run.rb

require "benchmark"

Benchmark.bm do |x|
  count = 10000
    mappings = { "a" => [1, 1], "t" => [-1, 1], "g" => [-1, -1], "c" => [1, -1] }
    x.report("hash.values.sample.size") do 
      _ = mappings.values.sample.size
    end

    x.report("hash.values[0].length  ") do
      _ = mappings.values[0].length
    end
end

	user	system	total	real
hash.values.sample.size	0.000033	0.000009	0.000042	( 0.000037)
hash.values[0].length	0.000003	0.000001	0.000004	( 0.000003)