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POHow to improve the performance of this Haskell program?
 

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  1. POHow to improve the performance of this Haskell program?
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    <p>I'm working through the problems in <a href="http://projecteuler.net/" rel="noreferrer">Project Euler</a> as a way of learning Haskell, and I find that my programs are a lot slower than a comparable C version, even when compiled. What can I do to speed up my Haskell programs?</p> <p>For example, my brute-force solution to <a href="http://projecteuler.net/index.php?section=problems&amp;id=14" rel="noreferrer">Problem 14</a> is:</p> <pre><code>import Data.Int import Data.Ord import Data.List searchTo = 1000000 nextNumber :: Int64 -&gt; Int64 nextNumber n | even n = n `div` 2 | otherwise = 3 * n + 1 sequenceLength :: Int64 -&gt; Int sequenceLength 1 = 1 sequenceLength n = 1 + (sequenceLength next) where next = nextNumber n longestSequence = maximumBy (comparing sequenceLength) [1..searchTo] main = putStrLn $ show $ longestSequence </code></pre> <p>Which takes around 220 seconds, while an "equivalent" brute-force C version only takes 1.2 seconds.</p> <pre><code>#include &lt;stdio.h&gt; int main(int argc, char **argv) { int longest = 0; int terms = 0; int i; unsigned long j; for (i = 1; i &lt;= 1000000; i++) { j = i; int this_terms = 1; while (j != 1) { this_terms++; if (this_terms &gt; terms) { terms = this_terms; longest = i; } if (j % 2 == 0) j = j / 2; else j = 3 * j + 1; } } printf("%d\n", longest); return 0; } </code></pre> <p>What am I doing wrong? Or am I naive to think that Haskell could even approach C's speed?</p> <p>(I'm compiling the C version with gcc -O2, and the Haskell version with ghc --make -O).</p>
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    1. USMartijn Pieters
    1. USstusmith
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      1. POHow to improve the performance of this Haskell program?
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    1. COYour `unsigned long` may be just 32-bit long. For fair comparison, use an `unsigned long long` or `uint64_t`.
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      1. POHow to improve the performance of this Haskell program?
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    2. CO@KennyTM - fair point - I was testing on 32-bit Ubuntu where a long happens to be 64-bits.
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    3. CO@stusmith: I see. That's fine then.
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