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POIs Perl's unpack() ever faster than substr()?
 

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  1. POIs Perl's unpack() ever faster than substr()?
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    <p>Several times I've read that <code>unpack()</code> is faster than <code>substr()</code>, especially as the number of substrings increases. However, this benchmark suggests otherwise. Is my benchmark flawed, or is the alleged performance advantage of <code>unpack()</code> a holdover from older versions of Perl?</p> <pre><code>use strict; use warnings; use Benchmark; my ($data, $format_string, $n_substrings); my %methods = ( unpack =&gt; sub { return unpack $format_string, $data }, substr =&gt; sub { return map {substr $data, $_, 1} 0 .. $n_substrings - 1 }, ); for my $exp (1 .. 5){ $n_substrings = 10 ** $exp; print $n_substrings, "\n"; $format_string = 'a1' x $n_substrings; $data = 9 x $n_substrings; Benchmark::cmpthese -2, \%methods; } </code></pre> <p>Output (on Windows):</p> <pre><code>10 Rate unpack substr unpack 131588/s -- -52% substr 276802/s 110% -- 100 Rate unpack substr unpack 13660/s -- -57% substr 31636/s 132% -- 1000 Rate unpack substr unpack 1027/s -- -68% substr 3166/s 208% -- 10000 Rate unpack substr unpack 84.4/s -- -74% substr 322/s 281% -- 100000 Rate unpack substr unpack 5.46/s -- -82% substr 30.1/s 452% -- </code></pre> <p>As pointed out in some answers, <code>unpack()</code> does poorly on Windows. Here's the output on a solaris machine -- not nearly so decisive, but <code>substr()</code> still wins the foot race:</p> <pre><code>10 Rate unpack substr unpack 202274/s -- -4% substr 210818/s 4% -- 100 Rate unpack substr unpack 22015/s -- -9% substr 24322/s 10% -- 1000 Rate unpack substr unpack 2259/s -- -9% substr 2481/s 10% -- 10000 Rate unpack substr unpack 225/s -- -9% substr 247/s 9% -- 100000 Rate unpack substr unpack 22.0/s -- -10% substr 24.4/s 11% -- </code></pre>
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    1. CONothing is faster than anything. You have to apply it to specific problem. Is a cheetah faster than a goose? Maybe over 100 meters, but not across an ocean. :)
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    2. CO@brian I take your point, but the comment seems off target. Here's a closer analogue: "Is a cheetah *ever* faster than a goose? Here's a *specific* example of them racing each other. Was my test biased?"? Answer: "Yes, biased. The goose was on steroids."
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