NoMethodError: 未定义方法 `[]=' 为 nil:NilClass

Question

Error in title. What's going wrong? Attempting to initialize Temperature object with a hash. If I just do

puts Temperature.from_celsius(50).in_fahrenheit

then it works fine and returns 122.0

But

Temperature.new(:f => 50)

returns an error.

class Temperature
  attr_accessor :f, :c

  @temp = {:f => 32, :c => 0}

  def initialize(params)
    if params[:f] != nil
      self.class.from_fahrenheit(params[:f])

    else 
      self.class.from_celsius(params[:c])
    end
  end

  def self.from_fahrenheit(temp)
   @temp[:f] = temp 
   @temp[:c] = ((temp - 32.0)/1.8).round(1)

    return @temp
  end

  def self.from_celsius(temp)
    @temp[:c] = temp
    @temp[:f] = (temp * 1.8 + 32).round(1)

    return @temp
  end  

  def in_fahrenheit
    @temp[:f]
  end

  def in_celsius
    @temp[:c]
  end


end

class Hash
  def in_fahrenheit
    self[:f]
  end

  def in_celsius
    self[:c]
  end
end

puts Temperature.from_celsius(50).in_celsius

tempo = Temperature.new(:f => 50)
tempo.in_fahrenheit

Answer 1

Just as the error message says. You are calling []= on @temp in a Temperature instance, which is nil by default because you have not assigned anything to it anywhere.

Answer 2

You can't initialize instance variable in a class body, as you did. You should do it in a constructor, and because you have three constructors, your code should look like this:

class Temperature
  def initialize(params)
    @temp = {:f => 32, :c => 0}
    if params[:f] != nil
      self.class.from_fahrenheit(params[:f])
    else 
      self.class.from_celsius(params[:c])
    end
  end

  def self.from_fahrenheit(temp)
    @temp = {}
    @temp[:f] = temp 
    @temp[:c] = ((temp - 32.0)/1.8).round(1)

    return @temp
  end

  def self.from_celsius(temp)
    @temp = {}
    @temp[:c] = temp
    @temp[:f] = (temp * 1.8 + 32).round(1)

    return @temp
  end  

  def in_fahrenheit
    @temp[:f]
  end

  def in_celsius
    @temp[:c]
  end


end