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sort_by-i.yaml
--- !ruby/object:RI::MethodDescription aliases: [] block_params: comment: - !ruby/struct:SM::Flow::P body: Sorts <em>enum</em> using a set of keys generated by mapping the values in <em>enum</em> through the given block. - !ruby/struct:SM::Flow::VERB body: " %w{ apple pear fig }.sort_by {|word| word.length}\n #=> ["fig", "pear", "apple"]\n" - !ruby/struct:SM::Flow::P body: The current implementation of <tt>sort_by</tt> generates an array of tuples containing the original collection element and the mapped value. This makes <tt>sort_by</tt> fairly expensive when the keysets are simple - !ruby/struct:SM::Flow::VERB body: " require 'benchmark'\n include Benchmark\n\n a = (1..100000).map {rand(100000)}\n\n bm(10) do |b|\n b.report("Sort") { a.sort }\n b.report("Sort by") { a.sort_by {|a| a} }\n end\n" - !ruby/struct:SM::Flow::P body: <em>produces:</em> - !ruby/struct:SM::Flow::VERB body: " user system total real\n Sort 0.180000 0.000000 0.180000 ( 0.175469)\n Sort by 1.980000 0.040000 2.020000 ( 2.013586)\n" - !ruby/struct:SM::Flow::P body: However, consider the case where comparing the keys is a non-trivial operation. The following code sorts some files on modification time using the basic <tt>sort</tt> method. - !ruby/struct:SM::Flow::VERB body: " files = Dir["*"]\n sorted = files.sort {|a,b| File.new(a).mtime <=> File.new(b).mtime}\n sorted #=> ["mon", "tues", "wed", "thurs"]\n" - !ruby/struct:SM::Flow::P body: "This sort is inefficient: it generates two new <tt>File</tt> objects during every comparison. A slightly better technique is to use the <tt>Kernel#test</tt> method to generate the modification times directly." - !ruby/struct:SM::Flow::VERB body: " files = Dir["*"]\n sorted = files.sort { |a,b|\n test(?M, a) <=> test(?M, b)\n }\n sorted #=> ["mon", "tues", "wed", "thurs"]\n" - !ruby/struct:SM::Flow::P body: This still generates many unnecessary <tt>Time</tt> objects. A more efficient technique is to cache the sort keys (modification times in this case) before the sort. Perl users often call this approach a Schwartzian Transform, after Randal Schwartz. We construct a temporary array, where each element is an array containing our sort key along with the filename. We sort this array, and then extract the filename from the result. - !ruby/struct:SM::Flow::VERB body: " sorted = Dir["*"].collect { |f|\n [test(?M, f), f]\n }.sort.collect { |f| f[1] }\n sorted #=> ["mon", "tues", "wed", "thurs"]\n" - !ruby/struct:SM::Flow::P body: This is exactly what <tt>sort_by</tt> does internally. - !ruby/struct:SM::Flow::VERB body: " sorted = Dir["*"].sort_by {|f| test(?M, f)}\n sorted #=> ["mon", "tues", "wed", "thurs"]\n" full_name: Enumerable#sort_by is_singleton: false name: sort_by params: | enum.sort_by {| obj | block } => array visibility: public
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