#ruby-202001091040.rb

months = ["2019-01-01", "2019-02-01", "2019-03-01", "2019-04-01"]
monthly_doc_count_for_topic = [
  ["foo","2019-02-01: 186904","2019-03-01: 196961"],
  ["bar","2019-01-01: 8876","2019-04-01: 8694"]
]

monthly_doc_count_for_topic.map { |array| array + months }
#=> [
#   ["foo", "2019-02-01: 186904", "2019-03-01: 196961", "2019-01-01", "2019-02-01", "2019-03-01", "2019-04-01"],
#   ["bar", "2019-01-01: 8876", "2019-04-01: 8694", "2019-01-01", "2019-02-01", "2019-03-01", "2019-04-01"]
# ]

# +monthly_doc_count_for_topic+ should be unchanged, since we
# only used non-mutating methods.


# Your solution also works, but mutates each array inside
# +monthly_doc_count_for_topic+. I've replaced #map with #each
# here. Which only effects the return value, which we aren't
# using here.
monthly_doc_count_for_topic.each do |topic_set|
  months.each { |month| topic_set << month }
end

monthly_doc_count_for_topic
#=> [
#   ["foo", "2019-02-01: 186904", "2019-03-01: 196961", "2019-01-01", "2019-02-01", "2019-03-01", "2019-04-01"],
#   ["bar", "2019-01-01: 8876", "2019-04-01: 8694", "2019-01-01", "2019-02-01", "2019-03-01", "2019-04-01"]
# ]