Ruby NoMethodError（未定义方法 ''...' for '...:Class'

Question

require_relative 'json_lookup'
require_relative 'csv_lookup'
require_relative 'error'

BASE_RATE = 'EUR'

class CurrencyExchange

  def initialize(file:, date:, from:, to:)
    @file = file
    @date = date
    @from = from
    @to = to
  end

  def rate
    lookup = find_lookup
    lookup.to_currency / lookup.from_currency
  end

  private
  def find_lookup
    case File.extname(@file)
    when ".json"
      JsonLookup.new(@file, @date, @from, @to)
    when ".csv"
      CsvLookup.new(@file, @date, @from, @to)
    else raise FileError
    end
  end
end

I keep on getting this error for when i run the CurrencyExchange.rate in irb so I am guessing something is going wrong with the rate method but can't figure it out as to why. But I may be missing something completely obvious... As I am a complete beginner at Ruby, would appreciate any help:)

The traceback is as follows..

irb(main):003:0> CurrencyExchange.rate(Date.new(2018, 11, 22), "USD", "GBP")                                            Traceback (most recent call last):
        5: from C:/Ruby26-x64/bin/irb.cmd:31:in `<main>'
        4: from C:/Ruby26-x64/bin/irb.cmd:31:in `load'
        3: from C:/Ruby26-x64/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        2: from (irb):3
        1: from (irb):3:in `rescue in irb_binding'
NoMethodError (undefined method `rate' for CurrencyExchange:Class)

Answer 1

rate is an instance method in your example but CurrencyExchange.rate tries to call a class method.

To solve this, initialize an instance first and call then rate on that instance. Furthermore rate doesn't accept arguments, you need to pass the variables to the initializing method.

currency_exchange = CurrencyExchange.new(
  file: file, date: Date.new(2018, 11, 22), from: "USD", to: "GBP"
)
currency_exchange.rate

Note take the initializer expects 4 named arguments. You will need to pass a file to the new method too.