Enumerable#group_by

Returns: Hash · Updated March 13, 2026 · Enumerable

ruby enumerable

The group_by method organizes collection elements into groups based on a block evaluation. It returns a hash where keys represent each unique group and values are arrays containing the elements belonging to that group.

Basic Usage

numbers = [1, 2, 3, 4, 5, 6]
grouped = numbers.group_by { |n| n.even? }
# => {false=>[1, 3, 5], true=>[2, 4, 6]}

Grouping by String Length

words = ["apple", "cat", "banana", "dog", "cherry"]
by_length = words.group_by { |word| word.length }
# => {5=>["apple"], 3=>["cat", "dog"], 6=>["banana", "cherry"]}

Grouping Strings by First Letter

names = ["Alice", "Bob", "Anna", "Bill"]
by_letter = names.group_by { |name| name[0] }
# => {"A"=>["Alice", "Anna"], "B"=>["Bob", "Bill"]}

With Nested Data

products = [
  { name: "Widget", price: 10 },
  { name: "Gadget", price: 25 },
  { name: "Gizmo", price: 10 }
]

by_price = products.group_by { |p| p[:price] }
# => {10=>[{name: "Widget", price: 10}, {name: "Gizmo", price: 10}], 
#     25=>[{name: "Gadget", price: 25}]}

Practical Example: Categorizing Test Results

scores = [85, 92, 78, 90, 55, 88]
grades = scores.group_by { |s|
  case
  when s >= 90 then "A"
  when s >= 80 then "B"
  when s >= 70 then "C"
  when s >= 60 then "D"
  else "F"
  end
}
# => {"B"=>[85, 88], "C"=>[78], "A"=>[92, 90], "F"=>[55]}

Return Value

group_by always returns a hash. Empty collections return an empty hash:

[].group_by { |x| x }
# => {}