Ruby初学者。未示出

Question

class Player 
    def initialize (name, age, start_year) 
    @name  = name.capitalize 
    @age  = age 
    @start_year = start_year 
    end 

    def playing_for 
    current_year = Time.new.strftime("%Y") 
    num_of_years = current_year - @start_year 
    @num_of_years = num_of_years 
    end 

    def to_s 
    "Hi, my name is #{@name}. I am #{@age} years old. I have been playing Ultimate for #{@num_of_years} years." 
    end 
end 

jon = Player.new("jon", 41, 2009) 
puts jon 

实例变量当运行在终端上面的代码，它显示为：

Hi my name is Jon. I am 41 years old. I have been playing Ultimate for years. 

和不显示#{@num_of_years}。

很明显，我定义@num_of_years的方式有问题，但我无法弄清楚什么。不打印

Answer 1

@num_of_years IST，因为变量未设置（未初始化的实例变量具有一个空字符串一个的nil和"#{nil}"结果值）。而另一方面在playing_for被设定，它永远不会被调用

jon = Player.new("jon", 41, 2009) # <- "new" calls Player#initialize 
puts jon       # <- "puts" calls Player#to_s 

@num_of_years（这一点：

@name，@age和@start_year在initialize这是通过调用Player.new调用都设置，并to_s由puts调用是什么魔杖制造商wanted to say）。

你可以调用puts之前手动调用方法：（记得用year更换strftime("%Y")修复the bug）

jon = Player.new("jon", 41, 2009) 
#=> #<Player:0x007fab8a8aa448 @name="Jon", @age=41, @start_year=2009> 
jon.to_s 
#=> "Hi, my name is Jon. I am 41 years old. I have been playing Ultimate for years." 
jon.playing_for 
#=> 7 
jon.to_s 
#=> "Hi, my name is Jon. I am 41 years old. I have been playing Ultimate for 7 years." 

puts jon 

打印：

Hi, my name is Jon. I am 41 years old. I have been playing Ultimate for 7 years. 

但记得要打电话每次变方法很快很乏味。

它更容易设置@num_of_years在初始化，就像其他的实例变量：

class Player 
    def initialize (name, age, start_year) 
    @name   = name.capitalize 
    @age   = age 
    @start_year = start_year 
    @num_of_years = Time.new.year - start_year 
    end 

    def to_s 
    "Hi, my name is #{@name}. I am #{@age} years old. I have been playing Ultimate for #{@num_of_years} years." 
    end 
end 

puts Player.new("jon", 41, 2009) 
#=> Hi, my name is Jon. I am 41 years old. I have been playing Ultimate for 7 years. 

或者你可以定义计算差值的方法。但你并不需要另一个实例变量在这种情况下，只返回值：

class Player 
    def initialize (name, age, start_year) 
    @name   = name.capitalize 
    @age   = age 
    @start_year = start_year 
    end 

    def playing_for 
    Time.new.year - @start_year 
    end 

    def to_s 
    "Hi, my name is #{@name}. I am #{@age} years old. I have been playing Ultimate for #{playing_for} years." 
    end 
end 

puts Player.new("jon", 41, 2009) 
#=> Hi, my name is Jon. I am 41 years old. I have been playing Ultimate for 7 years. 

看看在to_s方法。我已将#{@num_of_years}替换为#{playing_for}。