enum.sort_by â an_enumerator
Sorts enum using a set of keys generated by mapping the values in enum through the given block.
If no block is given, an enumerator is returned instead.
1 2 | %w{apple pear fig}.sort_by { |word| word.length} #=> ["fig", "pear", "apple"] |
The current implementation of sort_by generates an array of
tuples containing the original collection element and the mapped value.
This makes sort_by fairly expensive when the keysets are
simple.
1 2 3 4 5 6 7 8 | require 'benchmark'a = (1..100000).map { rand(100000) }Benchmark.bm(10) do |b| b.report("Sort") { a.sort } b.report("Sort by") { a.sort_by { |a| a } }end |
produces:
1 2 3 | user system total realSort 0.180000 0.000000 0.180000 ( 0.175469)Sort by 1.980000 0.040000 2.020000 ( 2.013586) |
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 sort method.
1 2 3 | files = Dir["*"]sorted = files.sort { |a, b| File.new(a).mtime <=> File.new(b).mtime }sorted #=> ["mon", "tues", "wed", "thurs"] |
This sort is inefficient: it generates two new File objects
during every comparison. A slightly better technique is to use the
Kernel#test method to generate the modification times
directly.
1 2 3 4 5 | files = Dir["*"]sorted = files.sort { |a, b| test(?M, a) <=> test(?M, b)}sorted #=> ["mon", "tues", "wed", "thurs"] |
This still generates many unnecessary Time 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.
1 2 3 4 | sorted = Dir["*"].collect { |f| [test(?M, f), f]}.sort.collect { |f| f[1] }sorted #=> ["mon", "tues", "wed", "thurs"] |
This is exactly what sort_by does internally.
1 2 | sorted = Dir["*"].sort_by { |f| test(?M, f) }sorted #=> ["mon", "tues", "wed", "thurs"] |
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