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Enumeration is the act of stepping through a collection (Array, Hash, etc) and performing some action on each object.

Enumeration is a key concept in Ruby and is used for sorting (sort_by), grouping (group_by), mapping (map), reducing (reduce), and much more. You’ll most frequently see enumeration as the idiomatic way iterating through collections rather than using loops.

A simple enumeration to print each word in an array would look like this:

words = %w[the cat sat on the mat]
words.each do |word|
  puts word
end

# Output:
# the
# cat
# sat
# on
# the
# mat

In this example, we have called the Array#each method and passed in a block, which takes one parameter (word) and prints it.

We can also chain enumerable methods. For example, we can chain .with_index onto each to print out the index of an object as well as it’s value:

words = %w[the cat sat on the mat]
list = words.map.with_index { |word, index| "#{index}. #{word}" }
puts list

# Output:
# 0. the
# 1. cat
# ...
# 5. mat

Enumerating Hash objects is exactly the same as enumerating Array objects, except that the block receives two arguments: the key and the value:

pet_names = {cat: "bob", horse: "caris", mouse: "arya"}
words.each { |animal, name| ... }

# The two arguments should be put in brackets when chaining
words.each.with_index { |(animal, name), index| ... }

The methods described above are part of the Enumerable module which is included in Array, Hash and other classes that require the ability to enumerate.

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Originally from Exercism ruby concepts