Enumerable#grep vs Enumerable#select
Often, Enumerable#select is the chosen method to obtain elements from an
Array for a given block. Without thinking twice, we may be doing more work than
necessary by not taking advantage of another method from the Enumerable module,
Enumerable#grep.
Enumerable#grep can not always be used instead of Enumerable#select, but
when it can, #grep provides better readability and a small speed improvement
over #select.
See the benchmark below for example, which uses the class matcher for #grep:
2.1.2 :041 > a = (1..100000).map { rand(100000) }
2.1.2 :042 > b = (1..100000).map { rand(100000).to_s }
2.1.2 :043 > c = a + b
2.1.2 :044 > Benchmark.bm(10) do |b|
2.1.2 :045 > b.report("select") { c.select { |x| x.is_a? Integer } }
2.1.2 :046?> b.report("grep") { c.grep(Integer) }
2.1.2 :047?> end
user system total real
select 0.040000 0.000000 0.040000 ( 0.037799)
grep 0.020000 0.000000 0.020000 ( 0.023749)
2.1.2 :048 > c.select { |x| x.is_a? Integer } == c.grep(Integer)
=> true
You can also see its power when using Regexp patterns, which is not only faster
but also less verbose, which is enough of a win to favor it over #select:
2.1.2 :049 > a = (1..100000).map { (65 + rand(26)).chr }
2.1.2 :050 > b = (1..100000).map { rand(10).to_s }
2.1.2 :051 > c = a + b
2.1.2 :052 > Benchmark.bm(10) do |b|
2.1.2 :053 > b.report("select") { c.select{|x| /^\d*$/ === x} }
2.1.2 :054?> b.report("grep") { c.grep(/^\d*$/) }
2.1.2 :055?> end
user system total real
select 0.020000 0.000000 0.020000 ( 0.021058)
grep 0.020000 0.000000 0.020000 ( 0.012626)
There are some pretty good use cases for #grep, and one last thing worth
keeping in mind: #grep can take a second parameter, which acts
as #map does and reads very nicely. For example:
2.1.2 :056 > hello = ['Hello', 'ello', 'llo', 'lo', 'o']
2.1.2 :057 > hello.select { |x| /[A-Z]\w+/ === x }.map(&:downcase)
=> ["hello"]
2.1.2 :058 > hello.grep(/[A-Z]\w+/, &:downcase)
=> ["hello"]