Enumerable.flat_map / group_by / tally

collection.flat_map { |element| expression }

# Groups elements by the block's return value as a key.
collection.group_by { |element| key }

# Returns a hash with the count of each element's occurrences.
collection.tally

# Flattens a nested array.
array.flatten
array.flatten(depth)

sample_enumerable_flat_map_group_by_tally.rb

sentences = ["member_a value_x", "member_b value_y"]
words = sentences.flat_map { |s| s.split(" ") }
puts words.inspect # ["member_a", "value_x", "member_b", "value_y"]

# Use group_by to group fighters by team.
fighters = [
  { name: "member_a", team: "team_x" },
  { name: "member_b", team: "team_y" },
  { name: "member_c", team: "team_z" },
  { name: "member_d", team: "team_z" }
]
by_team = fighters.group_by { |f| f[:team] }
puts by_team["team_x"].map { |f| f[:name] }.inspect # ["member_a"]
puts by_team["team_z"].map { |f| f[:name] }.inspect # ["member_c", "member_d"]

# Use tally to count occurrences of each element.
teams = ["team_x", "team_y", "team_x", "team_z", "team_y", "team_x"]
counts = teams.tally
puts counts.inspect # {"team_x"=>3, "team_y"=>2, "team_z"=>1}

# Get the most frequent element.
most_common = counts.max_by { |_, count| count }
puts "Most common: #{most_common[0]} (#{most_common[1]} times)" # Most common: team_x (3 times)

Running the code produces the following output:

ruby enumerable_flat_map_group_by_tally.rb
["member_a", "value_x", "member_b", "value_y"]
["member_a"]
["member_c", "member_d"]
{"team_x"=>3, "team_y"=>2, "team_z"=>1}
Most common: team_x (3 times)

flat_map combines map and flatten(1) into a single operation. It is useful when you want to generate an array from each element and collect the results into a flat list. group_by is handy for classifying data, and it returns a hash.

tally was added in Ruby 2.7, making it easy to count element occurrences. In earlier versions, you had to implement the same logic using each_with_object or inject. It is particularly useful for tasks like log analysis or vote tallying where frequency counting is required.