Ruby flat_map vs. map.flatten

benchmark.rb

require 'benchmark/ips'

Benchmark.ips do |bm|
  list = (0..10_000).to_a

  bm.report "flat_map" do
    list.flat_map do |x|
      [x, x * x]
    end
  end

  bm.report "map.flatten" do
    list.map do |x|
      [x, x * x]
    end.flatten
  end

  bm.compare!
end

results

tobi@happy ~/github/ruby_playground $ ruby benchmark/flat_map.rb 
Warming up --------------------------------------
            flat_map    95.000  i/100ms
         map.flatten    42.000  i/100ms
Calculating -------------------------------------
            flat_map    972.133  (± 5.1%) i/s -      4.845k in   5.002625s
         map.flatten    423.280  (± 2.4%) i/s -      2.142k in   5.063801s

Comparison:
            flat_map:      972.1 i/s
         map.flatten:      423.3 i/s - 2.30x slower