Three ways to use Ruby's group_by greg park

Ruby’s #group_by method provides a way to (wait for it) group things by some arbitrary property. It’s part of the Enumerable module, so you can generally use it anywhere you’d be using #each or some iteration. To use #group_by, you first need to know two things: - The collection of objects that needs grouping (e.g., an array) - The rule by which you want to group those objects #group_by returns a hash where the keys are defined by our grouping rule, and the values are the corresponding objects from our original collection. Now let’s look at three examples of #group_by in action! ## Example #1: Grouping strings - The collection: last names stored as strings in an array - The rule: group names according to the first letter # Create a list of names names = ["Ripley", "McClane", "Ryerson", "Murphy"] # group by first letter names_by_letter = names.group_by { |name| name[0] } #{ # "R" => [ # [0] "Ripley", # [1] "Ryerson" # ], # "M" => [ # [0] "McClane", # [1] "Murphy" # ] #} ## Example #2: Grouping integers - The collection: an array of 50 random integers between 1 and 99 - The rule: group integers into 5 bins (e.g, 1-19, 20-39…) # generate random numbers numbers = (1..99).to_a.sample 50 # group by quotient (number divided by 20) data.group_by { |number| number / 20 } #{ # 3 => [ # [0] 78, # [1] 61, # [2] 67, # [3] 65, # [4] 68, # [5] 71, # [6] 76, # [7] 70, # [8] 64, # [9] 60 # ], # 1 => [ # [ 0] 24, # [ 1] 39, # [ 2] 28, # [ 3] 20, # [ 4] 26, # [ 5] 31, # [ 6] 29, # [ 7] 35, # [ 8] 32, # [ 9] 33, # [10] 30 # ], # 2 => [ # [0] 49, # [1] 57, # [2] 46, # [3] 51, # [4] 52, # [5] 42, # [6] 55, # [7] 58 # ], # 4 => [ # [0] 84, # [1] 80, # [2] 93, # [3] 91, # [4] 92, # [5] 86 # ], # 0 => [ # [ 0] 10, # [ 1] 9, # [ 2] 13, # [ 3] 14, # [ 4] 8, # [ 5] 17, # [ 6] 18, # [ 7] 11, # [ 8] 2, # [ 9] 7, # [10] 4, # [11] 3, # [12] 19, # [13] 15, # [14] 5 # ] #} ## Example #3: Grouping various objects - The collection: an array of various classes of objects - The rule: group objects by class These are pretty contrived examples, but when combined with other methods, #group_by can be a handy way to get your objects in order. Now go get grouping! # Create a list of objects array_of_fun = ["Bob", 89, [1, 2, 3], Hash.new, (0..10).to_a, "Gene", 5] # group objects by class array of fun.group_by { |thing| thing.class } #{ # String < Object => [ # [0] "Bob", # [1] "Gene" # ], # Fixnum < Integer => [ # [0] 89, # [1] 5 # ], # Array < Object => [ # [0] [ # [0] 1, # [1] 2, # [2] 3 # ], # [1] [ # [ 0] 0, # [ 1] 1, # [ 2] 2, # [ 3] 3, # [ 4] 4, # [ 5] 5, # [ 6] 6, # [ 7] 7, # [ 8] 8, # [ 9] 9, # [10] 10 # ] # ], # Hash < Object => [ # [0] {} # ] #}