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POProject Euler #14 and memoization in Clojure
 

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  1. POProject Euler #14 and memoization in Clojure
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    <p>As a neophyte clojurian, it was <a href="https://stackoverflow.com/questions/2933051/clojure-for-a-lisp-illiterate">recommended to me</a> that I go through the <a href="http://projecteuler.net/" rel="nofollow noreferrer">Project Euler</a> problems as a way to learn the language. Its definitely a great way to improve your skills and gain confidence. I just finished up my answer to <a href="http://projecteuler.net/index.php?section=problems&amp;id=14" rel="nofollow noreferrer">problem #14</a>. It works fine, but to get it running efficiently I had to implement some memoization. I couldn't use the prepackaged <code>memoize</code> function because of the way my code was structured, and I think it was a good experience to roll my own anyways. My question is if there is a good way to encapsulate my cache within the function itself, or if I have to define an external cache like I have done. Also, any tips to make my code more idiomatic would be appreciated.</p> <pre><code>(use 'clojure.test) (def mem (atom {})) (with-test (defn chain-length ([x] (chain-length x x 0)) ([start-val x c] (if-let [e (last(find @mem x))] (let [ret (+ c e)] (swap! mem assoc start-val ret) ret) (if (&lt;= x 1) (let [ret (+ c 1)] (swap! mem assoc start-val ret) ret) (if (even? x) (recur start-val (/ x 2) (+ c 1)) (recur start-val (+ 1 (* x 3)) (+ c 1))))))) (is (= 10 (chain-length 13)))) (with-test (defn longest-chain ([] (longest-chain 2 0 0)) ([c max start-num] (if (&gt;= c 1000000) start-num (let [l (chain-length c)] (if (&gt; l max) (recur (+ 1 c) l c) (recur (+ 1 c) max start-num)))))) (is (= 837799 (longest-chain)))) </code></pre>
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    1. COJust wanted to thank you for exposing me to project euler. I too am trying to learn clojure.
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    2. COExtremely minor point, but (inc c) is probably more idiomatic than (+ 1 c).
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      1. USAdrian Mouat
 

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