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U:RDoc::AnyMethod[iI" sort_by:ETI"Enumerable#sort_by;TF: publico:RDoc::Markup::Document: @parts[ o: RDoc::Markup::Paragraph; [I"DSorts <i>enum</i> using a set of keys generated by mapping the ;TI"3values in <i>enum</i> through the given block.;To: RDoc::Markup::BlankLine o; ; [I"JThe result is not guaranteed to be stable. When two keys are equal, ;TI">the order of the corresponding elements is unpredictable.;T@o; ; [I"=If no block is given, an enumerator is returned instead.;T@o:RDoc::Markup::Verbatim; [I"7%w{apple pear fig}.sort_by { |word| word.length } ;TI"0 #=> ["fig", "pear", "apple"] ;T: @format0o; ; [ I"EThe current implementation of <code>sort_by</code> generates an ;TI"Harray of tuples containing the original collection element and the ;TI"Imapped value. This makes <code>sort_by</code> fairly expensive when ;TI" the keysets are simple.;T@o; ; [ I"require 'benchmark' ;TI" ;TI"*a = (1..100000).map { rand(100000) } ;TI" ;TI" Benchmark.bm(10) do |b| ;TI"& b.report("Sort") { a.sort } ;TI"3 b.report("Sort by") { a.sort_by { |a| a } } ;TI" end ;T; 0o; ; [I"<em>produces:</em>;T@o; ; [I",user system total real ;TI"=Sort 0.180000 0.000000 0.180000 ( 0.175469) ;TI"=Sort by 1.980000 0.040000 2.020000 ( 2.013586) ;T; 0o; ; [I"JHowever, consider the case where comparing the keys is a non-trivial ;TI"Ioperation. The following code sorts some files on modification time ;TI".using the basic <code>sort</code> method.;T@o; ; [I"files = Dir["*"] ;TI"Lsorted = files.sort { |a, b| File.new(a).mtime <=> File.new(b).mtime } ;TI"2sorted #=> ["mon", "tues", "wed", "thurs"] ;T; 0o; ; [ I"FThis sort is inefficient: it generates two new <code>File</code> ;TI"Hobjects during every comparison. A slightly better technique is to ;TI"Juse the <code>Kernel#test</code> method to generate the modification ;TI"times directly.;T@o; ; [ I"files = Dir["*"] ;TI""sorted = files.sort { |a, b| ;TI"# test(?M, a) <=> test(?M, b) ;TI"} ;TI"2sorted #=> ["mon", "tues", "wed", "thurs"] ;T; 0o; ; [ I"HThis still generates many unnecessary <code>Time</code> objects. A ;TI"Fmore efficient technique is to cache the sort keys (modification ;TI"Etimes in this case) before the sort. Perl users often call this ;TI"Aapproach a Schwartzian transform, after Randal Schwartz. We ;TI"Aconstruct a temporary array, where each element is an array ;TI"Jcontaining our sort key along with the filename. We sort this array, ;TI"3and then extract the filename from the result.;T@o; ; [ I"%sorted = Dir["*"].collect { |f| ;TI" [test(?M, f), f] ;TI"!}.sort.collect { |f| f[1] } ;TI"2sorted #=> ["mon", "tues", "wed", "thurs"] ;T; 0o; ; [I"?This is exactly what <code>sort_by</code> does internally.;T@o; ; [I"3sorted = Dir["*"].sort_by { |f| test(?M, f) } ;TI"1sorted #=> ["mon", "tues", "wed", "thurs"];T; 0: @fileI" enum.c;T:0@omit_headings_from_table_of_contents_below0I"]enum.sort_by { |obj| block } -> array enum.sort_by -> an_enumerator ;T0[ I"();T@`FI"Enumerable;TcRDoc::NormalModule00
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