109 lines
2.9 KiB
Markdown
109 lines
2.9 KiB
Markdown
-----
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isHidden: false
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menupriority: 1
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kind: article
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created_at: 2010-02-23T10:09:52+02:00
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title: When regexp is not the best solution
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tags:
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- programming
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- regexp
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- regular expression
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- extension
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- file
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-----
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Regular expression are really useful. Unfortunately, they are not always the best way of doing things.
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Particularly when transformations you want to make are easy.
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I wanted to know how to get file extension from filename the fastest way possible. There is 3 natural way of doing this:
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<div><code class="ruby">
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# regexp
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str.match(/[^.]*$/);
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ext=$&
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# split
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ext=str.split('.')[-1]
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# File module
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ext=File.extname(str)
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</code></div>
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At first sight I believed that the regexp should be faster than the split because it could be many `.` in a filename. But in reality, most of time there is only one dot and I realized the split will be faster. But not the fastest way. There is a function dedicated to this work in the `File` module.
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Here is the Benchmark ruby code:
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<div><code class="ruby" file="regex_benchmark_ext.rb">
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#!/usr/bin/env ruby
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require 'benchmark'
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n=80000
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tab=[ '/accounts/user.json',
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'/accounts/user.xml',
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'/user/titi/blog/toto.json',
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'/user/titi/blog/toto.xml' ]
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puts "Get extname"
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Benchmark.bm do |x|
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x.report("regexp:") { n.times do
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str=tab[rand(4)];
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str.match(/[^.]*$/);
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ext=$&;
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end }
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x.report(" split:") { n.times do
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str=tab[rand(4)];
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ext=str.split('.')[-1] ;
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end }
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x.report(" File:") { n.times do
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str=tab[rand(4)];
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ext=File.extname(str);
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end }
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end
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</code></div>
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And here is the result
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<pre class="twilight">
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Get extname
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user system total real
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regexp: 2.550000 0.020000 2.570000 ( 2.693407)
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split: 1.080000 0.050000 1.130000 ( 1.190408)
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File: 0.640000 0.030000 0.670000 ( 0.717748)
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</pre>
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Conclusion of this benchmark, dedicated function are better than your way of doing stuff (most of time).
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## file path without the extension.
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<div><code class="ruby" file="regex_benchmark_strip.rb">
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#!/usr/bin/env ruby
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require 'benchmark'
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n=80000
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tab=[ '/accounts/user.json',
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'/accounts/user.xml',
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'/user/titi/blog/toto.json',
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'/user/titi/blog/toto.xml' ]
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puts "remove extension"
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Benchmark.bm do |x|
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x.report(" File:") { n.times do
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str=tab[rand(4)];
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path=File.expand_path(str,File.basename(str,File.extname(str)));
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end }
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x.report("chomp:") { n.times do
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str=tab[rand(4)];
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ext=File.extname(str);
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path=str.chomp(ext);
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end }
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end
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</code></div>
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and here is the result:
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<pre class="twilight">
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remove extension
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user system total real
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File: 0.970000 0.060000 1.030000 ( 1.081398)
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chomp: 0.820000 0.040000 0.860000 ( 0.947432)
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</pre>
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Conclusion of the second benchmark. One simple function is better than three dedicated functions. No surprise, but it is good to know.
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