reverse.detect vs select { ... }.last

[nerdrew]

Third approach?

require 'benchmark/ips'

ARRAY = [*1..100]

def faster
  ARRAY.reverse_each { |x| break x if (x % 10).zero? }
end

def fast
  ARRAY.reverse.detect { |x| (x % 10).zero? }
end

def slow
  ARRAY.select { |x| (x % 10).zero? }.last
end

Benchmark.ips do |x|
  x.report('Enumerable#reverse_each + break') { faster }
  x.report('Enumerable#reverse.detect') { fast }
  x.report('Enumerable#select.last')    { slow }
  x.compare!
end

% ruby reverse.rb
Calculating -------------------------------------
Enumerable#reverse_each + break
                       115.335k i/100ms
Enumerable#reverse.detect
                        72.531k i/100ms
Enumerable#select.last
                        11.243k i/100ms
-------------------------------------------------
Enumerable#reverse_each + break
                          3.311M (± 7.2%) i/s -     16.493M
Enumerable#reverse.detect
                          1.255M (± 9.3%) i/s -      6.238M
Enumerable#select.last
                        129.592k (± 3.4%) i/s -    652.094k

Comparison:
Enumerable#reverse_each + break:  3310776.9 i/s
Enumerable#reverse.detect:  1255082.9 i/s - 2.64x slower
Enumerable#select.last:   129592.0 i/s - 25.55x slower

[PeterCamilleri]

This test is biased in favor of reverse each because it bails on the first element, 100. A fairer test would be to search for (x % 51).zero?, since we expect to traverse about half the elements on average.

[nerdrew]

Results with x % 51

Comparison:
Enumerable#reverse_each + break:   257525.8 i/s
Enumerable#reverse.detect:   189938.5 i/s - 1.36x slower
Enumerable#select.last:   129624.6 i/s - 1.99x slower

[PeterCamilleri]

Wow. That sure seems to level the playing field.

[parkerfinch]

I'd propose replacing reverse.detect with reverse_each.detect in the comparison, rather than adding it as a third option, for all the reasons listed in the reverse.each vs reverse_each section.

If there's support behind this, I'm happy to whip up a PR for it!

[corsonknowles]

Yes, reverse_each.detect and reverse_each.any? are going to be more performant than select.last or reverse.detect. This is even documented in another benchmark on the same page. But reference material by definition usually means you look at the one relevant entry.

It's worth an update!