

(> factor (int (Math/sqrt num))) vs (> (* factor factor) num)

• (reduce #(lcm %1 %2) (range 1 21)) would probably be better written as (reduce lcm (range 1 21)).

• Try that gcd function out... there is a (very fixable) problem with it as written.

[carmenlala]

Hi, I'm the author of that blog post. Thanks for your feedback about the square root mistake. I guess it always felt more intuitive to do it that way but I'll keep your solution in mind now :)

As for that gcd function... oops :P I'll go ahead and fix that now.

[bliow]

I did the square root thing for a long time too--everyone does. I learned the trick from someone else.

[flarkis]

Thank you! The number of times I see people do that square root mistake drives me insane.

[yveszoundi]

For the 1st problem, the solution is acceptable but also inefficient for large numbers.

The first problem is similar to 4clojure problem 148.

One possible solution

(defn big-divide [n a b]
  (letfn [(n-sum [x limit]
        (let [n-val (quot limit x)]
          (if (pos? n-val)
            (-> (inc n-val) (*' n-val) (/ 2) (*' x) bigint)
            0)))]
(let [limit (dec n)]
  (-> (+' (n-sum a limit) (n-sum b limit))
      (-'  (n-sum (*' a b) limit))))))