gistfile1.rb

#!/usr/bin/env ruby

Payment  = Struct.new(:payee, :amount, :tags)
Dateline = Struct.new(:date)
TRANSACTIONS = [
  Dateline.new("2015-01-01"),
  Payment.new("Milliways", 25476, "dining travel"),
  Payment.new("Sirius Cybernetics", 7839),
  Payment.new("Old Pink Dog Bar", 2790, "dining"),
  Dateline.new("2015-01-02"),
  Payment.new("Magrathea", 49900000000000000, "real-estate"),
  Payment.new("Big Bang Burger Bar", 4780),
  Payment.new("Megadodo", 7850, "travel"),
]

# So you have an array of objects that contain 2 different object types.
# So you want just the payments?


puts TRANSACTIONS.grep(Payment)

# Think about that for a second...
# You did not pass grep a string, you passed in an struct object class.