选择阵列 使用选择获得 >= 4 个容量

Question

我正在尝试使用select来获取容量>= 4:

class Home
  attr_reader(:name, :city, :capacity, :price)
  def initialize(name, city, capacity, price)
    @name = name
    @city = city
    @capacity = capacity
    @price = price
  end
end
homes = [
  Home.new("Nizar's place", "San Juan", 2, 42),
  Home.new("Fernando's place", "Seville", 5, 47),
  Home.new("Josh's place", "Pittsburgh", 3, 41),
  Home.new("Gonzalo's place", "Málaga", 2, 45),
  Home.new("Ariel's place", "San Juan", 4, 49)
]

I tried:

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
  puts high_capacities_homes
end

没有任何成功，我没有想法，有人可以帮助吗?(

Answer 1

Try

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
end
puts high_capacities_homes

puts返回nil，因此您的所有块计算为false。

Answer 2

让我们仔细看看发生了什么。当你执行你的代码时，你会得到以下内容:

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
  puts high_capacities_homes
end


 => [] 

返回一个空数组，但是为什么所有这些空行?这是因为您的代码对homes的每个元素执行puts nil。想想看:

high_capacities_homes = homes.select do |hm|
  hm.capacity >= 4
  puts "high_capacities_homes is nil: #{high_capacities_homes.nil?}"
end
high_capacities_homes is nil: true
high_capacities_homes is nil: true
high_capacities_homes is nil: true
high_capacities_homes is nil: true
high_capacities_homes is nil: true
 => [] 

当Ruby看到

high_capacities_homes = <anything>

她做的第一件事是创建一个局部变量high_capacities_homes并将其赋值为nil。只有在select枚举完homes之后，high_capacities_homes才会赋给select所在块的返回值[]。

还有一个问题。方法的第一行

hm.capacity >= 4 

返回true或false，但没有对该返回值进行操作。这就好像它被射入了太空，再也看不见了，使得你的代码相当于:

high_capacities_homes = homes.select do |hm|
  puts high_capacities_homes
end

由于puts返回nil(在输出输出之后)，为了构造数组high_capacities_homes，您的代码相当于:

high_capacities_homes = homes.select do |hm|
  nil
end

这就是为什么没有选择homes的元素。

您是否希望创建一个包含homes和hm.capacity >= 4的元素hm的数组(以及打印这些实例的信息)?如果是，首先选择homes的元素:¹。

selected_homes = homes.select { |hm| hm.capacity >= 4 }
  # => [#<Home:0x007ff81981e138 @name="Fernando's place", @city="Seville",
  #       @capacity=5, @price=47>,
  #     #<Home:0x007ff81981dda0 @name="Ariel's place", @city="San Juan",
  #       @capacity=4, @price=49>] 

注意，array# select保留homes的所有hm元素，select的block返回true。

现在，你想打印什么?如果它是selected_homes的每个元素的实例变量的值，您可以这样做(作为一个示例)。

selected_homes.each { |hm|
  puts "#{hm.name} in #{hm.city} has capacity #{hm.capacity} and price $#{hm.price}" }

打印

Fernando's place in Seville has capacity 5 and price $47
Ariel's place in San Juan has capacity 4 and price $49

^{1当然，你也可以写成selected_homes = homes.reject { |hm| hm.capacity < 4 } .}